3.2.1.1. Death

One report was located regarding death in humans following acute inhalation exposure to xylene (composition unspecified) (Morley et al. 1970). One of three men died after breathing paint fumes for several hours that contained an estimated atmospheric concentration of 10,000 ppm xylene. Xylene comprised 90% of the solvent in the paint (small amounts of toluene were also present), with the total solvent comprising 34% of the paint by weight. An autopsy of the man who died showed severe pulmonary congestion, interalveolar hemorrhage, and pulmonary edema; the brain showed hemorrhaging and evidence of anoxic damage. Clinical signs noted in the two exposed men who survived included solvent odor of the breath, cyanosis of the extremities, and neurological impairment (temporary confusion, amnesia). Both men recovered completely. The authors hypothesized that anoxia did not contribute to the effects observed in the survivors because the flow of oxygen into the area in which the men were working should have been adequate. The study was inconclusive for evaluating the toxic effects of xylene because the subjects were concurrently exposed to other chemicals in the paint. No studies were located regarding mortality in humans after intermediate or chronic inhalation exposure to mixed xylene or xylene isomers.

Acute inhalation LC₅₀ values have been determined in animals for xylene and its isomers (Bonnet et al. 1979; Carpenter et al. 1975a; Harper et al. 1975; Hine and Zuidema 1970; Ungvary et al. 1980b). The 4-hour LC₅₀ value for mixed xylene in rats ranged from 6,350 ppm (Hine and Zuidema 1970) to 6,700 ppm (Carpenter et al. 1975a). The 4-hour LC₅₀ value for p-xylene in rats was reported to be 4,740 ppm (Harper et al. 1975). In mice, the 6-hour LC₅₀ values for m-, o-, and p-xylene were determined to be 5,267, 4,595, and 3,907 ppm, respectively (Bonnet et al. 1979). These data suggest that p-xylene may be slightly more toxic than the other xylene isomers. According to the toxicity classification system of Hodge and Sterner (1949), these values indicate that mixed xylene and its isomers are slightly toxic by acute inhalation.

Mice appear to be more sensitive than rats to the lethal effects of the m- and o-isomers of xylene (Cameron et al. 1938). While no rats died following a 24-hour exposure to 2,010 ppm m-xylene, 6 of 10 mice died as a result of a similar exposure. Similarly, a 24-hour exposure of rats to 3,062 ppm o-xylene resulted in a death rate of only 1 in 10, whereas in mice, 4 of 10 died. It is unclear whether differential sensitivities exist for the p-isomer of xylene in mice and rats (Cameron et al. 1938).

Information regarding lethality following intermediate-duration exposures is limited to the results of a single study examining mortality in rats, guinea pigs, monkeys, and dogs following intermittent and continuous exposure to o-xylene (Jenkins et al. 1970). Continuous exposure to 78 ppm o-xylene for 90–127 days resulted in the death of only 1 of 15 rats. Intermittent exposure to 780 ppm o-xylene resulted in deaths of 3 of 15 rats; none of the 15 guinea pigs, 3 monkeys, or 2 dogs died. No data were located regarding death following chronic-duration exposure to mixed xylene or its isomers.

All LC₅₀ values and LOAEL values from each reliable study for death in each species and duration category are recorded in Table 3-1 and plotted in Figure 3-1.

3.2.1.2. Systemic Effects

No human or animal data were available regarding dermal effects following inhalation exposure to mixed xylene or xylene isomers. The systemic effects observed after inhalation exposure to xylene are discussed below. The highest NOAEL value and all LOAEL values from each reliable study for systemic effects in each species and duration category are recorded in Table 3-1, and are plotted in Figure 3-1.

Respiratory Effects. Self-reported symptoms of respiratory irritation and impaired performance in tests of pulmonary function have been observed in studies of volunteers exposed to xylene for short periods of time under controlled conditions. In humans, nose and throat irritation has been reported following exposure to mixed xylene at 200 ppm for 3–5 minutes (Nelson et al. 1943), to m-xylene at 50 ppm for 2 hours (Ernstgard et al. 2002), and to p-xylene at 100 ppm for 1–7.5 hours/day for 5 days (NIOSH 1981). However, no increase in reports of nose and throat irritation and no change in respiratory rate were seen in a study of subjects exposed to mixed xylene at a concentration of 396 ppm for 30 minutes (Hastings et al. 1986). Slight, but statistically significant increases in the average rating for subjective symptoms of respiratory effects were observed following exposure to m-xylene at 50 ppm (Ernstgard et al. 2002) for discomfort in the nose in both sexes after 60 and 118 minutes, in discomfort in the throat or airways in women after 60 minutes, and in breathing difficulty in men at 118 minutes and women at both timepoints. Small but statistically significant changes in objective tests of pulmonary function were reported in women, but not men, measured 3 hours after the end of the 2-hour exposure: decreased forced vital capacity (FVC), increased forced expiratory flow at 75% FVC (FEF₇₅), and increased ratio of forced expiratory volume in 1 minute (FEV₁) to forced vital capacity (FEV₁/FVC). This study is the basis for the acute-duration inhalation MRL for which respiratory and neurologic toxicity are the critical effects. Chest x-rays obtained from volunteers exposed to a time-weighted-average (TWA) concentration of 200 ppm m-xylene for 3.67 hours/day for 4 days showed no adverse effects on the lungs (Seppalainen et al. 1989). Also, no effects on pulmonary ventilation volume were observed in volunteers exposed to 150 ppm p-xylene for 5 days/week in a 4-week trial (NIOSH 1981).

Table 3-1

Levels of Significant Exposure to Xylene - Inhalation.

Figure 3-1

Levels of Significant Exposure to Xylene - Inhalation.

At much higher concentrations, however, the lung may be adversely affected. An autopsy revealed that exposure to an estimated 10,000 ppm of xylene produced severe lung congestion with focal intra-alveolar hemorrhage and pulmonary edema in one worker who died following exposure to xylene fumes for several hours while painting (Morley et al. 1970). Another worker exposed in the same incident exhibited patchy diffuse opacities in radiograms and moist rales in both lungs; a third exposed worker showed no evidence of lung effects. Case reports indicate that acute-duration inhalation exposure to mixed xylene and p-xylene has been associated with irritation of the nose and throat (Carpenter et al. 1975a; Klaucke et al. 1982; Nelson et al. 1943; Nersesian et al. 1985; NIOSH 1981). A worker at a chemical company who was exposed to heated xylene from a pressurized hose experienced throat pain and dyspnea (Narvaez and Song 2003).

Chronic occupational exposure of workers to an unspecified concentration of vapors of mixed xylene has also been associated with labored breathing and impaired pulmonary function (Hipolito 1980; Roberts et al. 1988). A significant (p<0.01) increase in the prevalence of nose and throat irritation was reported by workers chronically exposed to mixed xylene vapors at a geometric mean TWA concentration of 14 ppm (Uchida et al. 1993). This study is the basis for the chronic-duration inhalation MRL for which respiratory and neurological toxicity are the critical effects.

Adverse respiratory effects noted in rats, mice, and guinea pigs following acute and intermediate inhalation exposure to xylene are similar to those observed in humans. They include decreased respiration, labored breathing, irritation of the respiratory tract, pulmonary edema, pulmonary hemorrhage, and pulmonary inflammation (Carpenter et al. 1975a; De Ceaurriz et al. 1981; Furnas and Hine 1958; Korsak et al. 1990). Exposure to concentrations of 2,440 ppm mixed xylene for 6 minutes (Korsak et al. 1988) to 1,467 ppm o-xylene for 5 minutes (De Ceaurriz et al. 1981), or to 1,361 ppm m-xylene for 6 minutes (Korsak et al. 1993) produced a 50% decrease in respiratory rate in mice. Comparison of the individual xylene isomers showed that the irritant effects of m- and o-xylene as quantified by measurements of respiratory rate in mice are more pronounced than those of p-xylene, with o-xylene having the most prolonged effect (Korsak et al. 1990). In rats that died as a result of exposure to 9,900 ppm mixed xylene for 4 hours, atelectasis, hemorrhage, and edema of the lungs were observed (Carpenter et al. 1975a). Biochemical changes detected in the lungs after acute-duration intermittent exposure include transiently decreased lung surfactant levels at 300 ppm p-xylene (Silverman and Schatz 1991) and decreased pulmonary microsomal enzyme activities at 2,000 ppm mixed xylene, 75–2,000 ppm m-xylene, 2,000 ppm o-xylene, or 1,000 or 3,400 ppm p-xylene (Day et al. 1992; Elovaara et al. 1980, 1987; Patel et al. 1978; Silverman and Schatz 1991; Toftgard and Nilsen 1982). The LOAEL of 75 ppm for m-xylene was based on decreased P-450 and 7-ethoxycoumarin O-deethylase activities noted in the lungs of rats exposed for 24 hours (Elovaara et al. 1987). The decrease in pulmonary microsomal activity by selective inactivation of enzymes can result from damage to lung tissue caused by the toxic metabolite of xylene, a methylbenzaldehyde (Carlone and Fouts 1974; Patel et al. 1978; Smith et al. 1982); the selective inactivation of enzymes may also result in anoxia.

No effect on absolute or relative lung weights was observed in male rats intermittently exposed to m-xylene at concentrations as high as 100 ppm for 13 weeks (Korsak et al. 1994). No histopathological changes in the lungs were evident in rats, dogs, guinea pigs, or monkeys following intermediate exposure for 90–127 days to concentrations of 78 ppm o-xylene on a continuous basis (Jenkins et al. 1970) or 13 weeks to 810 ppm mixed or 6 weeks to 780 ppm o-xylene, 5 weeks to 300 ppm m-xylene, or for 5 days to 300 ppm p-xylene on an intermittent basis (Carpenter et al. 1975a; Elovaara et al. 1987; Jenkins et al. 1970; Silverman and Schatz 1991).

No animal studies were located that evaluated the respiratory effects of mixed xylene or single xylene isomers following chronic inhalation exposure.

An acute-duration inhalation MRL of 2 ppm was calculated for mixed xylenes based on a LOAEL for neurological and respiratory effects in human subjects exposed to 50 ppm m-xylene for 2 hours (Ernstgard et al. 2002; see footnote in Table 3-1). A chronic-duration inhalation MRL of 0.05 ppm was calculated for mixed xylenes based on a LOAEL of 14 ppm for subjective neurological and respiratory symptoms in workers exposed to mixed xylene 8 hours/day, 5 days/week for an average of 7 years (Uchida et al. 1993; see footnote in Table 3-1).

Cardiovascular Effects. Limited human data are available regarding the cardiovascular effects of xylene following inhalation exposure. Although tachycardia was reported by one of nine persons exposed to unidentified levels of xylene as a result of its use in a sealant in a heating duct, no effects on heart rate, blood pressure, or cardiac function were noted in humans exposed to ≤299 ppm mixed xylene for an acute duration (70 minutes to 7 hours) (Gamberale et al. 1978), 200 ppm m-xylene (Ogata et al. 1970; Seppalainen et al. 1989), or 150 ppm p-xylene (NIOSH 1981; Ogata et al. 1970). Furthermore, two survivors exposed to an estimated 10,000 ppm xylene in an industrial accident had normal pulse, blood pressure, and heart sounds upon hospitalization. Chronic occupational exposure to xylene along with other chemical agents has resulted in complaints of heart palpitations, chest pain, and an abnormal electrocardiogram (ECG) (Hipolito 1980; Kilburn et al. 1985). However, the contribution of other chemical exposures to these effects cannot be eliminated.

Data regarding cardiovascular effects in animals are limited. Morphological changes in coronary microvessels (increased wall thickness) were noted in rats exposed to 230 ppm xylene (unspecified composition) for 4 weeks (Morvai et al. 1987). Other effects seen in rats inhaling unspecified (lethal) concentrations of xylene of unknown composition included ventricular repolarization disturbances and occasional arrhythmias; the toxicity of unknown components was not reported (Morvai et al. 1976). However, no adverse effects on the heart were observed upon histopathological examination of rats and dogs exposed intermittently for 10–13 weeks to mixed xylene at concentrations as high as 810 ppm (Carpenter et al. 1975a) or rats, guinea pigs, dogs, or monkeys exposed to o-xylene at 78 ppm on a continuous basis for 90–127 days or 780 ppm on an intermittent basis for 6 weeks (Jenkins et al. 1970). No effect on absolute or relative heart weights was observed in male rats intermittently exposed to m-xylene at concentrations as high as 100 ppm for 13 weeks (Korsak et al. 1994). No information was located regarding cardiovascular effects in animals after chronic exposure to mixed xylene or its individual isomers.

Gastrointestinal Effects. Symptoms of nausea, vomiting, and gastric discomfort have been noted in workers exposed to xylene vapors (concentration unspecified) (Goldie 1960; Hipolito 1980; Klaucke et al. 1982; Nersesian et al. 1985; Uchida et al. 1993). These symptoms subsided after cessation of the xylene exposure. Anorexia and vomiting were also observed in a patient admitted to the hospital after sniffing paint containing xylene and other unknown substances over a 2-week period in an effort to become intoxicated (Martinez et al. 1989) and nausea was more frequently reported in males acutely exposed to m-xylene for 2 hours, as compared to controls (Ernstgard et al. 2002).

Limited data were located regarding gastrointestinal effects in animals. No lesions were observed in the gastrointestinal tract of rats and dogs exposed to concentrations as high as 810 ppm mixed xylene for 13 weeks (Carpenter et al. 1975a). No studies were located regarding gastrointestinal effects in animals after acute or chronic inhalation exposure to mixed xylene or the isomers of xylene.

Hematological Effects. Human data are limited regarding the effects of xylene on the blood. Female volunteers had normal blood counts after exposure to 100 ppm p-xylene for 1–7.5 hours/day for 5 days (NIOSH 1981). Hemoglobin content of the blood was unaffected in two workers exposed to an estimated 10,000 ppm of mixed xylene in an industrial accident (Morley et al. 1970). Decreased white blood cell counts were observed in two women with chronic occupational exposure to xylene (Hipolito 1980; Moszczynski and Lisiewicz 1983, 1984a), but exposure to other chemicals cannot be ruled out as an alternative explanation for the effects observed.

Previously, chronic occupational exposure to xylene by inhalation was thought to be associated with a variety of hematological effects (NIOSH 1975). However, exposure in all cases was to solvent mixtures known or suspected to contain benzene as well. Because benzene is an agent known to cause leukemia and other blood dyscrasias in humans (Agency for Toxic Substances and Disease Registry 2005), these effects cannot be solely attributed to xylene.

An occupational study in which no benzene exposure was involved (Uchida et al. 1993) found no hematological effects (red blood cell, white blood cell and platelet counts, and hemoglobin concentrations were unchanged). Workers (175) were exposed to a geometric mean TWA of 14 ppm xylene for an average of 7 years, and mixed xylene exposure accounted for 70% or more of the total exposure (Uchida et al. 1993). This study suggests that occupational exposure to relatively low concentrations of xylenes does not cause hematological effects.

No effect on erythrocyte fragility was observed in rats exposed to 15,000 ppm mixed xylene for 45 minutes (Carpenter et al. 1975a). No adverse hematological effects have been observed in rats exposed to 2,764 ppm mixed xylene for 5 hours/day for 9 days (Wronska-Nofer et al. 1991). In rats intermittently exposed to 100 ppm m-xylene for 90 days, erythrocyte counts were reduced by 18.5% and leukocyte counts were increased by 35% (Korsak et al. 1994). Increases in leukocyte count were reported in rats and dogs exposed intermittently to 780 ppm o-xylene for 6 weeks (Jenkins et al. 1970), but it is unknown whether these increases were statistically significant. However, no effects on hematological parameters were observed in rats or dogs following intermediate-duration intermittent exposure to concentrations as high as 810 ppm of mixed xylene (Carpenter et al. 1975a) or in guinea pigs exposed to 78 ppm o-xylene continuously or 780 ppm o-xylene intermittently (Jenkins et al. 1970) for an intermediate duration.

Musculoskeletal Effects. A 1993 occupational study indicates that workers exposed to xylenes (geometric mean TWA 14 ppm) reported reduced grasping power and reduced muscle power in the extremities more frequently than the unexposed controls (Uchida et al. 1993). This effect was a neurological effect rather than a direct effect on the muscles. No additional data were available regarding musculoskeletal effects in humans following inhalation exposure to mixed xylene or its individual isomers. Animal data regarding musculoskeletal effects following xylene inhalation are limited but provide no indication that xylene produces musculoskeletal effects. No lesions were observed in the skeletal muscle of rats and dogs exposed for an intermediate exposure to concentrations as high as 810 ppm mixed xylene (Carpenter et al. 1975a).

Hepatic Effects. Human data regarding hepatic effects following inhalation of xylene are limited to several case and occupational studies (Klaucke et al. 1982; Morley et al. 1970; Uchida et al. 1993); other occupational studies involve exposure to other compounds such as toluene (Dolara et al. 1982; Kurppa and Husman 1982). Two of these studies suggest that acute-duration exposure to high levels of xylene may result in hepatic toxicity. Two painters who survived exposure to an estimated 10,000 ppm of xylene and several workers who were exposed to an estimated 700 ppm of xylene had transiently elevated serum transaminase levels (Klaucke et al. 1982; Morley et al. 1970). The one painter who died had hepatocellular vacuolation following exposure to xylene for 18.5 hours. An occupational study in which workers were exposed an average of 7 years to >70% mixed xylenes (geometric mean TWA 14 ppm) found no changes in serum biochemistry values that reflect liver function (total bilirubin, aspartate aminotransferase, alanine aminotransferase, gamma glutamyl transpeptidase, alkaline phosphatase, and leucine aminopeptidase) (Uchida et al. 1993). This study suggests that low-level occupational exposure to xylenes does not result in hepatic effects.

Animal studies using rats indicate that mixed xylene, m-xylene, o-xylene, or p-xylene generally induce a wide variety of hepatic enzymes, as well as increased hepatic cytochrome P-450 content in rats (Elovaara 1982; Elovaara et al. 1980; Patel et al. 1979; Savolainen et al. 1978; Selgrade et al. 1993; Toftgard and Nilsen 1981, 1982; Toftgard et al. 1981; Ungvary et al. 1980a). Following acute exposures to mixed xylene (Savolainen et al. 1978; Ungvary 1990; Wisniewska-Knypl et al. 1989), m-xylene (Elovaara 1982; Ungvary et al. 1980b), o-xylene (Tatrai and Ungvary 1980; Ungvary et al. 1980a), or p-xylene (Patel et al. 1979; Simmons et al. 1991; Ungvary et al. 1980b), effects have been observed including increased relative liver weight (Simmons et al. 1991; Tatrai and Ungvary 1980; Ungvary et al. 1980a, 1980b), cytochrome P-450 content (Simmons et al. 1991; Ungvary 1990; Ungvary et al. 1980a; Wisniewska-Knypl et al. 1989), microsomal protein (Elovaara 1982), microsomal enzyme activity (Elovaara 1982; Savolainen et al. 1978; Ungvary 1990; Ungvary et al. 1980a; Wisniewska-Knypl et al. 1989), proliferation of the endoplasmic reticulum (Ungvary 1990; Wisniewska-Knypl et al. 1989), and decreased hexobarbital sleep time (Ungvary 1990; Ungvary et al. 1980a). Similar changes were observed in rabbits and mice (Ungvary 1990). Although histopathological examination of livers in most studies showed no adverse effects (Elovaara 1982; Simmons et al. 1991; Ungvary et al. 1980b), minor histopathological changes suggesting mild hepatic toxicity included decreased glycogen content, dilation of the cisterns of the rough endoplasmic reticulum, separation of ribosomes from the membranes, variously shaped mitochondria, and increased autophagous bodies (Tatrai and Ungvary 1980; Ungvary 1990). Also, increased serum transaminases were observed following a 4-hour exposure of rats to 1,000 ppm p-xylene (Patel et al. 1979).

Many similar hepatic effects appear after intermediate-duration exposure to mixed xylene or o-xylene. They include increased absolute and/or relative hepatic weight in rats (Kyrklund et al. 1987; Tatrai and Ungvary 1980; Tatrai et al. 1981; Toftgard et al. 1981; Ungvary 1990; Ungvary et al. 1980a), increased cytochrome P-450 (Tatrai et al. 1981; Ungvary 1990; Ungvary et al. 1980a); increased microsomal enzyme activity (Elovaara et al. 1980, 1987; Tatrai et al. 1981; Toftgard et al. 1981; Ungvary 1990; Ungvary et al. 1980a), proliferation of the smooth and rough endoplasmic reticulum (Rydzynski et al. 1992; Tatrai and Ungvary 1980; Tatrai et al. 1981; Ungvary 1990) and decreased hexobarbital sleeping time because of enhanced metabolism of the drug (Tatrai et al. 1981; Ungvary 1990; Ungvary et al. 1980a). Similar effects were observed in rabbits and mice (Ungvary 1990). As in the acute studies, several intermediate-duration studies in rats, guinea pigs, monkeys, or dogs, reported no effect on serum transaminases (Carpenter et al. 1975a; Tatrai et al. 1981) or hepatic morphology (Carpenter et al. 1975a; Jenkins et al. 1970). Ultrastructural examination of livers showed only minor changes: decreased hepatic glycogen in rats (Tatrai and Ungvary 1980; Ungvary 1990; Ungvary et al. 1980b), ultrastructural changes in hepatic rough endoplasmic reticulum and mitochondria in rats (Tatrai and Ungvary 1980; Ungvary 1990), increased autophagous bodies (Tatrai et al. 1981; Ungvary 1990), and changes in the distribution of hepatocellular nuclei in rats (Tatrai and Ungvary 1980). Some authors have characterized the hepatic changes as adaptive rather than adverse (Tatrai and Ungvary 1980; Ungvary 1990). No effects on hepatic microsomal proteins, cytochrome P-450, lipid peroxidation (as indicated by levels of malondialdehyde), triglycerides, serum enzymes (AST, ALT, SDH), or absolute or relative liver weights were observed in rats exposed to m-xylene at concentrations as high as 100 ppm for 13 weeks (Korsak et al. 1994). No changes in the levels of lipid peroxidation (malondialdehyde levels), glutathione, or glutathione-S-transferase activity were observed in rats intermittently exposed to 92 ppm m-xylene for 5 months (Jajte et al. 2003).

Increased liver weight and microsomal enzyme activity were reported in a study in which rats were exposed to 1,096 ppm o-xylene for 1 year (Tatrai et al. 1981). Electron microscopic examination of liver revealed a proliferation of the endoplasmic reticulum and only very minor effects on mitochondria as exemplified by increased numbers of peroxisomes.

Renal Effects. Although urinalyses (using a dip-stick technique) of volunteers exposed to p-xylene at 100 ppm for 5 days or up to 150 ppm in a multi-week exposure paradigm showed no adverse effects on the kidneys (NIOSH 1981), limited data from case reports and occupational studies suggest that inhalation exposure to solvent mixtures containing xylene may be associated with adverse renal effects in humans (Martinez et al. 1989; Morley et al. 1970). These effects included increased blood urea (Morley et al. 1970), distal renal tubular acidemia (Martinez et al. 1989), and decreased urinary clearance of endogenous creatinine (Morley et al. 1970). Other studies that reported increased urinary levels of β-glucuronidase (Franchini et al. 1983), or increased urinary excretion of albumin, erythrocytes, and leukocytes (Askergren 1981, 1982) are confounded by concurrent exposure to substantial amounts of toluene, a known renal toxicant.

In an occupational study in which the exposure was predominantly to mixed xylenes (geometric mean TWA 14 ppm) for an average of 7 years (Uchida et al. 1993), no effects on measures of kidney function (serum creatinine or urinalysis for urobilinogen, sugar, protein, and occult bleeding) were noted. This study suggests that low-level occupational exposure to xylenes does not result in kidney effects.

The renal effects of mixed xylene and o-xylene following inhalation exposure have been evaluated in acute and intermediate studies with rats, guinea pigs, dogs, and monkeys (Carpenter et al. 1975a; Elovaara 1982; Jenkins et al. 1970; Toftgard and Nilsen 1982). Effects noted in these studies at xylene concentrations of 50–2,000 ppm have included increased renal enzyme activity, increased renal cytochrome P-450 content, and increased kidney-to-body weight ratios (o-xylene-exposed rats) (Elovaara 1982; Toftgard and Nilsen 1982). However, histopathologic examination of rats, guinea pigs, dogs, and monkeys did not reveal any renal lesions after inhalation of 810 ppm mixed xylene or 78 ppm o-xylene for an intermediate period of 13 weeks and 90–127 days, respectively (Carpenter et al. 1975a; Jenkins et al. 1970). No effect on absolute or relative kidney weights was observed in male rats intermittently exposed to m-xylene at concentrations as high as 100 ppm for 13 weeks (Korsak et al. 1994).

No studies were located regarding renal effects following chronic inhalation exposure to mixed xylene or its isomers.

Endocrine Effects. No human data were available regarding endocrine effects following inhalation exposure to mixed xylene or xylene isomers. Inhalation exposure to 810 ppm mixed xylene for 13 weeks produced no adverse adrenal, thyroid, or parathyroid effects in the dog (Carpenter et al. 1975a). No effect on absolute or relative adrenal weights was observed in male rats intermittently exposed to m-xylene at concentrations as high as 100 ppm for 13 weeks (Korsak et al. 1994).

Ocular Effects. Human data indicate that acute inhalation exposures to 460 ppm mixed xylene and 100 ppm p-xylene vapors produce mild and transient eye irritation (Carpenter et al. 1975a; Hastings et al. 1986; Klaucke et al. 1982; Nelson et al. 1943; Nersesian et al. 1985; NIOSH 1981). This effect is probably the result of direct contact of the xylene vapor with the eye and as such is described under Ocular Effects in Section 3.2.3.2.

No animal data were available regarding ocular effects following inhalation exposure to mixed xylenes or xylene isomers.

Body Weight Effects. No studies were located regarding body weight effects in humans following inhalation exposure to mixed xylenes or xylene isomers.

A number of intermediate-duration intermittent inhalation studies of xylene have examined body weight effects in animals (Carpenter et al. 1975a; Gagnaire et al. 2001, 2006; Gralewicz and Wiaderna 2001; Jajte et al. 2003; Korsak et al. 1992, 1994; Rosengren et al. 1986; Tatrai et al. 1981). Except for the study by Tatrai et al. (1981) in which a 12% decrease in body weight was observed in rats exposed to 1,096 ppm o-xylene for 6 months, no significant adverse effects on body weight were noted.

Metabolic Effects. Metabolic acidosis was reported in a man who sniffed paint containing xylenes, but other solvents in the paint may have contributed to the effect (Martinez et al. 1989). No data were located concerning metabolic effects in animals following inhalation exposure to xylenes.

3.2.1.3. Immunological and Lymphoreticular Effects

Limited data were available regarding immunological and lymphoreticular effects of xylene in humans. Decreased lymphocytes (Moszczynski and Lisiewicz 1983, 1984a) and decreased serum complement (Smolik et al. 1973) have been observed in workers exposed to xylene. However, no determination can be made regarding the association between inhalation of xylene and immunological effects from the available human studies, because workers were concurrently exposed to other chemical agents.

Acute exposure (4 days, 4 hours/day) of mice to 1,208 ppm p-xylene had no effect on natural killer cell activity, although mortality from murine cytomegalovirus was increased (Selgrade et al. 1993). The investigators (Selgrade et al. 1993) attributed the enhanced virus susceptibility to increased liver toxicity rather than to an effect on the immune system. Intermittent exposure of rats and dogs to mixed xylenes for 10 or 13 weeks at concentrations as high as 810 ppm resulted in no effect on spleen weight (Carpenter et al. 1975a).

3.2.1.4. Neurological Effects

The neurological effects of xylene in humans following inhalation exposure have been evaluated in a number of experimental studies, case reports, and occupational studies. Results of experimental studies with humans indicate that acute inhalation exposure to mixed xylene or m-xylene causes impaired short-term memory, impaired reaction time, performance decrements in numerical ability, and alterations in equilibrium and body balance (Carpenter et al. 1975a; Dudek et al. 1990; Gamberale et al. 1978; Riihimaki and Savolainen 1980; Savolainen and Linnavuo 1979; Savolainen and Riihimaki 1981a; Savolainen et al. 1979b, 1984, 1985a).

Dizziness was reported by the majority of subjects exposed to 690 ppm mixed xylene for 15 minutes, but in only one of six persons exposed at 460 ppm (Carpenter et al. 1975a). In objective measures of neurological function, exposure to 100 ppm mixed xylene for 4 hours resulted in prolonged reaction time (Dudek et al. 1990) and exposure to 299 ppm mixed xylene for 70 minutes during exercise resulted in impaired short-term memory and reaction time (Gamberale et al. 1978). No impairment in performance tests was observed in sedentary subjects exposed at 299 ppm for 70 minutes (15 men) (Gamberale et al. 1978) or at 396 ppm for 30 minutes (10 men) (Hastings et al. 1986). The difference between the effects in the absence and presence of exercise may be due to increased xylene respiratory uptake during exercise.

Slight, but statistically significant, increases in the average rating for subjective symptoms of neurological effects were observed following exposure to 50 ppm m-xylene vapor compared to controls (Ernstgard et al. 2002). After 60 and 118 minutes of exposure, severity ratings for feelings of intoxication were elevated in men and women, and ratings for headache were elevated in men. The ratings for dizziness were increased in exposed men after 118 minutes of exposure. (This study was chosen as the basis for the acute-duration inhalation MRL, for these subjective neurological effects and changes in objective and subjective measures of respiratory function.) Electroencephalograms obtained from nine men exposed to m-xylene at 200 ppm (TWA) for 4 hours showed only minor changes (Seppalainen et al. 1991). These changes were characterized as a slight increase in alpha-wave frequency and percentage early in the exposure period and a decrease in exercise-induced increases in theta and delta waves indicating central nervous system effects. Studies using the m-isomer of xylene have also indicated that some tolerance may occur during acute exposures. While exposure to stable concentrations of m-xylene for 7 hours or 4 hours, twice a week in the range of up to approximately 280 ppm had no effect on body sway, coordination, or reaction time (Ogata et al. 1970; Savolainen 1980; Savolainen et al. 1980b), exposure for 6 hours or 6–9 days to levels fluctuating between 64 and 400 ppm produced impairment in human body balance and/or reaction time (Savolainen and Linnavuo 1979; Savolainen and Riihimaki 1981a; Savolainen et al. 1979b, 1980a, 1984, 1985a). A 3-hour exposure of nine male volunteers to m-xylene at 200 ppm during exercise resulted in a slight but significant (p<0.05) change in the N135 component of a pattern visual evoked potential (Seppalainen et al. 1989). Laine et al. (1993) saw no clear effects on visual reaction times or auditive choice reaction times in nine male volunteers exposed to levels of m-xylene fluctuating between 135 and 400 ppm (TWA 200 ppm) with or without exercise. Levels of m-xylene fluctuating between 135 and 400 ppm produced a slight decrease in the latency of visual evoked potentials (Seppalainen et al. 1989), but no clear effects on visual reaction times or auditory choice reaction times (Laine et al. 1993).

Objective measures of neurological function (electroencephalography, tests of motor activity and cognitive performance) in humans are not affected by acute or intermediate, intermittent or continuous inhalation exposure to p-xylene for 4 hours or up to 7 hours for 5 days at concentrations ranging from 69 to 150 ppm (NIOSH 1981; Olson et al. 1985). Differences in such factors as the xylene isomer, the neurological parameter, exposure conditions and concentrations, rapid development of tolerance, and total xylene uptake may account for the variability in results. However, some sex difference in subjective reports of central nervous system effects was observed (NIOSH 1981). Three women exposed to p-xylene at 100 ppm for 1–7.5 hours/day, for 5 days, showed no effects on electroencephalograms, evoked potentials, or cognitive performance, but frequently reported headache and dizziness as a result of exposure (NIOSH 1981). In contrast, four men exposed at concentrations of up to 150 ppm p-xylene under the same exposure conditions reported no increase in headaches or dizziness.

Available case reports and occupational studies together provide suggestive evidence that acute and chronic inhalation exposure to xylene or solvent mixtures containing xylene may be associated with neurological effects; however, most studies are difficult to evaluate because the exposure conditions either have not been well characterized or the subjects may have been exposed to other chemicals in addition to xylene. The neurological symptoms observed in these studies include headache, nausea, dizziness, difficulty concentrating, impaired memory, slurred speech, ataxia, fatigue, agitation, confusion, tremors, labored breathing, and sensitivity to noise (Arthur and Curnock 1982; Goldie 1960; Gupta et al. 1990; Hipolito 1980; Klaucke et al. 1982; Martinez et al. 1989; Morley et al. 1970; Nersesian et al. 1985; Roberts et al. 1988). In several case reports, isolated instances of unconsciousness, amnesia, brain hemorrhage, and epileptic seizure have been associated with acute inhalation exposure to solvent mixtures containing xylene (Arthur and Curnock 1982; Goldie 1960; Martinez et al. 1989; Morley et al. 1970). Long-term occupational exposure (≥10 years) to mixed solvents among spray painters was associated with an increase in depression and “loss of interest,” but no significant effects on psychological performance tests or CAT-scan measures of brain atrophy (Triebig et al. 1992a, 1992b). Workers exposed to mixed solvents for <10–>30 years exhibited significantly reduced conduction velocities in the radial and tibial nerves, as well as duration-related increases in symptoms of numbness, cramps, and weakness (Jovanovic et al. 2004). Because other chemicals were present with xylenes in many of these studies, the effects observed cannot be conclusively attributed to xylene exposure.

Another occupational study in which xylene exposure was most well defined and represented 70% of the solvent exposure (Uchida et al. 1993) reported an increase in subjective symptoms including an increased prevalence of anxiety, forgetfulness, inability to concentrate, and dizziness among workers exposed to an average TWA concentration of 21 ppm (14 ppm geometric mean) of mixed xylenes for an average of 7 years. No objective measures of neurological impairment were tested in this study. Subjective symptoms of neurological and respiratory toxicity in this study were selected as co-critical effects for the chronic-duration inhalation MRL.

Results of experimental studies with animals also provide evidence that mixed xylene and its isomers are neurotoxic following inhalation exposure. Signs of neurotoxicity observed in rats, mice, dogs, cats, and gerbils following acute and intermediate inhalation exposure to the various xylene isomers include narcosis, prostration, incoordination, tremors, muscular spasms, labored breathing, behavioral changes, hyperreactivity to stimuli, altered visual evoked potentials, elevated auditory thresholds, hearing loss, and decreased acetylcholine in midbrain and norepinephrine in hypothalamus (suggestive of effect on motor control, sleep, and memory maintenance) (Andersson et al. 1981; Bushnell 1989; Carpenter et al. 1975a; De Ceaurriz et al. 1983; Furnas and Hine 1958; Ghosh et al. 1987; Honma et al. 1983; Korsak et al. 1988, 1990; Kyrklund et al. 1987; Molnar et al. 1986; Pryor et al. 1987; Rank 1985; Rosengren et al. 1986; Savolainen and Seppalainen 1979; Savolainen et al. 1978, 1979b; Wimolwattanapun et al. 1987).

Exposure levels associated with neurological effects in animals are well defined. A comparative study determined that the minimal alveolar concentrations needed to induce anesthesia in rats were similar for all three isomers (0.00118, 0.00139, and 0.00151 atm, respectively, for o-, m-, and p-xylene), but only p-xylene also induced excitation (strong tremors) (Fang et al. 1996). Acute exposure to unspecified levels of mixed xylene resulted in respiratory paralysis (Morvai et al. 1976), 1,600 ppm p-xylene produced hyperactivity (Bushnell 1989), and 1,300 ppm mixed xylene produced incoordination in rats, which did not persist after exposure ended; no overt signs of toxicity were noted at 580 ppm (Carpenter et al. 1975a). All three xylene isomers produced narcosis in rats after 1–4 hours of exposure to concentrations of approximately 2,000 ppm (Molnar et al. 1986). No behavioral signs of xylene intoxication were observed in dogs or monkeys exposed continuously to 78 ppm o-xylene for up to 127 days, but dogs exposed to 780 ppm o-xylene intermittently for 6 weeks exhibited tremors during exposure (Jenkins et al. 1970).

The neurotoxicity of xylenes has been evaluated in neurobehavioral tests on animals exposed by inhalation. Mice exposed for 30 minutes by inhalation to any of the isomers exhibited impaired operant performance at the same minimal effective concentration, 1,400 ppm, but the median effective concentrations varied to a limited degree—5,179 ppm for o-xylene, 5,611 ppm for p-xylene, and 6,176 ppm for m-xylene (Moser et al. 1985). The order of potency was different for impairment of motor coordination in mice undergoing the inverted screen test, with a minimal effective concentration of 2,000 ppm for p-xylene and 3,000 ppm for the two other isomers (Moser et al. 1985); the median effective concentrations were 2,676, 3,640, and 3,790 ppm, respectively, for p-, o-, and m-xylene. Acute exposures to concentrations inducing behavioral changes in rats and mice ranged from 114 ppm for effects of mixed xylene on operant conditioning or self-stimulation behavior (Ghosh et al. 1987; Wimolwattanapun et al. 1987), 500 ppm for reduced response rate in schedule-controlled operant behavior in mice exposed to m-xylene (Bowen et al. 1998), to 1,010 ppm for o-xylene-induced immobility in a “behavioral despair swimming test” (De Ceaurriz et al. 1983). Exposure of male rats to 230 ppm o-xylene for 4 hours shortened the duration of response (extension of hindlimbs) to an applied electrical shock by 18.8% (Vodickova et al. 1995); doubling the exposure concentration correspondingly doubled the magnitude of the response. In the same report, exposure of female mice to 320 ppm o-xylene for 2 hours shortened the duration of response (velocity of tonic extension, i.e., the reciprocal of the latency) was reduced by 11%; unlike rats, exposure at twice the concentration, increased the magnitude of the response by a factor of 3.7. Impaired rotarod performance was observed in rats acutely exposed to mixed xylene and the individual xylene isomers at concentrations of ≥3,000 ppm (Korsak et al. 1990). In intermediate-duration inhalation studies with rats, exposure to 100 ppm m-xylene intermittently for 3 or 6 months or to 1,000 ppm for 3 months showed decreased rotarod performance and decreased spontaneous activity (Korsak et al. 1992, 1994). The effect was greater following the 3-month exposure at 1,000 ppm than the 6-month exposure at 100 ppm suggesting that for effects on motor activity, concentration is more important than duration of exposure. The persistence of neurological effects of xylene was examined in some studies that evaluated tested animals some weeks after the last exposure. Rats intermittently exposed to 100 ppm m-xylene for 4 weeks exhibited impaired passive avoidance learning (tested 5 weeks after exposure) and impaired acquisition, but not retention, of the two-way active avoidance response (tested 9 weeks after exposure) (Gralewicz and Wiaderna 2001). Exposure had no lasting effect in tests for short-term memory, responsiveness to a thermal stimulus (paw-lick latency), or spontaneous activity (open-field test) or the retention of a learned active avoidance response.

Sensory deficits resulting from xylene exposure have been observed under controlled testing conditions. Acute exposure to 1,600 ppm, but not 800 ppm p-xylene depressed the amplitude of visual evoked potentials (Dyer et al. 1988). Hearing deficits have been reported in animals exposed to xylene for acute or intermediate durations. Hearing loss occurred in rats exposed to 1,450 ppm mixed xylene for 8 hours, whereas exposure to 1,700 ppm for 4 hours produced no effects on hearing (Pryor et al. 1987) indicating that the duration of exposure is important for the observation of ototoxic effects in conditioned avoidance tests. In rats acutely exposed to 1,800 ppm mixed xylenes, 18–30 dB hearing losses were observed at mid-range frequencies (Crofton et al. 1994). The amplitude of brainstem auditory evoked potentials was reduced by 50% in rats acutely exposed at 2,000 ppm, but not at 1,700 ppm (Rebert et al. 1995). Hearing loss was also evident after exposure for 6 weeks to 800 ppm mixed xylene (Pryor et al. 1987). In rats exposed intermittently to 900 ppm p-xylene for 13 weeks, there was some hair cell loss in the organ of Corti, but no effect on auditory neurophysiology (Gagnaire et al. 2001). At 1,800 ppm, extensive ototoxicity (altered brainstem auditory evoked potentials, measurable hearing loss [deficits of 35–42 dB] and significant hair cell loss in the organ of Corti) was observed as early as the fourth week and did not improve during an 8-week recovery period; neither o-xylene nor m-xylene were ototoxic in this study. All rats exposed for 13 weeks to 1,000 or 2,000 ppm mixed xylenes containing ethylbenzene exhibited significant irreversible ototoxicity including hearing deficits and loss of hair cells (Gagnaire et al. 2006). Exposures to these mixtures at 250 or 500 ppm resulted in a small loss of hair cells in one of eight rats per group, but no effect on auditory thresholds. According to the authors, ethylbenzene contributes significantly to the ototoxicity of technical xylene.

Changes in neuronal cells and brain biochemistry have been noted in animals following exposure to xylenes. Acute exposure to p-xylene caused decreased axonal transport at concentrations as low as 800 ppm (Padilla and Lyerly 1989); however, no such decrease was apparent 3 days after exposures had ceased. At 1,600 ppm, however, the decrease in axonal transport persisted for 13 days after exposure. Acute inhalation of 2,000 ppm mixed xylene produced increased dopamine and/or noradrenaline levels in the hypothalamus of rats; no behavioral changes were assessed (Andersson et al. 1981). Levels of these catecholamines in the hypothalamus of rats were also increased following inhalation of 2,000 ppm m-, o-, or p-xylene (Andersson et al. 1981). Brain concentrations of deoxyribonucleic acid (DNA) and/or astroglial proteins increased in rats (at 300–320 ppm) and gerbils (at 160 ppm) after intermediate continuous exposure of 3–4.5 months to xylene (Rosengren et al. 1986; Savolainen and Seppalainen 1979). In addition, increased levels of brain enzymes, changes in axon membranes, and behavioral changes occurred in rats after exposure to 300 ppm of mixed xylene for 18 weeks (Savolainen and Seppalainen 1979; Savolainen et al. 1979a). Alterations in neurotransmitter levels were observed in some brain areas at 800 ppm mixed xylene for 30 days (Honma et al. 1983). However, no significant long-term alterations in fatty acid levels were noted in the brains of rats after intermediate-duration exposure of 30 or 90 days to 320 ppm mixed xylene (Kyrklund et al. 1987). At 1,600 ppm m-xylene for 7 weeks, decreased α-adrenergic binding compared to the controls was observed in the hypothalamus of exposed mice (Rank 1985). No persistent changes in brain weight or weights of the caudate-putamen or subcortical limbic areas, or in agonist binding to the dopamine D₂ receptor in those regions occurred in rats exposed to 80 ppm p-xylene for 4 weeks and examined 5 weeks after the last exposure (Hillefors-Berglund et al. 1995).

No animal studies were located regarding neurological effects following chronic inhalation exposure to mixed xylene or its isomers.

The highest NOAEL values and all LOAEL values for each reliable study for neurological effects in each species and duration category are recorded in Table 3-1 and plotted in Figure 3-1. An acute-duration inhalation MRL of 2 ppm was calculated for mixed xylenes based on a LOAEL for subjective neurological effects and measured and subjective respiratory effects in human subjects exposed to 50 ppm m-xylene for 2 hours (Ernstgard et al. 2002; see footnote in Table 3-1). An intermediate-duration inhalation MRL of 0.6 ppm was calculated for mixed xylenes based on a minimal LOAEL of 50 ppm for decreased mean latency of the paw-lick response in rats exposed to m-xylene for 6 hours/day, 5 days/week for 3 months (Korsak et al. 1994; see footnote in Table 3-1). A chronic-duration inhalation MRL of 0.05 ppm was calculated for mixed xylenes based on a LOAEL of 14 ppm for subjective neurological and respiratory symptoms in workers exposed to mixed xylene 8 hours/day, 5 days/week for an average of 7 years (Uchida et al. 1993; see footnote in Table 3-1).

3.2.1.5. Reproductive Effects

A few occupational studies evaluated reproductive effects in workers exposed to xylenes. A case-control study of spontaneous abortions among Finnish workers for whom there was biomonitoring data for exposure to organic solvents found no statistically significant increase in the odds ratio associated with exposure to xylene (Lindbohm et al. 1990). Spontaneous abortions in early pregnancy were significantly increased among 37 women exposed to xylene and formalin in pathology or histology laboratories (Taskinen et al. 1994). Although the increased odds ratio (OR=3.1; 95% confidence interval=1.3–7.5) for spontaneous abortion was statistically significant for women exposed to xylene at least 3–4 days/week, the analysis cannot be considered definitive because of simultaneous exposure to other solvents and chemicals. A cross-sectional study among 1,408 petrochemical workers in China reported an increased prevalence of oligomenorrhea among workers exposed to organic solvents, but the contribution of xylene cannot be definitively determined as all workers who were exposed to xylene were also exposed to other solvents such as benzene, toluene, and styrene (Cho et al. 2001).

Continuous exposure of CFY rats for 8 days on days 7–14 during pregnancy to 775 ppm mixed xylene produced an increased number of resorptions without any maternal toxicity; reduced fertility was also observed (Balogh et al. 1982). However, no adverse reproductive effects were noted following inhalation exposure of male and female CD rats to mixed xylene at concentrations as high as 500 ppm during premating, mating, pregnancy, and lactation (Bio/dynamics 1983). No effect on absolute or relative testicular weights was observed in rats intermittently exposed to m-xylene at concentrations as high as 100 ppm for 13 weeks (Korsak et al. 1994). Inhalation exposure of male Sprague-Dawley rats to 1,000 ppm mixed xylene for 61 days produced no alterations in testes, accessory glands, or circulating male hormone levels (Nylen et al. 1989). Strain differences may account for the differential response to mixed xylene in these studies. The highest NOAEL and LOAEL values for each reliable study for reproductive effects in rat for each duration category are recorded in Table 3-1 and plotted in Figure 3-1.

3.2.1.6. Developmental Effects

Although the human data regarding the developmental effects of xylene suggest a possible relationship between solvent (unspecified) exposure and developmental toxicity, these data are limited for assessing the relationship between inhalation of xylene and developmental effects because the available studies involved concurrent exposure to other solvents in addition to xylene in the workplace (Holmberg and Nurminen 1980; Kucera 1968; Taskinen et al. 1989; Windham et al. 1991), and because of the small number of subjects ranging from 9 to 61 (Taskinen et al. 1989; Windham et al. 1991).

Both mixed xylene and the individual isomers produce fetotoxic effects in laboratory animals. Effects of mixed xylene observed in rats, mice, and rabbits included increased incidences of skeletal variations in fetuses, delayed ossification, fetal resorptions, hemorrhages in fetal organs, and decreased fetal body weight (Balogh et al. 1982; Bio/dynamics 1983; Hass and Jakobsen 1993; Hudak and Ungvary 1978; Litton Bionetics 1978a; Mirkova et al. 1983; Ungvary 1985; Ungvary and Tatrai 1985). Balogh et al. (1982) reported delayed ossification in all exposure levels (53–775 ppm), but as the incidences were inversely related to dose, this end point is not included in LSE Table 3-1. The levels at which these effects were observed depended upon the composition and concentration of mixed xylene, the time of exposure, and the choice of strain and test species used. In addition, animals in a number of studies were exposed 24 hours/day (Balogh et al. 1982; Hudak and Ungvary 1978; Ungvary 1985; Ungvary and Tatrai 1985), whereas animals in other studies (Bio/dynamics 1983; Hass and Jakobsen 1993; Litton Bionetics 1978a; Mirkova et al. 1983) were exposed 6 hours/day. The study conducted by Litton Bionetics (1978a) used a formulation of mixed xylene with a comparatively high percentage (36%) of ethylbenzene. Developmental effects were reported following maternal exposure to concentrations as low as 12 ppm mixed xylene in rats (Mirkova et al. 1983), but the health of the test animals may have been compromised due to poor animal husbandry. This is suggested by the relatively low conception rates and the high incidence of fetal hemorrhages seen in the controls. Maternal toxicity was observed at 775 ppm in the study by Balogh et al. (1982) and at 138 ppm in the study by Ungvary (1985); however, no maternal toxicity occurred at exposure levels of 100–400 ppm in the studies by Bio/dynamics (1983), Hass and Jacobsen (1993), Hudak and Ungvary (1978), and Litton Bionetics (1978a). Insufficient evidence was presented to determine whether maternal toxicity occurred in the studies by Mirkova et al. (1983) and Ungvary and Tatrai (1985). Many of the studies (Balogh et al. 1982; Bio/dynamics 1983; Hudak and Ungvary 1978; Mirkova et al. 1983; Ungvary 1985; Ungvary and Tatrai 1985) had limitations that made them difficult to assess (e.g., unknown composition of xylene and insufficient number of doses to form a dose-response relationship; lack of detail with regard to both methods and data obtained). Furthermore, several of these studies (Balogh et al. 1982; Ungvary and Tatrai 1985; Ungvary et al. 1980b) reported the incidence of fetal effects, such as delayed skeletal ossification and extra ribs, on the basis of total fetuses per exposure group, but not on a per-litter basis. Without litter-specific results, it is not possible to adjust for possible covariance with litter size, as reported by Litton Bionetics (1978a). Given the uncertainty of the significance of these effects, increases in the incidences of delayed ossification or skeletal variants were not used to establish NOAELs or LOAELs unless litter-specific data were provided. Based on the reliable data in these reports (fetal weight, fetal death, implantation losses), developmental toxicity in rats occurred at concentrations of ≥350 ppm; these effects are noted in Table 3-1.

A number of studies reported neurobehavioral deficits in rats resulting from exposure to xylene during gestation. ATSDR considers these effects to be serious. Hass and Jakobsen (1993) reported decreased rotarod performance in 1- and 2-day-old rat pups exposed to 200 ppm mixed xylenes 6 hours/day on gestation days 4–20. Female offspring of rats exposed to 500 ppm mixed xylenes for 6 hours/day on gestation days 7–20 showed significant reductions in absolute brain weight after weaning and neurobehavioral deficits (delayed air righting reflex, nonsignificant trends for reduced motor coordination in the Rotarod test, and increased latencies in the Morris water maze test) (Hass et al. 1995). Hass et al. (1995) suggested that the delayed air righting reflex and impaired spatial navigation in the water maze (locating a re-situated platform) may have been related to ototoxicity, since both tasks involve the vestibular system and cochlear hair cells. The latency in locating a moved platform was significantly increased in the first learning trial by exposed offspring tested at week 28, but was not significantly different from controls at 55 weeks (Hass et al. 1997).

Inhalation of o- or p-xylene at concentrations similar to those at which mixed xylene caused fetal toxicity, produced decreased fetal weight, skeletal retardation, and post-implantation loss in rats, mice, and rabbits following maternal acute exposure during gestation days 7–14/15 (Ungvary and Tatrai 1985; Ungvary et al. 1980b, 1981); no maternal toxicity was observed. A NOAEL value of 1,612 ppm p-xylene for developmental effects was determined from one study with rats (Rosen et al. 1986). The large variation in concentrations of xylene producing developmental effects and those producing no developmental effects may be influenced by a number of factors (e.g., strain and species of animal, purity of xylene, method of exposure, exposure pattern and duration, etc.). For example, Rosen et al. (1986) exposed animals for 6 hours/day, whereas animals were exposed 24 hours/day in studies by Ungvary and Tatrai (1985) and Ungvary et al. (1980b, 1981). No information on maternal toxicity was available for the studies by Ungvary and Tatrai (1985) or Ungvary et al. (1981); however, in the studies by Rosen et al. (1986) and Ungvary et al. (1980b), signs of maternal toxicity in rats following inhalation of the isomers included decreased weight gain, decreased food consumption, and increased liver-to-body weight ratios. m-Xylene was the only isomer that resulted in lasting maternal growth inhibition or maternal mortality (Ungvary et al. 1980b). Thus, it is difficult to determine from these studies whether mixed xylenes are selectively toxic to the fetus or the observed developmental toxicity was secondary to maternal toxicity. As mentioned above, retardation in skeletal growth or variations, in the absence of litter-specific data, were not used to determine NOAELs or LOAELs for exposure to specific isomers of xylene (Ungvary et al. 1980b).

Results of a well-documented comparative standard developmental toxicity assay in rats exposed for 6 hours/day on gestational days 6–20, suggest that developmental toxicities of m- and p-xylene are secondary to maternal toxicity (Saillenfait et al. 2003). Statistically significant maternal effects (reduced ‘corrected weight gain’—body weight gain exclusive of gravid uterus weight) and fetal toxicity (reduced fetal weight) occurred at exposures of ≥1,000 ppm, but not 500 ppm for these two isomers. Both m- and p-xylene at 2,000 ppm induced significant increases in delayed ossification, a skeletal variation related to retarded growth, although the affected bones were different for the two isomers. Dams exposed at ≥1,000 ppm to o-xylene or mixed xylenes showed similar effects, although the reduced corrected weight gain for mixed xylenes was only significant for gestational days 6–13. However, both o-xylene and mixed xylene caused significantly reduced fetal body weights at 500 ppm, the maternal NOAEL. Exposure to mixed xylenes did not elicit any developmental structural anomalies at concentrations as high as 2,000 ppm, whereas o-xylene elicited increases in the percent of fetuses per litter with skeletal variations (incomplete ossification) at 2,000 ppm and in the percent of fetuses with any variation at 1,000 ppm. This assay did not include testing for postnatal neurobehavioral effects.

The highest NOAEL value and all LOAEL values from each reliable study for developmental effects in each species and duration category are recorded in Table 3-1 and plotted in Figure 3-1.

3.2.1.7. Cancer

Human data regarding cancer are limited to three occupational studies and one population-based study. Two studies examined the cancer and leukemia risks among solvent-exposed workers and suggested a possible relationship between coal-based xylene exposure and leukemia (Arp et al. 1983; Wilcosky et al. 1984). Both contain limitations (e.g., small number of subjects ranging from 9 to 85 male workers, no exposure concentrations, unknown composition of xylene) that preclude a definitive conclusion regarding inhalation of xylene and cancer. A retrospective cohort mortality study evaluated 14,457 workers at an aircraft maintenance facility, 108 of whom had been exposed to xylene (Spirtas et al. 1991; Stewart et al. 1991). In this study, evaluations of deaths from multiple myeloma or non-Hodgkin’s lymphoma revealed no cases among xylene-exposed workers, representing 1,837 and 444 person-years of exposure, for men and women, respectively (Spirtas et al. 1991; Stewart et al. 1991). A population-based case-control study that estimated job-related exposure to xylene reported limited evidence of increased risks for cancers of the colon and rectum following high exposure to xylene (Gerin et al. 1998). The authors indicated that the marginally significant excesses in rectal cancer in xylene-exposed workers may have been due to the confounding effect of exposure to styrene or another material associated with styrene exposure. Limitations of this study included a lack of information on exposure concentrations and the composition of xylene, a small number of cases, and the fact that most of the xylene-exposed workers were also exposed to toluene and benzene.

No studies were located regarding cancer in animals exposed via inhalation to mixed xylene or xylene isomers.

3.2.2.1. Death

Death in humans following accidental or intentional ingestion of xylene was reported by Abu Al Ragheb et al. (1986). Levels of xylene found in blood and gastric and duodenal contents were 110, 8,800, and 33,000 mg/L, respectively, indicating ingestion of a large, but undetermined, quantity of xylene. Death was attributed to respiratory failure secondary to depression of the respiratory center in the brain.

Mortality was observed in laboratory animals following the ingestion of mixed xylene or isomers of xylene. Reported acute oral LD₅₀ values in rats for mixed xylene range from 3,523 mg/kg when administered in corn oil (NTP 1986) to 8,600 mg/kg when administered undiluted (Hine and Zuidema 1970). It appears that the absorption of xylene was enhanced by corn oil due to its greater lipophilicity. The acute oral LD₅₀ values for mixed xylene administered in corn oil to male and female mice were 5,627 and 5,251 mg/kg, respectively (NTP 1986). The acute oral LD₅₀ for m-xylene (neat) in rats was 6,661 mg/kg (Smyth et al. 1962). According to a study by Gerarde (1959), m-xylene may be slightly less toxic than the other two isomers, since a single oral dose of 4,320 mg/kg of m-xylene resulted in death in 3/10 rats, whereas 4,400 mg/kg of o-xylene or 4,305 mg/kg of p-xylene produced death in 7/10 and 6/10 rats, respectively. In acute-duration repeated-dose gavage studies, mortality was observed in 8 of 10 rats given 2,000 mg/kg mixed xylene and 10 of 10 mice given 4,000 mg/kg mixed xylene in corn oil for 14 days (NTP 1986) and in 2 females from a group of 10 male and 10 female rats that received 2,000 mg/kg/day p-xylene for 10 days (Condie et al. 1988).

Survival was significantly lowered in male rats exposed to mixed xylene 5 days/week at chronic oral doses of 500 mg/kg but not at 250 mg/kg (NTP 1986); females were not affected. Although mortality appeared to be dose related in the treated rats, many of the early deaths were related to an error in gavage methodology; non-accidental deaths occurred with an incidence of 22% in controls, 32% at 250 mg/kg/day, and 38% at 500 mg/kg/day.

All LD₅₀ values and LOAEL values from each reliable study for death in each species and duration category are recorded in Table 3-2 and plotted in Figure 3-2.

3.2.2.2. Systemic Effects

The systemic effects observed after oral exposure to xylene are discussed below. The highest NOAEL value and all LOAEL values from each reliable study for systemic effects in each species and duration category are recorded in Table 3-2 and plotted in Figure 3-2.

Respiratory Effects. Limited information was located regarding respiratory effects in humans following oral exposure to mixed xylene or xylene isomers. Postmortem examination of a man who committed suicide by ingesting xylene showed pulmonary congestion and edema (Abu Al Ragheb et al. 1986). Death resulted from centrally mediated respiratory depression.

A single oral dose of 4,000 mg/kg in mice or daily oral dosing of rats and mice by gavage with mixed xylene for 14 days at 2,000 mg/kg/day resulted in shallow and labored breathing immediately after dosing, but no compound-related effects were observed in the lungs at necropsy (NTP 1986). Mice given daily oral doses of 2,000 mg/kg/day, 5 days/week, for 13 weeks exhibited similar effects 15–60 minutes after dosing (NTP 1986). Histopathological examination of the lungs and mainstem bronchi of rats and mice administered mixed xylene 5 days/week at doses as high as 1,000 mg/kg in rats and 2,000 mg/kg in mice for 13 weeks or 500 mg/kg in rats and 1,000 mg/kg in mice for up to 2 years revealed no adverse effects (NTP 1986). Gross and histopathological examination of rats administered m- or p-xylene for 13 weeks at doses as high as 800 mg/kg/day revealed no treatment-related effects (Wolfe 1988a, 1988b).

Table 3-2

Levels of Significant Exposure to Xylene - Oral.

Figure 3-2

Levels of Significant Exposure to Xylene - Oral.

Decreased pulmonary microsomal enzyme activity was observed in rats after a single oral dose of 1,000 mg/kg of p-xylene (Patel et al. 1978) and decreased pulmonary cytochrome P-450 content were observed in rats after gavage dosing with 800 mg/kg/day, 5 days/week, for 3 weeks (Elovaara et al. 1989), suggesting some direct toxicity of xylene in the lungs. Selective inactivation of enzymes can result in damage to tissue caused by the toxic metabolite of xylene, a methylbenzaldehyde (Carlone and Fouts 1974; Patel et al. 1978; Smith et al. 1982). The formation of the methylbenzaldehydes has not been confirmed in humans.

Cardiovascular Effects. Limited information was located regarding cardiovascular effects in humans following oral exposure to mixed xylene or its isomers. Postmortem examination showed no adverse effects on the heart or coronary arteries of a man who committed suicide by ingesting a large but unknown quantity of xylene (Abu Al Ragheb et al. 1986). No adverse cardiovascular effects were noted following histopathological examination of the heart in rats and mice exposed 5 days/week to mixed xylene at ≈63–2,000 mg/kg for 13 or 103 weeks (NTP 1986). No treatment-related effects were noted upon gross or histopathological examination of the heart in rats administered m- or p-xylene at doses as high as 800 mg/kg/day for 13 weeks (Wolfe 1988a, 1988b).

Gastrointestinal Effects. No superficial erosions, deep ulcerations, or other lesions were observed during postmortem examination of the gastric mucosa of a person who died following ingestion of a “large quantity” of xylene (Abu Al Ragheb et al. 1986). Histopathological examination of rats administered doses 5 days/week as high as 1,000 mg/kg of mixed xylene and mice administered doses as high as 2,000 mg/kg of mixed xylene for 13 weeks or in rats and mice administered doses as high as 500 and 1,000 mg/kg, respectively, for 2 years revealed no adverse effects on the stomach, small intestine, or colon (NTP 1986). Administration of p-xylene up to 800 mg/kg/day for 13 weeks also had no significant effect on gastrointestinal organs of rats (Wolfe 1988a, 1988b).

Hematological Effects. No studies were located regarding hematological effects in humans following oral exposure to mixed xylene or xylene isomers. Exposure of rats to 2,000 mg/kg/day p-xylene for 10 days resulted in no effects detectable in routine hematological analysis (Condie et al. 1988). Exposure to o- and m-xylene at 2,000 mg/kg/day for 10 days produced a decrease in the spleen weight of male rats (Condie et al. 1988); however, hematological analyses in these rats were normal. Mild polycythemia and leukocytosis in both male and female rats and an increase in spleen weight in females were observed in rats exposed to 1,500 mg/kg/day mixed xylene for 90 days (Condie et al. 1988). No effects were observed upon histopathological examination of the bone marrow following exposure to 800 mg/kg/day of p-xylene in rats and mice (Wolfe 1988a), or on administration 5 days/week of 1,000 mg/kg mixed xylene in rats for 13 weeks (NTP 1986), 2,000 mg/kg mixed xylene in mice for 13 weeks (NTP 1986), or 500 mg/kg mixed xylene in rats and 1,000 mg/kg in mice for 2 years (NTP 1986).

Musculoskeletal Effects. No studies were located regarding musculoskeletal effects in humans following oral exposure to mixed xylene or xylene isomers. In two animal bioassays, no musculoskeletal effects were observed in rats and mice upon histopathological examination of the femur, sternebrae, or vertebrae following intermediate or chronic exposure 5 days/week to mixed xylene up to 2,000 mg/kg for mice and 1,000 mg/kg for rats for 13 weeks and 1,000 mg/kg for mice or 500 mg/kg for rats for 103 weeks (NTP 1986). No adverse effects were observed in the sternum (with marrow), thigh musculature, or femur upon histopathological examination of rats administered m- or p-xylene at doses up to 800 mg/kg/day for 13 weeks (Wolfe 1988a, 1988b).

Hepatic Effects. No studies were located regarding hepatic effects in humans following oral exposure to mixed xylene or xylene isomers. In general, studies in animals have shown mild changes in the liver in response to oral exposure to mixed xylene. These changes included increased activity of liver enzymes and ultrastructural changes indicative of increased metabolic activity, but no evidence of histopathological changes in the liver tissue (Ungvary 1990). In acute and intermediate studies with rats, oral exposure to mixed xylene (Condie et al. 1988; Ungvary 1990) and its isomers (Condie et al. 1988; Elovaara et al. 1989; Pyykko 1980) has been associated with hepatic enzyme induction and increased hepatic weight. In the study by Condie et al. (1988), acute exposure to p-xylene at 250 mg/kg/day and m- and o-xylene at 1,000 mg/kg/day for 10 days caused increases in liver weight. Administration of doses of 1,060 mg/kg/day of all three xylene isomers for 3 days also produced increased cytochrome b₅ content and increased activities of liver enzymes in rats (Pyykko 1980), with the different isomers showing different enzyme induction potencies. Increased liver weight was observed with m- and o-xylene, but not p-xylene, and increased cytochrome P-450 was observed only with m-xylene. Administration of mixed xylene to rats for 90 days caused 13.8–27% increases in relative liver weight ratios at doses between 750 and 1,500 mg/kg/day (Condie et al. 1988); a statistically significant 5.7% increase in relative liver weight in male rats at 150 mg/kg/day is not biologically significant. No treatment-related histopathological changes were observed in the liver, but mild increases in serum transaminases were observed at 750 mg/kg/day. Similar increases in serum alanine aminotransferase were observed following ingestion of 800 mg/kg/day of m-xylene for 3 or 13 weeks (Elovaara et al. 1989; Wolfe 1988a). No effects were noted upon histopathological examination of the liver of rats and mice that were administered mixed xylene 5 days/week for a chronic or intermediate period of time with doses as high as 2,000 mg/kg for mice; 1,000 mg/kg for rats for 13 weeks; and 1,000 mg/kg for mice and 500 mg/kg for rats for 103 weeks (NTP 1986). Administration of doses as high as 800 mg/kg/day of p-xylene in rats for 13 weeks produced no adverse hepatic effects (Wolfe 1988b).

Renal Effects. No studies were located regarding renal effects in humans following oral exposure to mixed xylene or xylene isomers. Data are available for renal effects in laboratory animals following acute-duration exposure to individual isomers, but not to mixed xylene (Condie et al. 1988; Pyykko 1980). At 1,060 mg/kg/day for 3 days, increases in kidney weight were observed with m-xylene and increases in microsomal enzyme content and activity were observed with all three isomers (Pyykko 1980). No effects on urine parameters were noted after a 10-day exposure to 2,000 mg/kg/day of any of the isomers (Condie et al. 1988). The majority of studies using mixed xylene or its isomers for intermediate or chronic durations also showed no adverse effects on the kidneys. The only toxic change observed was increased hyaline droplet change in males (not relevant to humans) and increased early chronic nephropathy in females at 150–1,500 mg/kg/day mixed xylene for 90 days; the incidence in females at 150 mg/kg/day was not statistically significant. Although urine from these rats were normal (Condie et al. 1988), continued hyaline droplet accumulation can result in cell damage. Increased relative kidney weight was also observed in male rats given mixed xylene at 750 mg/kg/day and in female rats at 1,500 mg/kg/day for 90 days (Condie et al. 1988). Similarly, increased kidney weight and microsomal enzyme activity were observed in rats exposed to 800 mg/kg/day of m-xylene, 5 days/week, for 3 weeks (Elovaara et al. 1989). Increased relative kidney weight was also observed in male rats administered 800 mg m-xylene/kg/day for 13 weeks (Wolfe 1988a, 1988b). Histopathology of the kidneys and urinary bladder were normal. Also, no adverse effects were noted upon histopathological examination of the kidneys of rats and mice following intermediate or chronic exposure 5 days/week to doses of mixed xylene as high as 2,000 mg/kg (for 13 weeks in mice) and 1,000 mg/kg (for 103 weeks in mice) (NTP 1986).

Endocrine Effects. No studies were located regarding endocrine effects in humans or animals following oral exposure to mixed xylene or xylene isomers.

Dermal Effects. No studies were located regarding dermal effects in humans following oral exposure to mixed xylene or xylene isomers. Limited information was located regarding dermal effects in animals. No adverse effects were noted during microscopic examination of the skin of rats and mice administered mixed xylene 5 days/week at doses as high as 2,000 mg/kg in mice and 1,000 mg/kg for rats for an intermediate (13 weeks) period of time or as high as 1,000 mg/kg for mice and 500 mg/kg for rats for a chronic (103 weeks) period of time (NTP 1986). The skin of rats administered doses as high as 800 mg/kg/day of m- or p-xylene for 13 weeks appeared normal upon histopathological examination (Wolfe 1988a, 1988b).

Ocular Effects. No studies were located regarding ocular effects in humans following oral exposure to mixed xylenes or xylene isomers. Histopathological examination of the eyes of rats and mice orally exposed to mixed xylenes (NTP 1986) or to m- or p-xylene (Wolfe 1988a, 1988b) for 13 or 103 weeks showed no effects. No additional data regarding ocular effects in animals following oral exposure to xylenes were available.

Body Weight Effects. Effects on body weight were observed in several acute studies of the effects of mixed xylene and its isomers (Condie et al. 1988; NTP 1986; Pyykko 1980). Exposure to m-, o-, and p-xylene for 3 days resulted in weight losses of between 2.5 and 3 times that observed in control rats (Pyykko 1980). A 14-day exposure of rats and mice to mixed xylene resulted in an 18% decrease in body weight gain in male rats at 1,000 mg/kg/day and an 89% decrease in bodyweight gain in male mice at 2,000 mg/kg/day (NTP 1986). Body weights of male rats given 2,000 mg/kg/day o- or p-xylene, but not m-xylene, for 10 days showed 14% and 13% decreases, respectively, relative to controls (Condie et al. 1988). No significant body weight changes were observed in male or female rats dosed daily with ≤1,500 mg/kg/day mixed xylenes for 13 weeks (Condie et al. 1988), but a 16% decrease was noted in female mice dosed 5 days/week for 13 weeks at 2,000 mg/kg (NTP 1986). Body weights were decreased by 25% in males (with reduced food consumption) and by 15.5% in females (with normal food consumption) treated by daily oral gavage with 800 mg m-xylene/kg/day for 13 weeks (Wolfe 1988a). In a parallel study with p-xylene, body weights were reduced by 21% in males and by 11% in females treated at 800 mg/kg/day, although food consumption was significantly higher than controls (Wolfe 1988b). Body weight in male rats at 1,000 mg/kg and in female mice at 2,000 mg/kg mixed xylene were decreased 15% and 16%, respectively, in a 13-week study (NTP 1986). After week 59 in a 2-year study, mean body weights were reduced by 5–8% in male rats exposed by oral gavage 5 days/week to 500 mg/kg mixed xylenes, but this change is not biologically significant (NTP 1986). No body weight effects were observed in female rats exposed at doses as high as 500 mg/kg or in mice exposed at doses as high as 1,000 mg/kg/day (NTP 1986).

3.2.2.3. Immunological and Lymphoreticular Effects

No studies were located regarding immunological or lymphoreticular effects in humans after oral exposure to mixed xylene or xylene isomers. The only information suggesting a possible toxic effect of mixed xylene or its isomers on the immune system was a decrease in spleen and thymus weight observed in rats exposed for 10 days to 2,000 mg/kg/day p-xylene (Condie et al. 1988). Organ weight changes were not accompanied by histopathological changes.

The NOAEL and LOAEL values for immunological effects of p-xylene in rats are recorded in Table 3-2 and plotted in Figure 3-2.

3.2.2.4. Neurological Effects

Information concerning possible neurological effects associated with the ingestion of xylene is limited. Xylene produced a coma that persisted for more than 26 hours in a person who accidentally ingested an unknown amount (Recchia et al. 1985). The composition of the xylene was also unknown.

Clinical signs consistent with central nervous system toxicity have been observed in rats and mice following oral exposure to mixed xylene. A single oral dose of 4,000 mg/kg caused incoordination, prostration, decreased hindleg movement, and hunched posture in rats and tremors, prostration, and/or slowed breathing in mice (NTP 1986). In 13-week gavage assays in rats, neurotoxic signs included hyperactivity, convulsions, salivation, and epistaxis following exposure to 800 mg/kg/day m- or p-xylene (Wolfe 1988a, 1988b) and increased aggression following exposure to 1,500 mg/kg/day mixed xylenes (Condie et al. 1988). Clinical signs observed in mice in the hour after gavage dosing with mixed xylenes included weakness, lethargy, unsteadiness, tremors, and partial paralysis of hindlimbs at 2,000 mg/kg in a 13-week assay and hyperactivity at 1,000 mg/kg beginning week 4 in 13-week and 2-year assays (NTP 1986). Mild sedation at 2,000 mg/kg and increases in latency of several peaks in flash-evoked potentials at doses of 250 mg/kg and higher were observed following single doses of p-xylene; no effect was seen at 125 mg/kg/day (Dyer et al. 1988). This NOAEL was used to derive an MRL of 1 mg/kg/day for acute oral exposure to mixed xylenes or individual xylene isomers.

Histological damage to the outer hair cells of the organ of Corti provided evidence of ototoxicity in rats exposed by oral gavage to p-xylene, but not m- or o-xylene, at a dose of 900 mg/kg/day, 5 days/week for 2 weeks (Gagnaire and Langlais 2005). The losses of hair cells occurred in the area of the cochlea responsive to medium frequencies (10–25 kHz). No histopathology of the brain or spinal cord was observed in rats or mice administered mixed xylenes 5 days/week at doses as high as 1,000 mg/kg (rats) or 2,000 mg/kg/day (mice) for 13 weeks or 1,000 mg/kg for 2 years (NTP 1986). Similarly, no neurohistopathology was observed in rats administered doses of m- or p-xylene as high as 800 mg/kg/day for 13 weeks, although the brain-to-body weight ratio was increased in males dosed with 800 mg/kg/day of m-xylene (Wolfe 1988a, 1988b).

An acute-duration oral MRL of 1 mg/kg/day for mixed xylenes or individual isomers was calculated based on a NOAEL of 125 mg/kg and a LOAEL of 250 mg/kg for altered visual evoked brain potentials in male Long-Evans rats given single gavage doses of p-xylene (Dyer et al. 1988; NTP 1986) as described in the footnote in Table 3-2. An intermediate-duration oral MRL of 0.4 mg/kg/day for mixed xylenes or individual isomers was calculated based on a NOAEL of 500 mg/kg and a LOAEL of 1,000 mg/kg for hyperactivity in male and female B6C3F₁ mice dosed with mixed xylenes by gavage 5 days/week for 4–51 weeks (NTP 1986) as described in the footnote in Table 3-2.

The highest NOAEL value and all LOAEL values from each reliable study for neurological effects in rats and mice and for each exposure duration are recorded in Table 3-2 and plotted in Figure 3-2.

3.2.2.5. Reproductive Effects

No studies were located regarding reproductive effects in humans following oral exposure to mixed xylene or individual isomers.

No studies in animals directly examining reproductive function following oral administration of mixed xylene or its isomers were located; however, histological examination of rats and mice administered mixed xylene 5 days/week at doses as high as 1,000 mg/kg in rats and 2,000 mg/kg in mice for 13 weeks revealed no adverse effects on the prostate/testes (male), ovaries/uterus, or mammary glands (female) (NTP 1986). The reproductive system organs of rats administered doses of m- or p-xylene as high as 800 mg/kg/day appeared comparable to controls after 13 weeks of treatment (Wolfe 1988a, 1988b). In chronic studies, no adverse histopathological changes were observed in the reproductive organs in rats at doses as high as 500 mg/kg and in mice at doses as high as 1,000 mg/kg administered 5 days/week for 103 weeks (NTP 1986). The highest NOAEL value from each reliable study for reproductive effects are recorded in Table 3-2 and plotted in Figure 3-2.

3.2.2.6. Developmental Effects

No studies were located regarding developmental effects in humans following oral exposure to mixed xylene or xylene isomers.

Significantly increased incidences of cleft palate and decreased fetal body weight were reported following maternal oral exposure during gestation days 6–15 to doses of 2,060 mg/kg/day mixed xylene in mice (Marks et al. 1982). Mixed xylene was also toxic to the dams, producing 31.5% mortality at 3,100 mg/kg/day. It is unclear whether the observation of cleft palate in this study is associated with maternal toxicity or a predisposition of mice under stress to give birth to offspring with this birth defect. In a teratology screening study, 2,000 mg/kg/day of m-xylene produced no evidence of fetal toxicity in mice (Seidenberg et al. 1986). Given the limited amount of animal data, no conclusion can be made regarding the relationship between oral exposure of xylene and adverse developmental effects. The highest NOAEL value and all LOAEL values from each reliable study for developmental effects are recorded in Table 3-2 and plotted in Figure 3-2.

3.2.2.7. Cancer

No data were located regarding cancer in humans following oral exposure to mixed xylene or xylene isomers.

The carcinogenicity of mixed xylene following oral exposure has been evaluated in chronic studies with rats and mice; however, no animal studies were available on the carcinogenic effects of m-, o-, or p-xylene following oral exposure. Results of the chronic oral studies with mixed xylene have been negative (NTP 1986) or equivocal (Maltoni et al. 1983, 1985). In a chronic bioassay, rats and mice of both sexes received mixed xylene by gavage at 0, 250, or 500 mg/kg and 0, 500, or 1,000 mg/kg, respectively, 5 days/week for 103 weeks. The interpretation of the results of the NTP bioassay was compromised by the large number of gavage-related deaths early in the study in the high-dose male rats. In the other chronic study (Maltoni et al. 1983, 1985), male and female rats were fed xylene (unspecified) by gavage at 0 or 500 mg/kg, 4–5 days/week for 104 weeks. The Maltoni studies were weakened because of methodological flaws such as failure to report site-specific neoplasia, insufficient toxicity data, and absence of statistical analyses. Therefore, given the limited data, no definitive conclusion can be made regarding the carcinogenicity of mixed xylene in animals following oral exposure.

3.2.3.1. Death

No reports of death in humans following dermal exposure to xylene were located. Limited animal data suggest that mixed xylene and m-xylene can cause death when applied dermally (Hine and Zuidema 1970; Smyth et al. 1962). The acute dermal LD₅₀ in rabbits has been determined to be 3,228 mg/kg/day for m-xylene and >114 mg/kg/day for mixed xylene for 4 hours or more (Hine and Zuidema 1970; Smyth et al. 1962).

The LD₅₀ value for death in rabbits as a result of acute-duration exposure to m-xylene is recorded in Table 3-3.

3.2.3.2. Systemic Effects

No studies were located regarding musculoskeletal, endocrine, or body weight effects in humans or animals following dermal exposure to mixed xylenes or xylene isomers. The systemic effects that were observed after dermal exposure to xylene are discussed below. All LOAEL values from each reliable study for systemic effects in each species and duration category are recorded in Table 3-3.

Respiratory Effects. Case reports of dryness of the throat (Goldie 1960) in painters and decreased pulmonary function and dyspnea in histology technicians with chronic exposure to xylene (Hipolito 1980) have been published. It is likely that these effects represent direct effects of xylene or other solvents on the respiratory tissues and they are discussed in more detail in Section 3.2.1.2. No studies were located regarding respiratory effects in animals following dermal exposure to mixed xylene or xylene isomers, although some of the inhalation studies also involved exposure via dermal route as well.

Table 3-3

Levels of Significant Exposure to Xylene - Dermal.

Cardiovascular Effects. Cases of flushing, chest pains, and palpitations in histology technicians have been reported (Hipolito 1980). These studies also involved exposure via inhalation route. It is unclear whether these effects are directly attributable to xylene exposure because of possible exposure to other chemicals. No studies were located regarding cardiovascular effects in animals after dermal exposure to mixed xylene or xylene isomers.

Gastrointestinal Effects. Gastric discomfort in painters (Goldie 1960) and nausea in histology technicians (1980) have been reported; these studies also involved exposure via inhalation route. However, other chemicals in the workplace may have contributed to these effects. No studies were located regarding gastrointestinal effects in animals following dermal exposure to mixed xylene or xylene isomers.

Hematological Effects. Decreased white blood cell count has been observed in histology technicians (Hipolito 1980) and in workers with occupational exposure to benzene, toluene, and xylene (Moszczynski and Lisiewicz 1983, 1984a); these studies also involved exposure via inhalation route. However, chemicals other than xylene may have caused these decreases. No studies were located regarding hematological effects in animals following dermal exposure to mixed xylene or xylene isomers.

Hepatic Effects. When compared with unexposed controls, workers with occupational exposure via dermal and inhalation routes to toluene, xylene, and pigments had significantly increased urinary D-glucaric acid content in the urine indicating hepatic microsomal enzyme induction (Dolara et al. 1982). Serum antipyrine half-life was increased, suggesting possible hepatotoxicity. No effect on serum aminotransferases was observed in workers exposed to a mixture of solvents (Kurppa and Husman 1982). These studies also involved exposure via inhalation route. These studies are limited in that multiple chemical exposures occurred and the effects observed cannot be directly attributed to xylene. No studies were located regarding hepatic effects in animals following dermal exposure to mixed xylene or xylene isomers.

Renal Effects. Occupational exposure to a mixture of mainly xylene and toluene resulted in elevated albumin, erythrocytes, and leukocytes in the urine (Askergren 1981, 1982). In addition, increased β-glucuronidase was observed in the urine of painters (Franchini et al. 1983). However, these studies are limited in that the effects observed may be attributable to exposure to toluene, which is a known renal toxicant, and that inhalation as well as dermal exposure could have occurred. No studies were located regarding renal effects in animals following dermal exposure to mixed xylene or xylene isomers.

Dermal Effects. Acute dermal exposure of human subjects to undiluted m-xylene in hand immersion studies has been associated with transient skin erythema (irritation), vasodilation of the skin, and dryness and scaling of the skin (Engstrom et al. 1977; Riihimaki 1979b). Urticaria was reported in a female cytology worker exposed predominantly to xylene vapors (Palmer and Rycroft 1993). Because this response probably had an immunological component, it is discussed further in the Immunological and Lymphoreticular Effects section.

Mild-to-severe skin irritation was noted in rabbits, guinea pigs, and mice treated topically with mixed xylene (2.3–114 mg/kg/day), m-xylene (65–199 mg/day) or o-xylene (66 mg/day) in acute studies (Ahaghotu et al. 2005, isomer identified in personal communication by Singh 2006; Anderson et al. 1986; Chatterjee et al. 2005; Consumer Products Testing 1976; Food and Drug Research Labs 1976a; Hine and Zuidema 1970; Pound and Withers 1963; Smyth et al. 1962). The extent of the irritation appeared to increase with duration of exposure; the most severe dermal irritation ratings were obtained in the longest exposures of 10-days (Hine and Zuidema 1970). Application of 0.250 mL (1,200 mg/kg) of m-xylene to the skin of rats for 1 hour resulted in a significant increase in oxidative species and DNA fragmentation (increased low-molecular-weight DNA) within 2 hours (Gunasekar et al. 2003; Rogers et al. 2001). Treatment with m- or o-xylene increased levels of inducible nitric oxide synthetase and tumor necrosis factor-alpha (TNF-alpha), a pro-inflammatory cytokine, in skin, as well as plasma levels of interleukin-1-alpha (IL-1alpha) (Ahaghotu et al. 2005; Chatterjee et al. 2005; Gunasekar et al. 2003). Dermal swelling following application of 69 mg m-xylene to the ear of mice resulted in significant increases in the thickness of the ear as measured 0.5–25 hours after application (Iyadomi et al. 2000); peak thickness was observed 6 hours after treatment. Histopathological effects of exposure to m- or o-xylene included swelling and disruption of the stratum corneum, granulocyte infiltration of the epidermis, and separation of the epidermis and dermis, with evidence of local inflammation (accumulation of mast cells, plasma cells) (Ahaghotu et al. 2005; Chatterjee et al. 2005; Gunasekar et al. 2003). Impairment of skin function was evident in the increase in transepidermal water loss in hairless rats following occlusive exposure (199 mg for 1 hour) or unocclusive exposure (64.8 mg/day in 5 divided daily fractions over 4 days) to m-xylene or unocclusive exposure (66 mg/day in 5 divided doses, for 4 days) to o-xylene and the reduction in skin moisture content observed in rats treated unocclusively as above with m-xylene (Ahaghotu et al. 2005; Chatterjee et al. 2005). Moderate-to-marked irritation and moderate necrosis were observed in rabbits with a 2–4-week dermal exposure to undiluted xylene (Wolf et al. 1956). No chronic animal studies evaluating the dermal effects of xylene were located.

Ocular Effects. There are a few case reports in humans describing the effects of direct contact of the eye with heated xylene from a pressurized hose in one case or with paint containing xylene as the solvent in two cases (Ansari 1997; Narvaez and Song 2003). Both kinds of exposures resulted in photophobia, redness of the conjunctiva, and partial loss of the conjunctival and corneal epithelia. The effects in the two men exposed to paint were characterized as equivalent to grade II chemical burns and, with medical therapy, largely reversed after about a week (Ansari 1997). The other case also experienced subconjunctival hemorrhage, more extensive loss of the corneal epithelium, and xylene keratopathy (melting of the corneal stroma), the latter of which persisted to 4 weeks following the initial injury (Narvaez and Song 2003). It is possible combined thermal, physical, and chemical injury increased the severity of ocular effects in this case.

Several studies in humans have reported on ocular effects of exposure to xylene vapor. In the controlled exposure situations, eye irritation was reported at concentrations of mixed xylene as low as 200 ppm for 3–5 minutes (Nelson et al. 1943) and of p-xylene as low as 100 ppm for 1–7.5 hours/day for 5 days (NIOSH 1981). Case reports have also demonstrated transient eye irritation in humans exposed to vapors of mixed xylene or p-xylene (Carpenter et al. 1975a; Hastings et al. 1986; Klaucke et al. 1982; Nelson et al. 1943; Nersesian et al. 1985; NIOSH 1981). Eye irritation was more frequently reported by workers exposed to mixed xylene (geometric mean TWA 14 ppm) than by the controls (Uchida et al. 1993).

Instillation of 0.1 mL (87 mg) of mixed xylene or 0.5 mL (432 mg) of m-xylene into the eyes of rabbits resulted in slight-to-moderate eye irritation (Consumer Products Testing 1976; Hine and Zuidema 1970; Kennah et al. 1989; Smyth et al. 1962). No corneal effects were observed in these studies.

3.2.3.3. Immunological and Lymphoreticular Effects

Limited data were located regarding immunological and lymphoreticular effects in humans following dermal exposure to xylene. Occupational exposure to benzene, toluene, and xylene resulted in decreased serum complement (Smolik et al. 1973) and in decreased lymphocytes, but there was no effect on lymphocyte reactions when stimulated with phytohemagglutinin (Moszczynski and Lisiewicz 1983, 1984a). Interpretation of these studies is limited in that chemicals other than xylene may have accounted for the effects observed. Exposures via inhalation and dermal routes also may have occurred. Contact urticaria was reported in a female cytology worker exposed for several months predominantly to <100 ppm xylene vapors (Palmer and Rycroft 1993). A closed patch test resulting in severe erythema and wealing provides evidence that the effect was a result of direct contact of xylene vapor with the skin and suggests that the reaction was immunological.

No studies were located regarding immunological or lymphoreticular effects in animals after dermal exposure to mixed xylene or xylene isomers.

3.2.3.4. Neurological Effects

Occupational exposure to xylene has been reported to result in headache, dizziness, malaise, a feeling of drunkenness, irritability, fine tremor, dysphasia, hyperreflexia, and/or impaired concentration and memory (Goldie 1960; Hipolito 1980; Kilburn et al. 1985; Roberts et al. 1988). These studies are limited, however, because other chemical exposures in the workplace may have been responsible for the effects observed and that exposures via inhalation and dermal routes may have occurred.

As described in a brief report, pregnant rats dermally exposed to xylene (form not specified) at 2,000 mg/kg/day throughout gestation showed a statistically significant 27% reduction, compared to controls, in motor activity in an open field test, suggesting a neurotoxic effect of xylene (Mirkova et al. 1979). In this study, dosing at 200 or 2,000 mg/kg/day reduced brain cholinesterase activities in dams by 35–38% compared to controls; fetal brain cholinesterase was also reported to be ‘inhibited’ at those doses. The report did not discuss whether any measures were taken to prevent ingestion of the test material.

3.2.3.5. Reproductive Effects

No studies were located regarding reproductive effects in humans or animals after dermal exposure to mixed xylene or xylene isomers.

3.2.3.6. Developmental Effects

The human data regarding the developmental effects of xylene suggest a possible relationship between occupational solvent exposure and developmental toxicity (Holmberg and Nurminen 1980; Kucera 1968; Taskinen et al. 1989; Windham et al. 1991). However, these data are limited for assessing the relationship between dermal exposure to xylene and developmental effects because the available studies involved concurrent exposure to other chemical agents in addition to xylene in the workplace (Holmberg and Nurminen 1980; Kucera 1968; Taskinen et al. 1989; Windham et al. 1991), because few subjects were tested (Taskinen et al. 1989; Windham et al. 1991), and because it is extremely difficult to have a pure dermal exposure since such exposure in the absence of respiratory protection is accompanied by inhalation exposure.

As described in a brief report, dermal exposure of pregnant rats to doses as low as 200 mg/kg/day of xylene (unspecified concentration and isomer) throughout gestation produced decreases in enzyme activity (cholinesterase, cytochrome oxidase) in fetal and maternal brain tissue (Mirkova et al. 1979).

3.2.3.7. Cancer

Studies of workers occupationally exposed to solvents have examined the cancer and leukemia risks and suggest a possible relationship between coal-based xylene exposure and leukemia (Arp et al. 1983; Wilcosky et al. 1984). Both contain limitations (e.g., small number of subjects, no exposure concentrations, unknown composition of xylene and possible exposure to benzene and other chemicals) that preclude a definitive conclusion regarding dermal exposure to xylene and cancer; development of these studies probably also involved exposure via inhalation.

Limited information was located regarding the carcinogenicity of dermal exposure to xylene in animals (Berenblum 1941; Pound 1970; Pound and Withers 1963). Application of xylene (concentration, purity, and amount unspecified) to the skin for 25 weeks resulted in no increase in skin tumors, and did not potentiate the number of skin tumors produced by benz[a]pyrene (Berenblum 1941). However, two studies showed that a single xylene pretreatment slightly enhanced the number of tumors produced by a combination of ultraviolet light irradiation (initiation) and croton oil (promotion) (Pound 1970) or urethane (initiation) and croton oil (promotion) (Pound and Withers 1963). These findings suggest that xylene may be a promoter for skin cancer and might also act as initiator or cocarcinogen. These studies are limited in that tumors other than skin tumors were not assessed and untreated controls were not used. Furthermore, it is not known whether the xylene was analyzed for the presence of other aromatic hydrocarbons such as benzene that are known to be tumorigenic.

3.4.1.1. Inhalation Exposure

Xylenes are very soluble in blood and are therefore absorbed easily into the systemic circulation during inhalation exposure (Astrand 1982). Evidence for absorption of xylene by humans following inhalation exposure is provided by the observation that urine metabolites increase in proportion to exposure (Inoue et al. 1993; Jonai and Sato 1988; Kawai et al. 1991; Ogata et al. 1970; Riihimaki and Pfaffli 1978; Riihimaki et al. 1979b; Sedivec and Flek 1976b; Senczuk and Orlowski 1978; Wallen et al. 1985) and in proportion to increased ventilatory rates during exercise (Astrand 1982; Astrand et al. 1978; Bergert and Nestler 1991; Engstrom and Bjurstrom 1978; Riihimaki and Savolainen 1980; Riihimaki et al. 1979b). Absorption of the retained isomers appears to be similar, regardless of exposure duration or dose. The alveolar air concentrations of xylenes in male volunteers exposed at rest to 100 ppm mixed xylenes for 70 minutes reached equilibrium within 10 minutes (Gamberale et al. 1978).

Many authors have measured the retention of xylene in the lungs following inhalation exposure. It is this retained xylene that is available for absorption into the systemic circulation. In experimental studies with human subjects, retention of the various isomers was similar following inhalation of m-, o-, or p-xylene, and averaged 63.6% (Sedivec and Flek 1976b). Other authors have estimated that between 49.8 and 72.8% of inhaled xylene is retained (David et al. 1979; Ogata et al. 1970; Riihimaki and Pfaffli 1978; Riihimaki and Savolainen 1980; Wallen et al. 1985). Pulmonary retention does not appear to differ on the basis of sex (Senczuk and Orlowski 1978). However, one study reported that uptake of m-xylene during a 2-hour exposure at 200 mg/m³ was slightly higher in women than in men (Ernstgard et al. 2003). The enhanced pulmonary ventilation and cardiac output associated with physical exertion can increase the amount of xylene retained and subsequently absorbed into the body (Astrand et al. 1978; Riihimaki et al. 1979b). The study by Astrand et al. (1978) suggests that retention efficiency decreases as exposure duration increases.

In pregnant mice, approximately 30% of an administered inhalation dose of 600 ppm p-xylene was absorbed following a 10-minute exposure period (Ghantous and Danielsson 1986). Absorption was not quantified in other animal studies, but absorption can be inferred from the observed effects on pulmonary microsomal enzymes and the appearance of methylhippuric acid in the urine following inhalation of xylene (Carlsson 1981; David et al. 1979; Elovaara 1982; Elovaara et al. 1987; Patel et al. 1978).

3.4.1.2. Oral Exposure

Limited information is available on the absorption of xylene in humans and animals following ingestion. Excretion of urinary metabolites indicated that absorption had occurred following oral doses of either 40 or 80 mg/kg of o-xylene or m-xylene in humans (Ogata et al. 1979). Although absorption was not fully quantified, recovery of specific metabolites demonstrated that at least 34% of o-xylene and 53% of m-xylene had been absorbed from the administered doses of 40 mg/kg.

Measurement of urinary metabolites in rats over 24 hours demonstrated almost complete absorption (87–92%) following oral gavage dosing with 1.8 g m-xylene, or 1.74 g o- or p-xylene (Bray et al. 1949). Experiments in which 0.15 mL of radiolabelled m-xylene (0.27 mg/kg) was administered by oral gavage in 5% aqueous gum acacia to male and female rats indicated that absorption was rapid (Turkall et al. 1992); peak blood levels of radioactivity were observed within 20 minutes. The half-life of absorption of m-xylene was twice as fast in females than in males, although the total amount absorbed over 24 hours—the area under the curve (AUC) for plasma radioactivity—was the same in both sexes (about 0.2% of the initial dose/mL/hour). When m-xylene was first adsorbed to 0.5 g of a sandy soil before mixing with the vehicle and gavage administration, the absorption half-life was twice as long in female rats but unaffected in males (Turkall et al. 1992); adsorption to clay soil did not produce a biologically significant difference from m-xylene administered alone. Adsorption to sandy soil doubled the peak blood levels of radioactivity and increased the plasma AUC by 75% in female rats, but had no such effects in males.

3.4.1.3. Dermal Exposure

Results of experimental studies with humans indicate that m-xylene is absorbed following dermal exposure; however, the extent of penetration and absorption of m-xylene through skin is not nearly as great as that resulting from inhalation (Engstrom et al. 1977; Riihimaki 1979b; Riihimaki and Pfaffli 1978). Dermal absorption may occur via exposure to m-xylene vapors, as well as through direct dermal contact with the solvent (Dutkiewicz and Tyras 1968; Engstrom et al. 1977; Kezic et al. 2000; Loizou et al. 1999; Riihimaki 1979b; Riihimaki and Pfaffli 1978). Absorption of m-xylene through the skin was measured at a maximal permeation rate, occurring between 15 and 25 minutes at a flux of 46 nmol/cm²/minute (Kezic et al. 2001). In humans, the estimated absorption rate following immersion of both hands in m-xylene for 15 minutes was approximately 2 μg/cm²/minute (Engstrom et al. 1977). Another study measured a rate of absorption of 75–160 μg/cm²/minute when xylene was applied to forearm skin (Dutkiewicz and Tyras 1968). Absorption of m-xylene vapor through the skin has been estimated at between 0.1 and 2% of the inhaled dose (Kezic et al. 2000; Loizou et al. 1999; Riihimaki and Pfaffli 1978). Dermal permeability constants averaging 0.25 cm/hour were calculated from exposure of subjects to 600 ppm m-xylene for 3.5 hours (McDougal et al. 1990; Riihimaki and Pfaffli 1978). In human subjects exposed to 6770 ppm m-xylene vapor on the hand and forearm the average flux through the skin was 0.091 mg/L/hour for a 20-minute exposure, falling to 0.061 mg/L/hour for a 180-minute exposure (Kezic et al. 2004). Simultaneously, the maximal flux into the blood was 0.034 mg/L/hour after 20 minutes, rising to 0.063 mg/L/hour after a 180-minute exposure. Maximum permeation rates of m-xylene vapor were achieved within 90 minutes (Kezic et al. 2004). Dermal absorption of m-xylene following topical administration or exposure to vapor was 3 times greater in subjects with atopic dermatitis compared to normal subjects (Engstrom et al. 1977; Riihimaki and Pfaffli 1978).

Limited information is available regarding the absorption of xylene following dermal exposure in animals. Permeability of rat skin to m-xylene was estimated from blood levels obtained during dermal exposure to liquid m-xylene (Morgan et al. 1991; Skowronski et al. 1990) or m-xylene vapors (McDougal et al. 1990). Peak blood levels of m-xylene were reached within 2 hours of topical application from sealed dermal enclosures in rats and slowly declined during the 24-hour observation period (Morgan et al. 1991); an average of 562 mg (0.65 mL) was absorbed across a 3.1 cm² area of skin (1% of total surface area) from the initial volume of 2 mL over 24 hours. The total amount absorbed was reduced when m-xylene was administered in aqueous solution (Morgan et al. 1991). Dermal permeability constants calculated for rats exposed to 5,000 ppm m-xylene averaged 0.723 cm/hour, about 3 times greater than those calculated for humans (McDougal et al. 1990); the flux was calculated as 0.0151 mg/cm²/hour. Skin:air partition values for a series of solvents including m-xylene correlated well with permeability constants (McDougal et al. 1990), but not with octanol-water partition coefficients. m-Xylene adsorbed on sandy soil or clay soils showed lower peak absorption than for m-xylene alone and clay soil significantly prolonged the absorption half-life, but the total amount absorbed over an unspecified period was unchanged (Abdel-Rahman et al. 1993; Skowronski et al. 1990).

Dermal penetration rates of xylene have been estimated using excised skin of rats exposed to liquid or vapor. The absorption of o-xylene by excised abdominal skin from rats increased with the time of contact (Tsuruta 1982); the penetration rate was estimated to be 0.006 mg/cm²/hour. The flux of o-xylene through the dorsal skin of hairless rats was measured as 0.22 mg/cm²/hour (Ahaghotu et al. 2005). The skin:air partition coefficient for m-xylene was found to be 50.4±1.7 using rat skin in vitro exposed to 203 ppm m-xylene vapor (Mattie et al. 1994); the concentration of m-xylene in the skin reached equilibrium in 2 hours. Dermal absorption studies using excised skin are not directly comparable to in vivo studies because of the lack of an intact blood supply, but diffusion through the skin appears to be the limiting factor rather than removal from the skin by the blood (McDougal et al. 1990).

3.4.2.1. Inhalation Exposure

In human subjects exposed by inhalation to ≤44 ppm deuterium-labeled xylene isomers for 2 hours, peak blood xylene concentrations were highest exactly at the end of exposure and then declined (Adams et al. 2005). Absorbed xylene in venous blood was distributed in cells (12%) and serum (88%), with about 9% of the total amount in blood associated with protein-free serum (Riihimaki et al. 1979b). Following systemic circulation, xylene is distributed primarily to adipose tissue. Estimates of the amount of xylene accumulated in human adipose tissue range from 4 to 10% of the absorbed dose (Astrand 1982; Engstrom and Bjurstrom 1978; Riihimaki et al. 1979b). It has been suggested that following prolonged occupational exposure to xylene, significant amounts of the solvent could accumulate in adipose tissue (Astrand 1982; Engstrom and Bjurstrom 1978).

Table 3-6

Partition Coefficients for Xylenes.

Studies in mice (Bergman 1983; Ghantous and Danielsson 1986) and rats (Carlsson 1981; Ito et al. 2002) indicate that the distribution of m- or p-xylene and their metabolites is characterized by high uptake in lipid-rich tissues, such as brain and fat. High uptake also occurs in well-perfused organs, such as the liver and kidney. Using low-temperature (cryogenic) whole-body autoradiography of male mice exposed by inhalation to ¹⁴C-labeled m-xylene for 10 minutes, Bergman (1983) observed a high level of unmetabolized m-xylene (volatile radioactivity) in body fat, bone marrow, white matter of the brain, spinal cord, spinal nerves, liver, and kidney immediately after exposure. High levels of metabolites (nonvolatile radioactivity) were present in the blood, liver, lung, kidney, and adrenal medulla. Unmetabolized m-xylene persisted in the central nervous system and spinal nerves for up to 1 hour post-exposure, whereas a considerable amount persisted in body fat at 4–8 hours after exposure. Metabolites were detectable post-exposure in the adrenal medulla for 0.5 hours, in the liver for 2 hours, in the kidney for up to 8 hours, in bile for 2–8 hours, in the intestinal lumen for up to 24 hours, and in the nasal mucosa and bronchi for 2–24 hours. No radioactivity was detectable by 48 hours. In male Sprague-Dawley rats exposed to 2,000 ppm m-xylene vapor 4 hours/day for 5 consecutive days, m-xylene concentrations were measured in tissues by gas chromatography (Ito et al. 2002). The highest concentrations of m-xylene were observed in peri-intestinal fat and subcutaneous fat, whereas concentrations more than 40 times lower were detected in lung, spleen, testis, heart, kidney, liver, and psoas muscle. In the brain subdivided into four parts, the concentration of m-xylene was 3–6 times lower than in subcutaneous fat. The uneven distribution of inhaled m-xylene in the brain was associated with changes in GABA_A receptor binding (discussed in Section 3.5.2).

According to an intermediate-duration animal study, the level of xylene stored in body fat may decrease as exposure continues due to an increase in metabolic rate possibly by inducing its own metabolism (Savolainen et al. 1979a). Levels of m-xylene in perirenal fat of rats exposed to 300 ppm technical xylene decreased from 67.6 to 36.6 μg/g tissue as exposure duration increased from 5 to 18 weeks (Savolainen et al. 1979a).

p-Xylene and o-xylene have been shown to readily cross the placenta and were distributed in amniotic fluid and embryonic and fetal tissues (Ghantous and Danielsson 1986; Ungvary et al. 1980b). The level detected in fetal tissues (brain, liver, lung, and kidney), which are low in lipids, was only 2% of that detected in the maternal brain tissue, which contains large amounts of lipids (Ghantous and Danielsson 1986). Also, higher levels were detected in fetal tissues than in amniotic fluid (Ungvary et al. 1980b).

3.4.2.2. Oral Exposure

No studies were located regarding distribution in humans following oral exposure to mixed xylene or xylene isomers. In rats administered m-xylene by gavage, fat contained the highest tissue concentration of radioactivity; approximately 0.3% of the administered dose was found per gram of fat in females and 0.1% per gram of fat in males (Turkall et al. 1992).

3.4.2.3. Dermal Exposure

No studies were located regarding distribution of xylene in humans following dermal exposure to mixed xylene or individual isomers. Extremely limited information was located regarding distribution in animals following dermal absorption. After topical administration, m-xylene was rapidly detectable in plasma, reaching peak concentrations in about 2 hours (Skowronski et al. 1990). About 0.003% of the initial dose was detected per gram of subcutaneous fat 48 hours after exposure.

3.4.4.1. Inhalation Exposure

In humans, about 95% of absorbed xylene is biotransformed and excreted as urinary metabolites, almost exclusively as methylhippuric acids; the remaining 5% is eliminated unchanged in the exhaled breath (Astrand et al. 1978; Ogata et al. 1979; Pellizzari et al. 1992; Riihimaki et al. 1979b; Sedivec and Flek 1976b; Senczuk and Orlowski 1978). Less than 0.005% of the absorbed dose of xylene isomers is eliminated unchanged in the urine, and <2% is eliminated as xylenols (Sedivec and Flek 1976b). The excretion of methylhippuric acids is rapid and a significant amount is detected in the urine within 2 hours of exposure. The amount of methylhippuric acid increases with time. Differences in the amount of the metabolites excreted depend on the interpersonal differences in lung ventilation and retention, not on the isomer of xylene (Sedivec and Flek 1976b).

There appear to be at least two distinct phases of elimination, a relatively rapid one (half-life, 1 hour) and a slower one (half-life, 20 hours). These phases of elimination are consistent with the distribution of xylene into three main tissue compartments; the rapid and slower elimination phases correspond to elimination from the muscles and the adipose tissue, respectively, whereas the elimination of xylene from the parenchymal organs is so rapid that the available studies could not monitor it (Ogata et al. 1970; Riihimaki et al. 1979a, 1979b). It is also possible that the renal excretion of the most common xylene metabolite, methylhippuric acid, takes place via the tubular active secretion mechanism of organic acids. Renal excretion is not a rate-limiting step in the elimination of absorbed xylene under normal physiological conditions (Riihimaki et al. 1979b). Physiologically based pharmacokinetic modeling suggests that the urinary excretion of m-methylhippuric acid following m-xylene exposure of humans is linear at concentrations up to 500 ppm, and that elimination of m-methylhippuric acid is slower in individuals with a greater percentage of body fat (e.g., women) (Kaneko et al. 1991a, 1991b). Systemic clearance rates in male subjects following inhalation of ≤44 ppm deuterium-labeled xylene isomers for 2 hours ranged between 116 and 129 L/hour (Adams et al. 2005). The terminal half-life averaged 34 hours for all three isomers among all subjects, but varied widely among individuals from 12 to 108 hours.

Volunteers acutely exposed by inhalation to 100 or 200 ppm m-xylene for 7 hours excreted 54 and 61%, respectively, of the administered dose by 18 hours after exposure ended (Ogata et al. 1970). Following intermittent acute exposure of men and women to 23, 69, or 138 ppm m-xylene, excretion of m-methylhippuric acid peaked 6–8 hours after exposure began. It decreased rapidly, regardless of exposure level or sex, after exposure had ended. Almost no xylene or m-methylhippuric was detected 24 hours later (Senczuk and Orlowski 1978). Following exposure to 200 mg/m³ m-xylene for 2 hours, small differences in elimination were noted between genders (Ernstgard et al. 2003). Women excreted 31% more (AUC) unmetabolized m-xylene in exhaled air than men, whereas men excreted 37% more (AUC) m-methylhippuric acid in urine than women.

Exercise increased the amount of xylene absorbed and thus increased the amount of m-methylhippuric acid and 2,4-xylenol eliminated in the urine of men exposed to m-xylene (Riihimaki et al. 1979b). No evidence of saturation of metabolism of xylene was observed at exposures as high as 200 ppm (Riihimaki et al. 1979b). The excretion of m-methylhippuric acid appeared to correspond very closely to the estimated xylene uptake and expired xylene represented about 4–5% of the absorbed xylene in all exposure groups (Riihimaki et al. 1979b). Urinary excretion of methylhippuric acid correlated well with exposure (Kawai et al. 1992; Lapare et al. 1993; Skender et al. 1993), and based on a study of workers occupationally exposed to mixed xylenes (geometric mean TWA 14 ppm), Inoue et al. (1993) estimated a slope of 13 mg methylhippuric acid/L/ppm (11.1 mg/g creatinine/ppm) for all three isomers. A sex-related difference in the urinary excretion of methylhippuric acids was not observed (Inoue et al. 1993).

A few studies have evaluated the kinetics of elimination of the metabolites of xylene following inhalation exposure of experimental animals. The primary urinary metabolite is methylhippuric acid (David et al. 1979). A dose-relationship in the urinary elimination of m-methylhippuric acid in rats was observed immediately after exposure to 50 or 500 ppm m-xylene for 6 hours (Kaneko et al. 2000). Another study found the increase of urinary elimination of methylhippuric acid metabolites to be linear in rats exposed for 5 hours to mixed xylene between 75 and 150 ppm, but non-linear between exposures at 150 and 225 ppm (Tardif et al. 1992). Concomitantly, there were non-linear increases in the amount of xylene excreted unchanged in exhaled air, indicating possible saturation of metabolism at 225 ppm.

Using a low-temperature (cryogenic), whole-body autoradiography technique for mice exposed by inhalation to radiolabeled m-xylene for 10 minutes, Bergman (1983) observed elimination of metabolites (non-volatile radioactivity) in bile and feces. Clearance of metabolites was completed in the adrenal medulla by 0.5 hours, in the liver after a little over 2 hours, in the kidney after 8 hours, in bile after 8 hours, in the intestinal lumen after 24 hours, and in the nasal mucosa and bronchi after 24 hours. No radioactivity was detectable 48 hours after exposure.

3.4.4.2. Oral Exposure

Limited information is available on the elimination of the metabolites of xylene following ingestion in humans. In an unspecified number of male volunteers given oral doses of 40 mg/kg/day of o-xylene or m-xylene, the molar (mol) excretion ratios (total excretion [mol] in urine during appropriate interval/dose administered [mol] × 100[%]) for o-methylhippuric acid and m-methylhippuric acid were 33.1 and 53.1, respectively (Ogata et al. 1979). More of the m-xylene is eliminated as methylhippuric acid than the ortho derivative for o-xylene. The molar excretion ratio for o-toluic acid glucuronide (o-toluyl-glucuronide) was 1.0 in men given o-xylene as an oral dose of 40 mg/kg/day. The amounts of o-methylhippuric acid (o-toluic acid) and of o-toluic acid glucuronide excreted in the urine attained maximal levels in 3–6 hours of exposure, while that of m-methylhippuric acid attained peak levels in 1–3 hours (Ogata et al. 1979). These results indicate that the major elimination pathway of o-xylene is the formation of o-methylhippuric acid in humans. The formation of o-toluic acid glucuronide is a minor pathway for the elimination of o-toluic acid, but would be available in the event of saturation of the major pathway.

Excretion of radioactivity by rats following an oral dose of m-xylene showed most excretion occurred in the urine during the first 12 hours after dosing (50–59% of the dose), with a total of 96.2% eliminated in urine in males and 73.7% in urine in females over 48 hours (Turkall et al. 1992). Approximately 8 and 22% of the total dose was excreted in exhaled air by males and females, respectively, during the first 12 hours. m-Methylhippuric acid comprised 67–75% of the urinary radioactivity, with xylenol comprising 2–18%, and unchanged xylene comprising approximately 1%. When m-xylene adsorbed onto sandy soil matrix was administered, the excretion of radioactivity in urine and exhaled air in female rats was significantly decreased compared to m-xylene during the first 12 hours (Turkall et al. 1992). In males given xylene in a sandy or clay soil matrix, the excretion of radioactivity was increased in exhaled air during the first 12 hours. In a study of aromatic hydroxylation products of xylene metabolism, rats administered 100 mg/kg doses of xylene isomers eliminated 1% or less of the administered dose as xylenols in the urine, along with trace amounts of methylbenzyl alcohols for o- and m-xylene only (Bakke and Scheline 1970).

3.4.4.3. Dermal Exposure

The elimination of m-xylene has been studied in human subjects exposed dermally either to liquid or to vapor. The elimination of liquid m-xylene absorbed dermally in humans following a 15-minute exposure was through the exhaled breath and urine (Engstrom et al. 1977; Riihimaki and Pfaffli 1978). Elimination in the exhaled breath followed a two-phase elimination curve with a rapid half-life of 1 hour and a longer half-life of 10 hours. In human subjects who immersed both hands in m-xylene for 15 minutes, the rate of excretion of m-methylhippuric acid was approximately 50 μmol/hour at 2 hours and 2 μmol/hour 3 hours later (Riihimaki 1979b). In male subjects whose hand and forearm were exposed dermally to 6,770 ppm m-xylene vapor for up to 3 hours, elimination of xylene in the exhaled breath was detected within 10 minutes and continued up to several hours after the exposure ended (Kezic et al. 2004).

In rats receiving a dermal application of 15 mg of radiolabeled m-xylene per cm², elimination over 48 hours was primarily in unmetabolized expired air (61.9% of the initial dose), with 42.7% excreted as metabolites in the urine and 0.1% in feces (Skowronski et al. 1990). The majority of the excretion in expired air occurred within the first 12 hours, with excretion in the urine occurring primarily during the first 24 hours; fecal excretion was detected the second day. If m-xylene was applied to the skin in the form of a sandy soil matrix, the excretion was similar to that seen with m-xylene alone, but if the m-xylene was applied adsorbed onto clay soil matrix, approximately equal amounts were excreted in exhaled air and in the urine (46 and 53%, respectively).

3.4.4.4. Other Routes of Exposure

Limited information was available on the elimination of xylene metabolites in rats following intraperitoneal injection (Ogata et al. 1979; Sugihara and Ogata 1978; van Doorn et al. 1980). The urinary metabolites of xylene are similar regardless of route of exposure; however, the amounts of the various metabolites differ. The elimination profile of xylene isomers is related more to absorption than it is with dose or duration of exposure. In rats, 49–62.6% of various doses of m-xylene or 64–75% of various doses of p-xylene were excreted in the urine as m-methylhippuric acid or p-methylhippuric acid, respectively (Sugihara and Ogata 1978). Urinary excretion of o-toluic acid glucuronide and o-methylhippuric acid accounted for 57 and 16%, respectively, of single intraperitoneal dose of 1,240 mg o-xylene/kg give to rats (Ogata et al. 1979). The amount of o-toluic acid glucuronide and o-methylhippuric acid excreted reached a maximum 8–24 hours after dosing. Mercapturic acid derivatives were present in the urine of rats following an intraperitoneal dose of m-, o-, or p-xylene (Tanaka et al. 1990; van Doorn et al. 1980). The percentages ranged from 0.6% (p-xylene) to 10–29% (o-xylene).