PHYSICAL AND CHEMICAL PROPERTIES

Hydrogen sulfide is a colorless flammable gas with a characteristic odor of rotten eggs (Budavari et al. 1989; Weil et al. 2006). Studies indicate that the odor threshold varies. Ruth (1986) reported an odor threshold of 0.5-10 ppb; others have reported much higher values (ATSDR 2006). The irritating concentration is reported as 10 ppm (Ruth 1986), and olfactory fatigue and nerve paralysis are reported at 100 and 150 ppm, respectively. Selected physical and chemical properties are shown in Table 5-1.

TABLE 5-1

Physical and Chemical Properties of Hydrogen Sulfide.

OCCURRENCE AND USE

Hydrogen sulfide is a component of the natural sulfur cycle and is produced endogenously in mammals (Weil et al. 2006). It arises from bacterial reduction of sulfates and decomposition of proteins. Hydrogen sulfide is a byproduct of many industrial processes and is used to make inorganic sulfides that are used to make such products as dyes, pesticides, polymers, and pharmaceuticals (Weil et al. 2006).

The ambient air concentration resulting from natural sources is estimated as 0.11-0.33 ppb (ATSDR 2006). Hydrogen sulfide has been measured in the submarine atmosphere. Data collected on three nuclear-powered attack submarines indicate a range of 2-22 ppb (Hagar 2008). Whether the reported concentrations are representative of the submarine fleet is not known; few details were provided about the conditions on the submarines when the samples were taken. No other exposure data were located. Hydrogen sulfide emissions are thought to arise from the sanitary system (Hagar 2008).

SUMMARY OF TOXICITY

Occupational exposure to hydrogen sulfide occurs in the agricultural, gas, oil-refining, and other industries, and workers often notice the characteristic rotten-egg odor associated with exposure. Most people readily perceive hydrogen sulfide because the olfactory detection limit for it is so low (for example, 0.3 ppb or greater as reported by Hoshika et al. 1993). The toxic effects of hydrogen sulfide are characteristically dose-related and most notably involve the nervous, cardiovascular, and respiratory systems (ATSDR 2006). People exposed at low concentrations of hydrogen sulfide and other sulfur gases often report head aches, nausea, and other symptoms (Glass 1990; Shusterman 1992). Hydrogen sulfide is a contact irritant, causing inflammatory and irritant effects on the moist membranes of the eyes and respiratory tract; respiratory tract inflammation can result with exposure at about 50 ppm. Eye irritation is the most common complaint associated with single or repeated hydrogen sulfide exposure. There is agreement in the literature that effects on the eye predominate at concentrations above 50 ppm (Beauchamp et al. 1984; ACGIH 2005). Adverse effects on the human eye after exposure at 20 ppm or less are often associated with concomitant exposure to other chemicals or irritants that would reduce the threshold of corneal irritation (ACGIH 2005).

People acutely exposed to hydrogen sulfide at about 100 ppm commonly experience lacrimation, photophobia, corneal opacity, tachypnea, dyspnea, tracheobronchitis, nausea, vomiting, diarrhea, and cardiac arrhythmias (ATSDR 2006). Those changes generally resolve on evacuation to fresh air. However, people recovering from hydrogen sulfide exposure can have cough and a variety of effects on the sense of smell, including diminished function (hyposmia), altered sensation (dysosmia), and false odor recognition (phantosmia), for a few days to weeks. In humans, inhalation of hydrogen sulfide at as low as 100-250 ppm for only a few minutes can result in incoordination, memory and motor dysfunction, and anosmia (so-called olfactory paralysis). Symptoms become more severe with longer exposure and sometimes lead to pulmonary edema. Exposure at much higher concentrations (about 500 ppm) may result in coma, which is often rapidly reversed when the victim is evacuated, or persistent headaches, equilibrium loss, and memory loss.

Laboratory studies generally indicate that toxic effects observed in animals exposed to hydrogen sulfide at high concentrations are identical with those observed in humans who exhibit acute toxicity of the gas. Compilations of the concentrations of hydrogen sulfide that produce serious systemic toxic effects and death in humans and laboratory animals indicate that species differences in toxicity are not significant (ATSDR 2006). Concentrations that produce effects in the respiratory, cardiovascular, and nervous systems are similar in rats and mice and humans, varying by a factor of less than 10. The mechanism of action to produce serious effects appears to be the same in all species because cellular respiration and energy production controlled by cytochrome oxidase in the mitochondria are inhibited by hydrogen sulfide in all species tested. That mechanism is consistent with effects observed in multiple organ systems in laboratory animals given acute, high exposures. Laboratory investigations have identified olfactory lesions in the nasal cavity of the rat after both acute high-concentration and subchronic low-concentration exposures to hydrogen sulfide.

As a direct-acting metabolic poison, hydrogen sulfide most profoundly affects organs that are critically dependent on oxidative metabolism, such as the brain and heart (ATSDR 2006). Like cyanide, hydrogen sulfide blocks the respiratory chain primarily by inhibiting cytochrome c oxidase.

Available data on animals suggest that hydrogen sulfide does not cause significant effects on fertility (ATSDR 2006). No studies of the carcinogenic potential of hydrogen sulfide were available for review. There are few studies on the genotoxic potential of hydrogen sulfide, but test results show no indication of toxicity.

Effects in Humans

Accidental Exposures

Hydrogen sulfide is associated with fatal exposures in the workplace. According to U.S. Occupational Safety and Health Administration records, there were 80 fatalities in 57 hydrogen sulfide incidents from 1984 to 1994 (Fuller and Suruda 2000). Exposure concentrations and durations in those accidents were generally poorly defined.

Most fatalities occur in confined spaces (such as sewers, animal processing plants, and manure tanks) and result from respiratory failure, noncardiogenic pulmonary edema, coma, and cyanosis (Adelson and Sunshine 1966, cited in ATSDR 2006; Winek et al. 1968, cited in ATSDR 2006; Arnold et al. 1985). Pulmonary edema is not uniformly seen in fatal hydrogen sulfide poisoning cases, and recovery from coma can be relatively rapid on evacuation of the person to fresh air and application of artificial respiration and oxygen. Some people exposed at about 1,000 ppm develop vagal-mediated apnea and hydrogen sulfide-induced central respiratory arrest (Almeida and Guidotti 1999), and increased blood sulfide concentrations are occasionally detected after exposure at high concentrations.

Many people exposed to hydrogen sulfide at 500 ppm or higher become unconscious rapidly and then appear to recover. This syndrome is often referred to as knockdown and may result in long-term neurologic deficits, including in-coordination, memory and motor dysfunction, personality changes, hallucinations, and anosmia. The clinical effects are consistent with organic brain disease resulting from hypoxia and may occasionally persist for several years after the initial hydrogen sulfide exposure (Arnold et al. 1985; Tvedt et al. 1991a; Tvedt et al. 1991b, cited in NRC 2002 and ATSDR 2006; Reiffenstein et al. 1992; Kilburn and Warshaw 1995). Vapor concentrations of about 500 ppm or higher are often fatal within minutes (Reiffenstein et al. 1992).

Signs and symptoms observed after acute exposure at 100-500 ppm include ocular and respiratory tract irritation, nausea, vomiting, diarrhea, headaches, loss of equilibrium, memory loss, olfactory paralysis, loss of consciousness, tremors, and convulsions (ATSDR 2006). Ocular effects include tearing, burning, and irritation of the cornea and conjunctivae (Lambert et al. 2006). The symptoms generally resolve without intervention after cessation of exposure (ATSDR 2006). The case reports below illustrate the effects of accidental exposure to hydrogen sulfide.

While drilling a pit to lay the foundation for a municipal sewage pumping station, 37 workers (24-50 years old) were accidentally exposed to hydrogen sulfide at an undetermined concentration (Snyder et al. 1995). Signs and symptoms included headache, dizziness, breathlessness, cough, burning and discomfort in the chest, throat and eye irritation, nausea, and vomiting. Most of the workers recovered uneventfully, but one worker died and another remained in a coma for 5 days. The comatose patient was aggressively treated with hyperbaric oxygen. He was discharged from the hospital on day 16 with slow speech, impaired attention span, easy distractibility, isolated retrograde amnesia, decreased ability to communicate, impaired visual memory, and poor acquisition, retention, and recall of new information. His condition was unchanged at 12 and 18 months after exposure. In another case report, six patients were examined 5-10 years after accidental exposures to hydrogen sulfide at an unknown concentration (Tvedt et al. 1991a; Tvedt et al. 1991b, cited in ATSDR 2006). The patients had been unconscious for 5-20 min in the hydrogen sulfide atmospheres. Despite rapid evacuation, neurologic symptoms persisted, including impaired vision, memory loss, decreased motor function, tremors, ataxia, abnormal learning and retention, and slight cerebral cortical atrophy. One patient was severely demented on long-term follow-up.

Numerous other reports of permanent or persistent neurologic effects of exposure to hydrogen sulfide have been published (Wasch et al. 1989; Kilburn 1993, cited in ATSDR 2006; Kilburn and Warshaw 1995; Kilburn 1997). The reports imply that exposure to hydrogen sulfide at relatively high concentrations can cause severe health effects, but as with most case studies, there is a lack of definitive exposure data.

Experimental Studies

Bhambhani and co-workers (Bhambhani and Singh 1991; Bhambhani et al. 1994, 1996a,b, 1997) performed a series of experiments to examine the dose-related effects of low-concentration exposures to hydrogen sulfide in exercising volunteers. Exercise increased the exposure to hydrogen sulfide by raising the respiratory rate, thereby approximating the exposure situation of an exercising worker. In the first series of experiments, 16 healthy male volunteers undertook increasing increments of bicycle exercise while inhaling hydrogen sulfide at 0 (control), 0.5, 2.0, or 5.0 ppm on separate occasions; multiple physiologic measurements were made (Bhambhani and Singh 1991). The subjects inhaled the hydrogen sulfide vapor through their mouths while their noses were plugged with an external clip, so the study does not provide useful information concerning the ocular or nasal toxicity of hydrogen sulfide. The results indicated that there were no significant changes in the cardiorespiratory and metabolic process at any exposure concentration and at any exercise level. Heart rate and expired ventilation rate were unaffected. Oxygen uptake increased slightly, carbon dioxide output decreased slightly, and during exposure to hydrogen sulfide at 5.0 ppm there was a significant increase in blood lactate. The results suggested that anaerobic metabolism is increased by the presence of the sulfide, but whether that is due to inhibition of cytochrome oxidase cannot be determined from the results.

In a second series of experiments, 13 male and 12 female healthy volunteers were exposed to hydrogen sulfide at 0 ppm (control) and 5.0 ppm while exercising for 30 min (Bhambhani et al. 1994, 1996a). The 5.0-ppm exposure did not change arterial blood gases, hemoglobin oxygen saturation, or cardiovascular and metabolic responses. Citrate synthetase, a marker of aerobic metabolism, was the only enzyme that showed a statistically significant decrease in activity. Lactate dehydrogenase and cytochrome oxidase were not altered in muscle tissue taken by biopsy. The results confirmed those from the previous study except that lactate concentrations in blood were slightly higher, but not statistically significantly different, in the 5.0-ppm group. Thus, healthy exercising men and women showed little response to hydrogen sulfide at 5 ppm.

Another study showed that healthy people (nine men and 10 women) exposed to hydrogen sulfide at 10 ppm for 15 min while exercising showed no changes from control values in a series of respiratory measurements (Bhambhani et al. 1996b). A final study in this series (Bhambhani et al. 1997) used 28 healthy volunteers (15 men and 13 women) exposed to hydrogen sulfide at 0 ppm (control) or 10 ppm while exercising for 30 min. Increased muscle lactate in men and women, decreased muscle cytochrome oxidase in men, and increased muscle cytochrome oxidase in women were found, although most changes were not statistically significant.

The controlled studies presented above were limited in a number of ways. The exercising volunteers were protected from exposure to their noses and eyes, so they could not smell the gas and were not subject to eye irritation, both sensitive outcomes of hydrogen sulfide exposure. In addition, the volunteers were not previously exposed to hydrogen sulfide at the concentrations used in the experiments.

Jappinen et al. (1990) exposed a group of 10 mildly asthmatic subjects (three men, a mean of 40.7 years old, and seven women, a mean of 44.1 years old) to hydrogen sulfide at 2 ppm for 30 min in a closed chamber. All subjects experienced unpleasant odor at the start of the exposure but rapidly became accustomed to it. Three of the 10 subjects experienced headache after exposure was completed. There were no changes in forced vital capacity (FVC), forced expiratory volume in 1 s (FEV₁), and forced expiratory flow (FEF) as a result of exposure. Calculated airway resistance was slightly decreased in two and increased in eight subjects with no significant change in mean values. Specific airway conductance was decreased in six and increased in four subjects. There was over a 30% change in airway resistance and specific airway conductance in two subjects, indicating bronchial obstruction. No adverse symptoms were noted.

Fiedler and colleagues (2008) exposed 74 healthy subjects (35 women and 39 men, a mean of 24.7 ± 4.2 years old) to hydrogen sulfide at 0.05, 0.5, or 5 ppm. On the basis of self-reporting, 47% (35) of the subjects were Asian, 35% (26) Caucasian, 8% (six) Hispanic, 7% (five) black, and 3% (two) other. Criteria that resulted in exclusion of subjects included neurologic disease or brain injury, substantial current or previous exposure to other neurotoxicants, stroke or cardiovascular disease, serious pulmonary disease (such as asthma), hepatic or renal disease, serious gastrointestinal disorders (such as colitis), major psychiatric conditions, pregnancy, lactation, or use of specified medications (such as anxiolytics, antidepressants, and beta blockers). Selected subjects completed a medical history and physical examination that included spirometry, electrocardiography, blood counts and routine chemistry measures, and visual-acuity testing. For each exposure session, subjects completed ratings and tests before exposure (baseline) and during the final hour of the 2-h exposure period. Subjects used analogue scales to rate pleasantness, intensity, and irritation of the hydrogen sulfide odor and to evaluate environmental qualities. Symptoms were rated on a ratio scale from 0 (barely detectable or no sensation) to 100 (strongest imaginable). Other tests included evaluation of postural sway, visual-acuity and visual-contrast sensitivity, simple reaction time and continuous-performance test, the finger-tapping test, the symbol-digit substitution test, and the auditory verbal learning test. Increased concentrations of hydrogen sulfide resulted in increased ratings of odor intensity, irritation, and unpleasantness. Total symptom severity, including eye irritation, was not significantly increased in any exposure condition, but anxiety symptoms were significantly greater in people exposed to hydrogen sulfide at 5 ppm than in people exposed at 0.05 ppm. “No dose-response effect was observed for sensory or cognitive measures. Verbal learning was compromised during each exposure condition.” The authors concluded that “although some symptoms increased with exposure, the magnitude of the changes was relatively minor. Increased anxiety was significantly related to ratings of irritation due to odor. Whether the effect on verbal learning represents a threshold effect of hydrogen sulfide or an effect due to fatigue could not be determined.”

Occupational and Epidemiologic Studies

A number of community and occupational studies have examined the effects of exposure to hydrogen sulfide. Because hydrogen sulfide has an objectionable odor that is apparent at concentrations below 10 ppb, it is readily recognized in ambient air and is often considered a “nuisance odor.” Thus, low concentrations of hydrogen sulfide can cause concern, particularly from the perspective of community exposure. For example, in the early 1980s, community concerns in Alberta, Canada, related to exposure to natural gas contaminated with hydrogen sulfide (that is, sour gas) resulted in a large cross-sectional survey (Dales et al. 1989; Spitzer et al. 1989) to determine whether there was a higher incidence of health effects in a community living close to a sour-gas facility than in an unexposed community. The results of the investigation were generally unremarkable, although some possible effects on the respiratory system were reported (Dales et al. 1989). No air measurements were taken, so no direct correlations between health effects and air concentrations of hydrogen sulfide were possible (Spitzer et al. 1989).

Data reported from studies of communities experiencing low exposures to hydrogen sulfide and other contaminants from nearby industrial or natural sources could provide an indication of a threshold for adverse health effects. The study results are equivocal because of the mixture of pollutants in the air, which usually includes organic sulfides, sulfur dioxide, and particles. In two communities in Finland that were exposed to pulp-mill emissions, self-reported respiratory symptoms (wheezing and shortness of breath) and eye irritation or conjunctivitis were more frequent than in a community that was not exposed (Jaakkola et al. 1990). Children in the exposed communities did not appear to be more sensitive to the effects of exposure than adults (Marttila et al. 1994). Information obtained from questionnaires in a later study in which exposure concentrations were measured indicated that average 24-h exposures to hydrogen sulfide at less than 0.007 ppm (range, 0-0.059 ppm) did not have significant respiratory or ocular effects, but respiratory symptoms were observed above 0.007 ppm (Marttila et al. 1995). Those studies cannot be given great weight in identifying a threshold exposure for hydrogen sulfide toxicity because, as previously mentioned, other air pollutants undoubtedly contributed to the reported effects. At best, the information suggests that hydrogen sulfide may contribute to respiratory symptoms when mixed with other air pollutants originating in pulp production.

Viscose-rayon workers (123 men) repeatedly exposed to hydrogen sulfide or carbon disulfide participated in a health survey that included questions on eye effects (Vanhoorne et al. 1995). Their responses to a questionnaire were compared with those of a reference group of 67 workers who were not exposed to either chemical. Workplace hydrogen sulfide exposure was determined by personal sampling devices and ranged from 0.14 to 6.4 ppm. The frequency of exposure is unknown, but participating workers had been employed for at least a year. Workers exposed to hydrogen sulfide at 3.6 ppm or less did not have significantly more ocular complaints than the unexposed workers. Workers exposed to hydrogen sulfide at 6.4 ppm did have significantly more eye complaints, including pain, burning, irritation, hazy sight, and photophobia.

Jappinen et al. (1990) studied a cohort of 26 male pulp-mill workers (mean age, 40.3 years; range, 22-60 years) to assess the possible effects of daily hydrogen sulfide exposure on respiratory function. Most of the exposures were at 2-7 ppm, with a range of 1-11 ppm. Bronchial responsiveness, FVC, and FEV₁ were measured at the end of a work day that followed at least 1 day off from work. No significant changes in respiratory function or bronchial responsiveness were observed after hydrogen sulfide exposure compared with control values.

In another study, 21 swine confinement-facility workers were tested with spirometry immediately before and after a 4-h work period (Donham et al. 1984). They had statistically significant (p < 0.05) reductions in pulmonary flow rates: a mean FEF of 3.3-11.9% after the 4-h work period. The work environment was sampled for particles and gases during the exposure period, and there was suggestive evidence of a concentration-response relationship between carbon dioxide and hydrogen sulfide exposure concentrations and lung-function decrements. However, the monitoring data were not presented in the study report.

Barthelemy (1939), using work-area sampling and wet-chemistry analytic methods, reported hydrogen sulfide concentrations of about 30 ppm in the presence of carbon disulfide and acid particles in a rayon-production facility. It was reported that coexposure to carbon disulfide and acid particles reduced the threshold for hydrogen sulfide-induced ocular irritation. Exposure to hydrogen sulfide at about 30 ppm under the mixed exposure conditions caused “quite a number” of workers to develop severe eye irritation sufficient to prevent their ability to work in the facility. Six years of monthly air-monitoring data and worker-complaint records showed that the number of reported conjunctivitis cases increased from zero in December 1932 to 332 by December 1933 (when the plant’s ventilation system failed). Ocular effects included intense photophobia, eyelid spasm, excessive tearing, intense congestion, pain, blurred vision, and sluggish pupil reaction. Typical hydrogen sulfide concentrations at the facility when it was operating without eye complaints were 9-18 ppm. Barthelemy concluded that maintaining hydrogen sulfide concentrations below 20 ppm (even in the presence of carbon disulfide at about 30 ppm) eliminated reports of eye irritation. Other investigators have reported that the incidence of eye irritation is “materially increased” at hydrogen sulfide concentrations of 20 ppm or slightly higher (Carson 1963). A hydrogen sulfide concentration of 20 ppm appears to be a threshold for obvious effects on the eye.

Table 5-2 summarizes the common clinical signs observed in experimental and occupational studies.

TABLE 5-2

Hydrogen Sulfide-Induced Effects Observed in People.

Effects in Animals

Acute Toxicity

ATSDR (2006) reported similar findings on acute toxicity in rats, mice, and rabbits. Zwart et al. (1990) reported that the hydrogen sulfide concentration that was lethal to 50% (LC₅₀) of rats and mice was 684 and 677 ppm, respectively, after a 50-min exposure. MacEwen and Vernot (1972) reported LC₅₀s in rats and mice of 712 and 634 ppm after a 1-h exposure. The similarity in response among mammalian species is typical for contact irritants.

Sprague-Dawley, Long Evans, and Fischer-344 rats were exposed to hydrogen sulfide in whole-body exposure chambers for 2, 4, and 6 h (72 males and 84 females at 2-h and 6-h intervals, 72 of each sex at 4 h) (Prior et al. 1988). The resulting LC₅₀s for 2-, 4-, and 6-h exposures were 587, 501, and 335 ppm, respectively. Six exposure concentrations were used in the 6-h study, and eight each in the 2- and 4-h studies. All deaths were attributed to severe pulmonary edema. Groups of five male and five female Wistar rats were exposed to hydrogen sulfide at various concentrations for 10, 30, or 50 min (Arts et al. 1989, cited in NRC 2002; Zwart et al. 1990). The 10-min LC₅₀ was 835 ppm, and the 30- and 50-min LC₅₀s were 727 and 684 ppm, respectively (see Table 5-3)

TABLE 5-3

Summary of Rat and Mouse LC₅₀s.

Brenneman et al. (2002) exposed groups of 10-week-old Sprague-Dawley rats (five rats per concentration per exposure time) to hydrogen sulfide at 0, 30, 80, 200, or 400 ppm 3 h/day for 1 or 5 consecutive days. A concentration of 30 ppm for a single day or 5 days was the no-observed-adverse-effect level (NOAEL) for nasal lesions. The most prominent nasal lesion was olfactory mucosal necrosis. Repair of that lesion was complete at 6 weeks after exposure (Brenneman et al. 2002). The primary differences observed in effects between exposure durations of 1 and 5 days were that animals exposed for 5 days had a higher lesion incidence (100%) and more extensive injury of the nasal cavity. Brenneman and co-workers also reported transient metaplasia in the nasal respiratory mucosa after exposure at 80 ppm or higher. That lesion is not seen after subchronic hydrogen sulfide exposure, and this finding suggests that the regenerated respiratory epithelium becomes resistant to further injury. No pulmonary lesions were observed even at the highest concentration tested. The lowest observed-adverse-effect level (LOAEL) in the study was 80 ppm for the reversible olfactory mucosa lesion.

Lopez et al. (1989) showed that the gross and histologic evidence of pulmonary edema caused by 5-min exposure to hydrogen sulfide at a lethal concentration (1,662 ppm) was a direct effect because intraperitoneal injections of 30 mg/kg did not affect the airways or lungs. Brenneman et al. (2000a) exposed 10-week-old male CD rats that had undergone unilateral nasal occlusion to hydrogen sulfide at 400 ppm for 3 h. A day after the exposure, rats developed olfactory neuronal loss on the side of the open nostril. That lesion was absent on the side that had the occluded nostril; this suggests that the observed injury was the result of a direct effect rather than of systemic delivery of the gas.

Lopez et al. (1988) exposed Fischer-344 rats to hydrogen sulfide at 0, 10, 200, or 400 ppm for 4 h. Animals were killed 1, 18, or 44 h after exposure. The authors found olfactory mucosal necrosis in the rats exposed at 400 ppm. Olfactory lesions were not seen after exposure at 10 or 200 ppm. The authors also reported lesions in the respiratory mucosa after exposure at 400 ppm; the lesions consisted of necrosis with ulceration 1 h after exposure and early regeneration with inflammation 18 h after exposure.

Numerous studies suggest that hydrogen sulfide can affect brain neurochemistry, physiology, and behavior in rodents. Several studies are repeated-exposure studies and are included here for completeness. Higuchi and Fukamachi (1977) evaluated conditioned-avoidance responses in well-trained Wistar rats during a 1-h inhalation exposure to hydrogen sulfide at 100-500 ppm. Rats exposed at greater than 200 ppm displayed inhibited discriminated avoidance responses immediately after the exposure. Avoidance responses returned to normal within 1 or 24 h after exposure at 200 or 500 ppm, respectively. Similarly, moderate exposure to hydrogen sulfide (125 ppm) can interfere with the ability of rats to learn a baited radial-arm maze task (Partlo et al. 2001).

Struve et al. (2001) exposed male Sprague-Dawley rats to hydrogen sulfide 3 h/day for 5 days. Groups of 10 rats each received whole-body exposure at 0, 30, or 80 ppm. Groups of 20 rats each received nose-only exposure at 0, 30, 80, 200, or 400 ppm. Motor activity and water-maze (spatial-learning) performance were evaluated. No treatment-related effects were observed in whole-body exposure groups. One of 20 rats died after exposure at 400 ppm. Hydrogen sulfide exposure produced statistically significant changes in the water-maze test (NOAEL, 200 ppm; LOAEL, 400 ppm) and motor activity (NOAEL, 30 ppm; LOAEL, 80 ppm) in the nose-only exposure groups.

Neonatal rats exposed to hydrogen sulfide at 75 ppm 7 h/day, beginning on postnatal day (PND) 5 and ending on PND 21, develop increased cerebellar serotonin and norepinephrine concentrations (Hannah and Roth 1991; Skrajny et al. 1992). Hannah and Roth (1991) exposed Sprague-Dawley rats to hydrogen sulfide at 0, 20, or 50 ppm 7 h/day from gestation day (GD) 5 through PND 21. The mean Purkinje cell terminal path length was significantly higher in animals exposed at 20 and 50 ppm than in controls; however, the biologic significance of this finding is unclear because no concentration-response relationship was observed. In a similar experiment, Hannah et al. (1989) exposed pregnant rats from GD 5 through PND 21 to hydrogen sulfide at 75 ppm 7 h/day. Brain concentrations of aspartate, γ-aminobutyric acid, glutamate, and taurine were significantly decreased, but no follow-up studies were conducted to determine whether the changes affected behavioral or structural development. In a repeated-exposure study, groups of five male Sprague-Dawley rats were exposed to hydrogen sulfide at 0 ppm (nitrogen air mixture) or 25, 50, 75, or 100 ppm 3 h/day for 5 days (Skrajny et al. 1996). The rats had hippocampal electrodes implanted in the dentate gyrus or CA1 region to determine the effects of hydrogen sulfide on electroencephalographic (EEG) activity in the hippocampus and neocortex. Exposure to hydrogen sulfide at 100 ppm resulted in increased hippocampal theta activity but did not change the basic behavior-EEG correlation. Total hippocampal theta activity increased in a cumulative manner in both the dentate gyrus and CA1 regions during exposure at 25 ppm or higher. The increase was significant (p < 0.05) after exposure on days 3, 4, and 5 and did not return to control values during the 24-h period between exposures. Complete recovery of the animals exposed at 100 ppm took about 2 weeks.

Li and co-workers (2008) exposed anesthetized, paralyzed, and mechanically ventilated piglets to hydrogen sulfide at 20, 40, 60, and 80 ppm over 6 h (exposure at each concentration for 1.5 h). A control group of four piglets were exposed to air for 6 h. Blood lactate concentrations did not change significantly in exposed piglets; this suggested a lack of a metabolic effect on aerobic respiration. The findings are inconsistent with the increased blood lactate concentrations reported in humans exposed to hydrogen sulfide at 5 ppm for up to 16 min (Bhambhani and Singh 1991).

Repeated Exposures and Subchronic Toxicity

Curtis et al. (1975) exposed three immature pigs continuously (24 h/day) to hydrogen sulfide at 0 or 8.5 ppm for 17 days. Pigs were subjected to a complete gross examination at necropsy and histologic examination of tissues from the respiratory tract, eyes, and viscera. The pigs weighed an average of 13.2 kg at the beginning of the study and gained an average of 0.53 kg/day. The results indicate that the exposure concentration was a NOAEL.

Brenneman et al. (2000b) exposed groups of 12 male Sprague-Dawley rats to hydrogen sulfide at 0, 10, 30, or 80 ppm 6 h/day, 7 days/week for 10 weeks. Multifocal, bilaterally symmetric olfactory neuron loss and basal cell hyperplasia, limited to the olfactory mucosa, were observed in rats exposed at 30 ppm or higher. Lesions were observed in the dorsal medial meatus and dorsal and medial areas of the ethmoid recess. Exposure to hydrogen sulfide at 80 ppm induced more severe and more frequent olfactory mucosal injury than exposure at 30 ppm. No treatment-related effects were noted at 10 ppm, which was the NOAEL for lesions in the olfactory mucosa.

In a subchronic study, groups of 10 male and 10 female Fischer-344 rats, B6C3F1 mice, and Sprague-Dawley rats were exposed to hydrogen sulfide at 0, 10, 30, or 80 ppm 6 h/day, 5 days/week for 90 days (Dorman et al. 2004). Exposure at 80 ppm was associated with reduced feed consumption for the first week in rats and throughout the study in mice. Male Fischer-344 rats, female Sprague-Dawley rats, and female B6C3F1 mice exposed at 80 ppm had lower terminal body weights and lower body weight gain than air-exposed controls. No treatment-related gross pathologic, hematologic, or serum-chemistry effects were observed. Rhinitis (100% incidence) was observed in mice exposed at 80 ppm. A concentration-related increase in incidence of olfactory neuronal loss occurred at 30 ppm or higher in both sexes of the mice and the Fischer-344 rats; in the Sprague-Dawley rats, lesions were observed only at 80 ppm. Bronchiolar epithe lial hypertrophy and hyperplasia were evident in male and female Sprague-Dawley rats at 30 ppm or higher and in male Fischer-344 rats at 80 ppm. Overall, a NOAEL of 10 ppm was demonstrated for the bronchiolar lesions.

Partlo et al. (2001) exposed groups of 16-24 Sprague-Dawley rats repeatedly to hydrogen sulfide at 125 ppm 4 h/day, 5 days/week for 5 or 11 weeks. A 16-arm maze was used to evaluate learning and memory. Exposure to hydrogen sulfide for 5 weeks had no effect on a previously learned task nor did exposure affect acquisition of a new task in an 11-week training session that coincided with daily exposure. However, the exposed rats’ ability to find all the reinforcements before the end of each trial period in the 11 weeks was impaired, and this suggested an effect on performance rate. When the learning task was changed by reversing the locations of reinforcement, hydrogen sulfide exposure had a detrimental effect on the rats’ ability to learn the new complex task; they required more arm entries than the controls to locate the reinforcements.

Chronic Toxicity

No chronic experimental studies of animals exposed to hydrogen sulfide were found.

TOXICOKINETIC AND MECHANISTIC CONSIDERATIONS

Sulfide is present as an endogenous substance in normal mammalian tissues (Mitchell et al. 1993; Kage et al. 1998, cited in ATSDR 2006; Boehning and Snyder 2003). Normal tissues contain relatively high concentrations (in the parts per million range) of endogenous sulfide ion (HS‾). Hydrogen sulfide has been measured in rat brain at 1-2 ppm, and background concentrations in the lung range from 50 to 140 ppm (Dorman et al. 2002). Volatile sulfur compounds are produced by oral bacteria (Persson et al. 1990); hydrogen sulfide and methyl mercaptan are the main components found in mouth air. Humans with severe halitosis may have oral cavity hydrogen sulfide concentrations that exceed 10 ng/mL (0.7 ppm; Tsai et al. 2008). Endogenous sulfide also arises from bacterial activity in the lower bowel (NRC 1979). There is some evidence that hydrogen sulfide participates in normal nerve transmission (Kimura 2000). Hydrogen sulfide added at physiologic concentrations facilitates the induction of long-term potentiation in the hippocampus and is accompanied by increases in cAMP, which indirectly activates postsynaptic N-methyl-D-aspartate receptors for glutamate (Boehning and Snyder 2003). Thus, like nitric oxide and carbon monox ide, hydrogen sulfide has been identified as a putative gaseous biologic molecule and neurotransmitter (Szabó 2007). Functioning as a metabolite or as a nerve transmitter, it has been shown to mediate an array of biologic effects, including cytotoxic effects to cytoprotective actions.

Hydrogen sulfide is formed from cysteine by cystathionine β-synthetase and cystathionine γ-lyase. The production of sulfide is closely associated with the catabolism of cysteine and methionine and with gluthathione metabolism (Wang 2002; Fiorucci et al. 2006). In tissue homogenates, sulfide is produced at 1-10 pmol/s-mg of protein (Doeller et al. 2005). That results in low micromolar extracellular concentrations of sulfide.

The major metabolic and excretory pathway of hydrogen sulfide involves oxidation to sulfate (Beauchamp et al. 1984). The exact mechanism of the oxidation is unknown, but both enzymatic (sulfide oxidase) and nonenzymatic catalytic systems have been proposed. Glutathione stimulates mitochondrial oxidation of thiosulfate to sulfite in vitro, and a sulfite intermediate may be converted to sulfate by sulfite oxidase. After oxidation, hydrogen sulfide is excreted as free sulfate or as a conjugated sulfate in the urine (Beauchamp et al. 1984).

Two other metabolic pathways for hydrogen sulfide have been identified: methylation of hydrogen sulfide to produce methanethiol and dimethylsulfide and reaction of hydrogen sulfide with metalloenzymes or disulfide-containing enzymes (Beauchamp et al. 1984). Data suggest that thiol S-methyltransferase catalyzes the methylation of hydrogen sulfide to yield less toxic methanethiol and dimethylsulfide (Beauchamp et al. 1984).

Hydrogen sulfide may also reduce disulfide bridges in proteins, and this reaction is probably responsible for hydrogen sulfide-induced inhibition of succinic dehydrogenase. Oxidized glutathione (but not reduced glutathione) is protective against hydrogen sulfide poisoning. The protection is probably due to the scavenging of hydrosulfide by the oxidized glutathione-disulfide linkage, which prevents the reaction of the sulfide with other enzymatic sites (Beauchamp et al. 1984).

Dorman et al. (2002) evaluated the relationship between the concentration of sulfide and cytochrome oxidase activity in target tissues after acute inhalation exposure to hydrogen sulfide at sublethal concentrations. Specifically, they exposed groups of six male CD rats to hydrogen sulfide at 0, 10, 30, 80, 200, or 400 ppm for 3 h and examined hindbrain, lung, liver, and nasal cytochrome oxidase activity and sulfide concentrations immediately after exposure. Lung sulfide and sulfide-metabolite concentrations were also analyzed at 0, 1.5, 3, 3.25, 3.5, 4, 5, and 7 h after the start of exposure to hydrogen sulfide at 400 ppm. Lung sulfide concentrations increased during exposure and returned to baseline values within 15 min after exposure at 400 ppm, and lung sulfide-metabolite concentrations were transiently increased immediately after the end of exposure. Decreased cytochrome oxidase activities were noted in the olfactory epithelium at 30 ppm or greater. Increased olfactory epithelial sulfide concentrations were noted after exposure at 400 ppm. No effects on hindbrain or nasal respiratory epithelial sulfide concentrations were noted. Hepatic sulfide concentrations were increased at 200 ppm or higher, and, surprisingly, hepatic cytochrome oxidase activity was increased in all treatment groups. The authors concluded that sulfide concentrations in the brain, lung, or nose are unlikely to increase after a single 3-h exposure to hydrogen sulfide at 30 ppm or lower.

Nasal extraction of hydrogen sulfide was measured in the isolated upper respiratory tracts of male Sprague-Dawley CD (Crl:CD[SD]BR) rats (Schroeter et al. 2006). Extraction was measured for constant unidirectional inspiratory flow at 75, 150, and 300 mL/min, which corresponded to 50, 100, and 200% of the predicted minute volume of the adult male CD rat. Nominal exposures to hydrogen sulfide at 10, 80, and 200 ppm were used. The concentration of hydrogen sulfide entering and leaving the upper respiratory tract was measured about every 8 min through the end of the 120-min exposure. Extraction was calculated as [(inlet concentration – exit concentration)/inlet concentration] and expressed as percent. Time-averaged extraction values were calculated as the average of the 15 samples for each animal. Time-averaged nasal extraction depended on the concentration of inspired hydrogen sulfide and the rate of airflow through the nasal cavity and ranged from 32% for a 10-ppm exposure at 75 mL/min to 7% for a 200-ppm exposure at 300 mL/min.

Hydrogen sulfide is a contact irritant and causes inflammatory and irritant effects on the moist membranes of the eyes and respiratory tract (Beauchamp et al. 1984). Eye irritation is the most common complaint associated with single or repeated exposure to hydrogen sulfide (Barthelemy 1939; Ahlborg 1951, cited in ATSDR 2006; Carson 1963).

The clinical picture resulting from acute lethal exposure to hydrogen sulfide (500-1,000 ppm) is almost identical with that of hydrogen cyanide poisoning. The symptoms are typically those of respiratory insufficiency accompanied by a period of hyperpnea followed by respiratory failure, noncardiogenic pulmonary edema, coma, and cyanosis (ATSDR 2006). In many cases, people lose consciousness after only one or two breaths of hydrogen sulfide (knockdown). A direct irritating effect on mucous membranes gives hydrogen sulfide a greater tendency than cyanide exposure to produce conjunctivitis and pulmonary edema.

Studies by Smith et al. (1977) and Khan et al. (1990) showed that under physiologic conditions, hydrogen sulfide acts to block the respiratory chain primarily by inhibiting cytochrome c oxidase and that the undissociated species (H₂S) is a more potent inhibitor than the anionic species (HS‾). Hydrogen sulfide blocks cytochrome c oxidase-dependent reduction of oxygen to water and thus impairs oxidative phosphorylation. Tissues with high oxygen demand (such as cardiac muscle and brain) are particularly sensitive to sulfide inhibition of electron transport, which is the same mechanism as has been shown for cyanide, and, as with cyanide poisoning, the presence of methemoglobin restores the activity of the cytochrome c oxidase enzyme system (ATSDR 2006). The enzyme blockage not only has a direct potent toxic effect but appears to cause indirectly hyperpnea through stimulation of the carotid and aortic body chemosensors by blocking the availability of oxygen (Ammann 1986).

Animal studies confirm that the olfactory system is especially sensitive to hydrogen sulfide inhalation. Acute, repeated exposure of rats to hydrogen sulfide at moderately high concentrations (80 ppm or higher) resulted in nasal lesions characterized by respiratory epithelial metaplasia and full-thickness necrosis of the olfactory mucosa (Brenneman et al. 2002). The mechanism by which hydrogen sulfide inhalation damages the nasal epithelium and results in adverse clinical signs is poorly understood. Direct inhibition of cellular enzymes is one mechanism of hydrogen sulfide toxicity (Beauchamp et al. 1984). As mentioned earlier, hydrogen sulfide-induced inhibition of cytochrome oxidase is believed to disrupt the electron-transport chain and impair oxidative metabolism. Although nasal cytochrome oxidase is a sensitive marker of hydrogen sulfide exposure, there is an incomplete correlation between hydrogen sulfide-induced nasal lesions and cytochrome oxidase inhibition (Dorman et al. 2002).

An alternative mechanism by which hydrogen sulfide inhalation could cause nasal lesions is dissociation of hydrogen sulfide that results in the release of free protons, which could alter intracellular pH and cause cytotoxicity. Roberts et al. (2006) treated nasal respiratory and olfactory epithelial cell isolates and explants from naïve rats with the pH-sensitive intracellular chromophore SNARF-1 and exposed them to air or hydrogen sulfide at 10, 80, 200, or 400 ppm for 90 min. “Intracellular pH was measured with flow cytometry or confocal microscopy…A modest but statistically significant decrease in intracellular pH occurred after exposure of respiratory and olfactory epithelium to hydrogen sulfide at 400 ppm.” However, decreased cytochrome oxidase activity was observed after exposure at over 10 ppm, so changes in intracellular pH might play a secondary role in hydrogen sulfide-induced nasal injury.

Injury to and regeneration of the nasal respiratory mucosa occurred in animals exposed to hydrogen sulfide 6 h/day for 7 days/week, and this suggests that the regenerated respiratory epithelium becomes resistant to further injury (Brenneman et al. 2002). To understand that response, Roberts et al. (2008) exposed 10-week-old male Sprague-Dawley rats nose-only to air or hydrogen sulfide at 200 ppm 3 h/day for 1 day or 5 consecutive days. “Nasal respiratory epithelial cells at the site of injury and regeneration were laser-capture microdissected, and gene-expression profiles were generated at 3, 6, and 24 h after the initial 3-h exposure and 24 h after the fifth exposure with the Affymetrix Rat Genome 230 2.0 microarray. Gene-ontology enrichment analysis showed that exposure to hydrogen sulfide altered gene expression associated with a variety of biologic processes, including cell-cycle regulation, protein kinase regulation, and cytoskeletal organization and biogenesis.”

Moulin et al. (2002) showed that the predicted regional flux of hydrogen sulfide correlates with the distribution of nasal olfactory lesions in the rat. In follow-up studies, Schroeter et al. (2006) used the olfactory-lesion incidence data from Brenneman et al. (2000b) to show a good correlation with hydrogen sulfide nasal-tissue dose predictions of an anatomically accurate computational fluid-dynamics model. An anatomic human model was used to estimate human nasal-tissue doses. The maximum 99th percentile flux value in the human model was used to estimate a human NOAEL of 5 ppm with the EPA (1994) inhalation reference concentration method.

INHALATION EXPOSURE LEVELS FROM THE NATIONAL RESEARCH COUNCIL AND OTHER ORGANIZATIONS

A number of organizations have established or proposed acceptable exposure limits or guidelines for inhaled hydrogen sulfide. Table 5-4 summarizes selected values.

TABLE 5-4

Selected Inhalation Exposure Levels for Hydrogen Sulfide from National Research Council and Other Agencies.

COMMITTEE RECOMMENDATIONS

The committee’s recommendations for EEGL and CEGL values for hydrogen sulfide are summarized in Table 5-5. The current U.S. Navy values are provided for comparison.

TABLE 5-5

Emergency and Continuous Exposure Guidance Levels for Hydrogen Sulfide.

DATA ADEQUACY AND RESEARCH NEEDS

Although there is an extensive literature on the health effects of hydrogen sulfide, questions remain about its possible effects and about concentrations at which effects may occur. Those questions apply particularly in connection with the end points of neurologic, respiratory, behavioral and developmental effects. At airborne concentrations above 20 ppm, the direct irritation effects of hydrogen sulfide are increasingly apparent. Acute respiratory effects are widely assumed to occur after even brief exposures at concentrations above 200 ppm, and acute neurotoxic responses occur at concentrations above 500 ppm. Questions now exist as to whether longer-term neurotoxic and respiratory or pulmonary deficits may occur after short-term high-concentration exposures. There are also concerns about human health effects in the low-exposure region, especially after chronic low-concentration exposure.

REFERENCES

• ACGIH (American Conference of Governmental Industrial Hygienists, Inc). 2005. Documentation of the Threshold Limit Values and Biological Exposure Indices; Supplement to the 7th Edition. American Conference of Governmental Hygienists, Cincinnati, OH.
• Adelson, L., and I. Sunshine. 1966. Fatal hydrogen sulfide intoxication. Report of three cases occurring in a sewer. Arch. Pathol. 81(5):375-380. [PubMed: 5933812]
• Ahlborg, G. 1951. Hydrogen sulfide poisoning in shale oil industry. A.M.A. Arch. Ind. Hyg. Occup. Med. 3(3):247-266. [PubMed: 14810252]
• Almeida, A.F. and T.L. Guidotti. 1999. Differential sensitivity of lung and brain to sulfide exposure: a peripheral mechanism for apnea. Toxicol. Sci. 50(2):287-293. [PubMed: 10478866]
• Ammann, H.M. 1986. A new look at physiologic respiratory response to H₂S poisoning. J. Hazard. Mater. 13(3):369-374.
• Arnold, I.M., R.M. Dufresne, B.C. Alleyne, and P.J. Stuart. 1985. Health implication of occupational exposures to hydrogen sulfide. J. Occup. Med. 27(5):373-376. [PubMed: 3159860]
• Arts, J.H., A. Zwart, E.D. Schoen, and J.M. Klokman-Houweling. 1989. Determination of concentration-time-mortality relationships versus LC50s according to OECD guideline 403. Exp. Pathol. 37(1-4):62-66. [PubMed: 2637172]
• ATSDR (Agency for Toxic Substances and Disease Registry). 2006. Toxicological Profile for Hydrogen Sulfide. U.S. Department of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry, Atlanta, GA. July 2006 [online]. Available: http://www​.atsdr.cdc​.gov/toxprofiles/tp114-p.pdf [accessed Apr. 1, 2009].
• ATSDR (Agency for Toxic Substances and Disease Registry). 2008. ATSDR Minimal Risk Levels (MRLs). U.S. Department of Health and Human Services, Agency for Toxic Substances and Disease Registry, Atlanta, GA. December 2008 [online]. Available: http://www​.atsdr.cdc​.gov/mrls/pdfs/atsdr_mrls_december_2008​.pdf [accessed Apr. 1, 2009].
• Attene-Ramos, M.S., E.D. Wagner, M.J. Plewa, and H.R. Gaskins. 2006. Evidence that hydrogen sulfide is a genotoxic agent. Mol. Cancer Res. 4(1):9-14. [PubMed: 16446402]
• Barthelemy, H.L. 1939. Ten years’ experience with industrial hygiene in connection with the production of viscose rayon. J. Ind. Hyg. Toxicol. 21(4):141-151.
• Beauchamp, R.O., J.S. Bus, J.A. Popp, C.J. Boreiko, and D.A. Andjelkovich. 1984. A critical review of the literature on hydrogen sulfide toxicity. CRC Crit. Rev. Toxicol. 13(1):25-97. [PubMed: 6378532]
• Bhambhani, Y., and M. Singh. 1991. Physiological effects of hydrogen sulfide inhalation during exercise in healthy men. J. Appl. Physiol. 71(5):1872-1877. [PubMed: 1761485]
• Bhambhani, Y., R. Burnham, G. Snydmiller, I. MacLean, and T. Martin. 1994. Comparative physiological responses of exercising men and women to 5 ppm hydrogen sulfide exposure. Am. Ind. Hyg. Assoc. J. 55(11):1030-1035. [PubMed: 7992795]
• Bhambhani, Y., R. Burnham, G. Snydmiller, I. MacLean, and T. Martin. 1996. a. Effects of 5 ppm hydrogen sulfide inhalation on biochemical properties of skeletal muscle in exercising men and women. Am. Ind. Hyg. Assoc. J. 57(5):464-468. [PubMed: 8638517]
• Bhambhani, Y., R. Burnham, G. Snydmiller, I. MacLean, and R. Lovlin. 1996. b. Effects of 10 ppm hydrogen sulfide inhalation on pulmonary function in healthy men and women. J. Occup. Environ. Med. 38(10):1012-1017. [PubMed: 8899577]
• Bhambhani, Y., R. Burnham, G. Snydmiller, and I. MacLean. 1997. Effects of 10 ppm hydrogen sulfide inhalation in exercising men and women: Cardiovascular, metabolic, and biochemical responses. J. Occup. Environ. Med. 39(2):122-129. [PubMed: 9048318]
• Boehning, D., and S.H. Snyder. 2003. Novel neural modulators. Ann. Rev. Neurosci. 26:105-131. [PubMed: 14527267]
• Brenneman, K.A., B.A. Wong, M.A. Buccellato, E.R. Costa, E.A. Gross, and D.C. Dorman. 2000. a. Direct olfactory transport of inhaled manganese (⁵⁴MnCl₂) to the rat brain: Toxicokinetic investigations in a unilateral nasal occlusion model. Toxicol. Appl. Pharmacol. 169(3):238-248. [PubMed: 11133346]
• Brenneman, K.A., R.A. James, E.A. Gross, and D.C. Dorman. 2000. b. Olfactory neuron loss in adult male CD rats following subchronic inhalation exposure to hydrogen sulfide. Toxicol. Pathol. 28(2):326-333. [PubMed: 10805151]
• Brenneman, K.A., D.F. Meleason, M. Sar, M.W. Marshall, R.A. James, E.A. Gross, J.T. Martin, and D.C. Dorman. 2002. Olfactory mucosal necrosis in male CD rats following acute inhalation exposure to hydrogen sulfide: Reversibility and possible role of regional metabolism. Toxicol. Pathol. 30(2):200-208. [PubMed: 11950163]
• Budavari, S., editor; , M.J. O’Neil, editor; , A. Smith, editor; , and P.E. Heckelman, editor. , eds. 1989. Hydrogen sulfide. P. 761 in The Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals, 11th Ed. Rahway, NJ: Merck.
• Carson, M.B. 1963. Hydrogen sulfide exposure in the gas industry. Ind. Med. Surg. 32:63-64. [PubMed: 14018925]
• Curtis, S.E., C.R. Anderson, J. Simon, A.H. Jensen, D.L. Day, and K.W. Kelly. 1975. Effects of aerial ammonia, hydrogen sulfide and swine-house dust on rate of gain and respiratory-tract structure in swine. J. Anim. Sci. 41(3):735-739. [PubMed: 1158807]
• Dales, R.E., W.O. Spitzer, S. Suissa, M.T. Schechter, P. Tousignant, and N. Steinmetz. 1989. Respiratory health of a population living downwind from natural gas refineries. Am. Rev. Respir. Dis. 139(3): 595:600. [PubMed: 2923356]
• Doeller, J.E., T.S. Isbell, G. Benavides, J. Koenitzer, H. Patel, R.P. Patel, J.R. Lancaster, Jr., V.M. Darley-Usmar, and D.W. Kraus. 2005. Polarographic measurement of hydrogen sulfide production and consumption by mammalian tissues. Anal. Biochem. 341(1):40-51. [PubMed: 15866526]
• Donham, K.J., D.C. Zavala, and J. Merchant. 1984. Acute effects of the work environment on pulmonary functions of swine confinement workers. Am. J. Ind. Med. 5(5):367-375. [PubMed: 6426296]
• Dorman, D.C., K.A. Brenneman, M.F. Struve, K.L. Miller, R.A. James, M.W. Marshall, and P.M. Foster. 2000. Fertility and developmental neurotoxicity effects of inhaled hydrogen sulfide in Sprague-Dawley rats. Neurotoxicol. Teratol. 22(1):71-84. [PubMed: 10642116]
• Dorman, D.C., F.J. Moulin, B.E. McManus, K.C. Mahle, R.A. James, and M.F. Struve. 2002. Cytochrome oxidase inhibition induced by acute hydrogen sulfide inhalation: Correlations with tissue sulfide concentrations in the rat brain, liver, lung and nasal epithelium. Toxicol. Sci. 65(1):18-25. [PubMed: 11752681]
• Dorman, D.C., M.F. Struve, E.A. Gross, and K.A. Brenneman. 2004. Respiratory tract toxicity of inhaled hydrogen sulfide in Fisher-344 rats, Sprague-Dawley rats, and B₆C₃F₁ mice following subchronic (90 day) exposure. Toxicol. Appl. Pharmacol. 198(1):29-39. [PubMed: 15207646]
• EPA (U.S. Environmental Protection Agency). 1994. Methods for Derivation of Inhalation Reference Concentrations and Application of Inhalation Dosimetry. EPA/600/8-90/066F. Environmental Criteria and Assessment Office, Office of Health and Environmental Assessment, Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, NC [online]. Avail able: http://www​.epa.gov/raf​/publications/pdfs/RFCMETHODOLOGY.PDF [accessed Apr. 21, 2009].
• EPA (U.S. Environmental Protection Agency). 2002. Hydrogen Sulfide Results. Interim Acute Exposure Guideline Levels (AEGLs) for Hydrogen Sulfide, 9/10/2002. National Advisory Committee/AEGL, U.S. Environmental Protection Agency, Washington, DC: [online]. Available: http://www​.epa.gov/oppt​/aegl/pubs/results57.htm [accessed Apr. 2, 2009].
• Fiedler, N., H. Kipen, P. Ohman-Strickland, J. Zhang, C. Weisel, R. Laumbach, K. Kelly-McNeil, K. Olejeme, and P. Lioy. 2008. Sensory and cognitive effects of acute exposure to hydrogen sulfide. Environ. Health Perspect. 116(1):78-85. [PMC free article: PMC2199294] [PubMed: 18197303]
• Fiorucci, S., E. Distrutti, G. Cirino, and J.L. Wallace. 2006. The emerging roles of hydrogen sulfide in the gastrointestinal tract and liver. Gastroenterology 131(1):259-271. [PubMed: 16831608]
• Fuller, D.C., and A.J. Suruda. 2000. Occupationally related hydrogen sulfide deaths in the United States from 1984 to 1994. J. Occup. Med. 42(9):939-942. [PubMed: 10998771]
• Glass, D.C. 1990. A review of the health effects of hydrogen sulphide exposure. Ann. Occup. Hyg. 34(3): 323-327. [PubMed: 2196846]
• Hagar, R. 2008. Submarine Atmosphere Control and Monitoring Brief for the COT Committee. Presentation at the First Meeting on Emergency and Continuous Exposure Guidance Levels for Selected Submarine Contaminants, June 17, 2008, Washington, DC.
• Hannah, R.S., and S.H. Roth. 1991. Chronic exposure to low concentrations of hydrogen sulfide produces abnormal growth in developing cerebellar Purkinje cells. Neurosci. Lett. 122(2):225-228. [PubMed: 2027523]
• Hannah, R.S., L.J. Hayden, and S.H. Roth. 1989. Hydrogen sulfide exposure alters amino acid content in developing rat CNS. Neurosci. Lett. 99(3):323-327. [PubMed: 2566964]
• Higuchi, Y., and M. Fukamachi. 1977. Behavioral studies on toxicity of hydrogen sulfide by means of conditioned avoidance responses in rats [in Japanese]. Folia Pharmacol. Jpn. 73(3):307-319. [PubMed: 914122]
• Hoshika, Y., T. Imamura, G. Muto, L.J. Van Gemert, J.A. Don, and J.I. Walpot. 1993. International comparison of odor threshold values of several odorants in Japan and in The Netherlands. Environ Res. 61(1):78-83. [PubMed: 8472679]
• HSDB (Hazardous Substances Data Bank). 2005. Hydrogen Sulfide (CASRN: 7783-06-4). TOXNET, Specialized Information Services, U.S. National Library of Medicine, Bethesda, MD: [online]. Available: http://toxnet​.nlm.nih.gov/ [accessed Apr. 6, 2009].
• Jaakkola, J.J., V. Vilkka, O. Marttila, P. Jappinen, and T. Haahtela. 1990. The South Karelia Air Pollution Study. The effects of malodorous sulfur compounds from pulp mills on respiratory and other symptoms. Am. Rev. Respir. Dis. 142(6 Pt 1):1344-1350. [PubMed: 2252252]
• Jappinen, P., V. Vilkka, O. Marttila, and T. Haahtela. 1990. Exposure to hydrogen sulfide and respiratory function. Br. J. Ind. Med. 47(12):824-828. [PMC free article: PMC1035289] [PubMed: 2271389]
• Kage, S., S. Ito, T. Kishida, K. Kudo, and N. Ikeda. 1998. A fatal case of hydrogen sulfide poisoning in a geothermal power plant. J. Forensic Sci. 43(4):908-910. [PubMed: 9670519]
• Khan, A.A., M.M. Schuler, M.G. Prior, S. Young, R.W. Coppock, L.Z. Florence, and L.E. Lillie. 1990. Effects of hydrogen sulfide exposure on lung mitochondrial respiratory chain enzymes in rats. Toxicol. Appl. Pharmacol. 103(3):482-490. [PubMed: 2160136]
• Kilburn, K.H. 1993. Case report: Profound neurobehavioral deficits in an oil field worker overcome by hydrogen sulfide. Am. J. Med. Sci. 306(5):301-305. [PubMed: 8238084]
• Kilburn, K.H. 1997. Exposure to reduced sulfur gases impairs neurobehavioral function. South. Med. J. 90(10):997-1006. [PubMed: 9347810]
• Kilburn, K.H., and R.H. Warshaw. 1995. Hydrogen sulfide and reduced-sulfur gases adversely affect neurophysiological functions. Toxicol. Ind. Health 11(2):185-197. [PubMed: 7491634]
• Kimura, H. 2000. Hydrogen sulfide induces cyclic AMP and modulates NMDA receptor. Biochem. Biophys. Res. Commun. 267(1):129-133. [PubMed: 10623586]
• Lambert, T.W., V.M. Goodwin, D. Stefani, and L. Strosher. 2006. Hydrogen sulfide (H₂S) and sour gas effects on the eye. A historical perspective. Sci. Total Environ. 367(1):1-22. [PubMed: 16650463]
• Li, J., G. Zhang, S. Cai, and A.N. Redington. 2008. Effect of inhaled hydrogen sulfide on metabolic responses in anesthetized, paralyzed, and mechanically ventilated piglets. Pediatr. Crit. Care Med. 9(1):110-112. [PubMed: 18477923]
• Lopez, A., M. Prior, S. Yong, L. Lillie, and M. Lefebvre. 1988. Nasal lesions in rats exposed to hydrogen sulfide for four hours. Am. J. Vet. Res. 49(7):1107–1111. [PubMed: 3421534]
• Lopez, A., M.G. Prior, R.J. Reiffenstein, and L.R. Goodwin. 1989. Peracute toxic effects of inhaled hydrogen sulfide and injected sodium hydrosulfide on the lungs of rats. Fundam. Appl. Toxicol. 12(2):367-373. [PubMed: 2714535]
• MacEwen, J.D., and E.H. Vernot. 1972. Acute toxicity of hydrogen sulfide. Pp. 66-70 in Toxic Hazards Research Unit Annual Technical Report: 1972. Report No. ARML-TR-72-62. Aerospace Medical Research Laboratory, Air Force Systems Command, Wright-Patterson Air Force Base, OH. August 1972.
• Marttila, O., J.J.K. Jaakkola, V. Vilkka, P. Jappinen, and T. Haahtela. 1994. The South Karelia Air-Pollution Study: The effects of malodorous sulfur-compounds from pulp-mills on respiratory and other symptoms in children. Environ. Res. 66(2):152-159. [PubMed: 8055837]
• Marttila, O., J.J. Jaakkola, K. Partti-Pellinen, V. Vilkka, and T. Haahtela. 1995. South Karelia Air Pollution Study: Daily symptom intensity in relation to exposure levels of malodorous sulfur compounds from pulp mills. Environ. Res. 71(2):122-127. [PubMed: 8977620]
• Mitchell, T.W., J.C. Savage, and D.H. Gould. 1993. High-performance liquid chromatography detection of sulfide in tissues from sulfide-treated mice. J. Appl. Toxicol. 13(6):389-394. [PubMed: 8288842]
• Moulin, F.J., K.A. Brenneman, J.S. Kimbell, and D.C. Dorman. 2002. Predicted regional flux of hydrogen sulfide correlates with distribution of nasal olfactory lesions in rats. Toxicol. Sci. 66(1):7-15. [PubMed: 11861968]
• Nesswetha, W. 1969. Eye lesions caused by sulphur compounds [in German]. Arbeitsmed. Sozialmed. Arbeitshyg. 4:288-290.
• NIOSH (National Institute of Occupational Safety and Health). 2005. NIOSH Pocket Guide to Chemical Hazards: Hydrogen Sulfide. U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute of Occupational Safety and Health, Cincinnati, OH. September 2005 [online]. Available: http://www​.cdc.gov/Niosh/npg/npgd0337​.html [accessed Apr. 21, 2008].
• NRC (National Research Council). 1979. Hydrogen Sulfide. Baltimore: University Park Press.
• NRC (National Research Council). 1985. Hydrogen sulfide. Pp. 55-68 in Emergency and Continuous Exposure Guidance Levels for Selected Airborne Contaminants, Vol. 4. Washington, DC: National Academy Press.
• NRC (National Research Council). 2002. Hydrogen sulfide. Pp. 178-218 in Review of Submarine Escape Action Levels for Selected Chemicals. Washington, DC: National Academy Press.
• Partlo, L.A., R.S. Sainsbury, and S.H. Roth. 2001. Effects of repeated hydrogen sulphide exposure on learning and memory in the adult rat. Neurotoxicology 22(2):177-189. [PubMed: 11405250]
• Persson, S., M.B. Edlund, R. Claesson, and J. Carlsson. 1990. The formation of hydrogen sulfide and methyl mercaptan by oral bacteria. Oral Microbiol. Immunol. 5(4):195-201. [PubMed: 2082242]
• Prior, M.G., A.K. Sharma, S. Yong, and A. Lopez. 1988. Concentration-time interactions in hydrogen sulfide toxicity in rats. Can. J. Vet. Res. 52(3):375-379. [PMC free article: PMC1255467] [PubMed: 3167719]
• Reiffenstein, R.J., W.C. Hulbert, and S.H. Roth. 1992. Toxicology of hydrogen sulfide. Annu. Rev. Pharmacol. Toxicol. 32:109-134. [PubMed: 1605565]
• Roberts, E.S., V.A. Wong, B.E. McManus, M.W. Marshall, S. Lancianese, and D.C. Dorman. 2006. Changes in intracellular pH play a secondary role in hydrogen sulfide-induced nasal cytotoxicity. Inhal. Toxicol. 18(3):159-167. [PubMed: 16399658]
• Roberts, E.S., R.S. Thomas, and D.C. Dorman. 2008. Gene expression changes following acute hydrogen sulfide (H₂S)-induced nasal respiratory epithelial injury. Toxicol. Pathol. 36(4):560-567. [PubMed: 18467678]
• Ruth, J.H. 1986. Odor thresholds and irritation levels of several chemical substances: A review. Am. Ind. Hyg. Assoc. J. 47(3):A142-A151. [PubMed: 3706135]
• Schroeter, J.D., J.S. Kimbell, A.M. Bonner, K.C. Roberts, M.E. Andersen, and D.C. Dorman. 2006. Incorporation of tissue reaction kinetics in a computational fluid dynamics model for nasal extraction of inhaled hydrogen sulfide in rats. Toxicol. Sci. 90(1):198-2007. [PubMed: 16344266]
• Shusterman, D. 1992. Community health and odor pollution regulation. Am. J. Public Health 82(11):1566-1567. [PMC free article: PMC1694630] [PubMed: 1443313]
• Skrajny, B., R.S. Hannah, and S.H. Roth. 1992. Low concentrations of hydrogen sulfide alter monoamine levels in the developing rat central nervous system. Can. J. Physiol. Pharmacol. 70(11):1515-1518. [PubMed: 1296865]
• Skrajny, B., R.J. Reiffenstein, R.S. Sainsbury, and S.H. Roth. 1996. Effects of repeated exposures of hydrogen sulfide on rat hippocampal EEG. Toxicol. Lett. 84(1):43-53. [PubMed: 8597177]
• Smith, L., H. Kruszyna, and R.P. Smith. 1977. The effect of methemoglobin on the inhibition of cytochrome c oxidase by cyanide, sulfide or azide. Biochem. Pharmacol. 26(23):2247-2250. [PubMed: 22333]
• Snyder, J.W., E.F. Safir, G.P. Summerville, and R.A. Middleberg. 1995. Occupational fatality and persistent neurological sequelae after mass exposure to hydrogen sulfide. Am. J. Emerg. Med. 13(2):199-203. [PubMed: 7893309]
• Spitzer, W.O., R.E. Dales, M.T. Schechter, S. Suissa, P. Tousiqnant, N. Steinmetz, and M.E. Hutcheon. 1989. Chronic exposure to sour gas emissions: Meeting a community concern with epidemiologic evidence. CMA J. 141(7):685-691. [PMC free article: PMC1452739] [PubMed: 2790604]
• Struve, M.F., J.N. Brisbois, R.A. James, M.W. Marshall, and D.C. Dorman. 2001. Neurotoxicological effects associated with short-term exposure of Sprague-Dawley rats to hydrogen sulfide. Neurotoxicology 22(3):375-385. [PubMed: 11456338]
• Subramaniam, R.P., R.B. Richardson, K.T. Morgan, J.S. Kimbell, and R.A. Guilmette. 1998. Computational fluid dynamics simulations of inspiratory airflow in the human nose and nasopharynx. Inhal. Toxicol. 10(2):91-120.
• Szabó, C. 2007. Hydrogen sulphide and its therapeutic potential. Nat. Rev. Drug Discov. 6(11):917-935. [PubMed: 17948022]
• Tansy, M.F., F.M. Kendall, J. Fantasia, W.E. Landin, R. Oberly, and W. Sherman. 1981. Acute and subchronic toxicity studies of rats exposed to vapors of methyl mercaptan and other reduced-sulfur compounds. J. Toxicol. Environ. Health 8(1-2):71-88. [PubMed: 7328716]
• Tsai, C.C., H.H. Chou, T.L. Wu, Y.H. Yang, K.Y. Ho, Y.M. Wu, and Y.P. Ho. 2008. The levels of volatile sulfur compounds in mouth air from patients with chronic periodontitis. J. Periodontal Res. 43(2):186-193. [PubMed: 18302621]
• Tvedt, B., K. Skyberg, O. Aaserud, A. Hobbesland, and T. Mathiesen. 1991. a. Brain damage caused by hydrogen sulfide: A follow-up study of six patients. Am. J. Ind. Med. 20(1):91-101. [PubMed: 1867221]
• Tvedt, B., A. Edlund, K. Skyberg, and O. Forberg. 1991. b. Delayed neuropsychiatric sequelae after acute hydrogen sulfide poisoning: Affection of motor function, memory, vision, and hearing. Acta Neurol. Scand. 84(4):348-351. [PubMed: 1772008]
• Vanhoorne, M., L. van den Berge, A. Devreese, E. Tijtgat, L. van Poucke, and C. van Peteghem. 1991. Survey of chemical exposures in a viscose rayon plant. Ann. Occup. Hyg. 35(6):619-631. [PubMed: 1768011]
• Vanhoorne, M., A. de Rouck, and D. de Bacquer. 1995. Epidemiological study of eye irritation by hydrogen sulfide and/or carbon disulphide exposure in viscose rayon workers. Ann. Occup. Hyg. 39(3):307-315. [PubMed: 7793750]
• Wang, R. 2002. Two’s company, three’s a crowd: Can H₂S be the third endogenous gaseous transmitter? FASEB J. 16(13):1792-1798. [PubMed: 12409322]
• Wasch, H.H., W.J. Estrin, P. Yip, R. Bowler, and J.E. Cone. 1989. Prolongation of the P-300 latency associated with hydrogen sulfide exposure. Arch. Neurol. 46(8):902-904. [PubMed: 2757531]
• Weil, E.D., S.R. Sandler, and M. Gernon. 2006. Sulfur compounds. Pp. 621-701 in Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 23, J.I. Kroschwitz, editor; and A. Seidel, editor. , eds. Hoboken, NJ: Wiley-Interscience.
• Winek, C.L., W.D. Collom, and C.H. Wecht. 1968. Death from hydrogen sulfide fumes. Lancet 1(7551):1096. [PubMed: 4171784]
• Zhang, H., L. Zhi, S.M. Moochhala, P.K. Moore, and M. Bhatia. 2007. Endogenous hydrogen sulfide regulates leukocyte trafficking in cecal ligation and puncture-induced sepsis. J. Leukoc. Biol. 82(4):894-905. [PubMed: 17599903]
• Zwart, A., J.H.E. Arts, J.M. Klokman-Houweling, and E.D. Schoen. 1990. Determination of concentration-time-mortality relationships to replace LC50 values. Inhal. Toxicol. 2(2):105-117.