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IARC Working Group on the Evaluation of Carcinogenic Risks to Humans. Some Chemicals Used as Solvents and in Polymer Manufacture. Lyon (FR): International Agency for Research on Cancer; 2017. (IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, No. 110.)
1.1. Identification of the agent
1.1.1. Nomenclature
- Chem. Abstr. Serv. Reg. No.: 75-09-2
- Chem. Abstr. Serv. Name: Dichloromethane
- IUPAC Systematic Name: Dichloromethane
- Synonyms: Methane dichloride; methylene bichloride; methylene chloride; methylene dichloride
1.1.2. Structural and molecular formulae, and relative molecular mass
- Molecular formula: CH2Cl2
- Relative molecular mass: 84.93
1.1.3. Chemical and physical properties of the pure substance
- Description: Colourless liquid with penetrating ether-like odour (O’Neil et al., 2006; Haynes, 2010)
- Boiling point: 40 °C
- Melting point: –97.1 °C
- Density: d420 1.327 g/mL
- Solubility: Slightly soluble (1.38 g/100 mL) in water at 20 °C; soluble in carbon tetrachloride; miscible in ethanol, diethyl ether, and dimethylformamide
- Volatility: Vapour pressure, 58.2 kPa at 25 °C; relative vapour density (air = 1), 2.93 (Verschueren, 1996)
- Stability: Vapour is nonflammable and is not explosive when mixed with air, but may form explosive mixtures in atmospheres with higher oxygen content (Sax, 1984)
- Reactivity: Reacts vigorously with active metals (lithium, sodium, potassium) and with strong bases (potassium tert-butoxide) (Sax, 1984)
- Octanol/water partition coefficient (P): log P, 1.25 (Hansch et al., 1995)
- Conversion factor: Assuming normal temperature (25 °C) and pressure (101 kPa), 1 mg/m3 = 3.53 ppm; calculated from: mg/m3 = (relative molecular mass/24.47) × ppm.
1.1.4. Technical products and impurities
Dichloromethane is available in several grades according to intended end use: technical grade; aerosol; vapour degreasing; special; urethane; and Food Chemicals Codex/National Formulary (food and pharmaceutical applications). Purity, when reported, ranges from 99% to 99.99%. Acidity (as hydrochloric acid) may be up to 5 mg/kg. The maximum concentration of water in these grades of dichloromethane is 100 mg/kg (Rossberg et al., 1986; Holbrook, 1993; Dow Chemical Co, 1995; Vulcan Chemicals, 1995, 1996a, b, c, d).
Small amounts of stabilizers are often added to dichloromethane at the time of manufacture to protect against degradation by air and moisture. The following substances in the listed concentration ranges are the preferred additives (wt%): ethanol, 0.1–0.2; methanol, 0.1–0.2; cyclohexane, 0.01–0.03; and amylene (2-methyl-2-butene), 0.001–0.01. Other substances have also been described as being effective stabilizers, including phenols (phenol, hydroquinone, para-cresol, resorcinol, thymol, 1-naphthol), amines, nitroalkanes (nitromethane), aliphatic and cyclic ethers, epoxides, esters, and nitriles (Rossberg et al., 1986; Holbrook, 1993).
1.1.5. Analysis
Methods for the analysis of dichloromethane in air, solids, liquids, water, and food have been reviewed by ATSDR (2000) and HSDB (2012). Selected methods for the analysis of dichloromethane in various matrices are presented in Table 1.1. Exposures to dichloromethane can also be monitored in air using a direct-reading infrared analyser, with a minimum detectable concentration of 0.7 mg/m3 (0.2 ppm) (Goelzer & O’Neill, 1985).
Table 1.1
Methods for the analysis of dichloromethane.
Exposure to dichloromethane can be monitored in samples of blood, breath, or urine (ATSDR, 2000; WHO, 2000; SCOEL, 2009). Urinary concentrations of dichloromethane in humans are reported to correlate well with exposure concentrations in air (Di Vincenzo et al., 1972; SCOEL, 2009). The concentration of dichloromethane or carboxyhaemoglobin (COHb) levels are measured in blood (SCOEL, 2009). Since the relationship between alveolar carbon monoxide (CO) and COHb has not been well established for workers exposed to dichloromethane, breath analysis for CO cannot be considered as providing definitive quantitative information regarding exposure to dichloromethane (WHO, 2000).
1.2. Production and use
1.2.1. Production
Dichloromethane was first prepared in 1840 by the chlorination of methyl chloride in sunlight. It became an industrial chemical of importance during the Second World War (Rossberg et al., 1986). Two commercial processes are currently used for the production of dichloromethane: hydrochlorination of methanol and direct chlorination of methane (Rossberg et al., 1986; Holbrook, 1993; ATSDR, 2000).
Global production of dichloromethane increased from 93 000 tonnes in 1960 to an estimated 570 000 tonnes in 1980 (IARC, 1986), and is estimated to range from 764 000 to 814 000 tonnes per year from 2005 to 2010 (OECD/SIDS, 2011). In 2009, dichloromethane was produced by 26 manufacturers worldwide and was available from 133 suppliers (NTP, 2011). Production and imports of dichloromethane in the USA totalled 45 000–227 000 tonnes between 1996 and 2006 (NTP, 2011). In the European Union, the total tonnage band for dichloromethane was reported to be 100 000 to 1 000 000 tonnes per year (ECHA, 2016). The production and import of dichloromethane reported in Japan was 58 000 tonnes in 2011 (METI, 2013).
1.2.2. Use
Most of the applications of dichloromethane are based on its solvent properties (IARC, 1999). The principal uses worldwide comprise paint stripper (23–50%), aerosol solvents and propellants (10–25%), process solvent in the chemical and pharmaceutical industry (10–20%), and metal degreasing (8–13%) (WHO, 1996; IARC, 1999). The distribution of uses varies considerably among countries (OECD, 1994). Dichloromethane has also been used in the production of cellulose fibre, in the manufacture of photographic film, in textile manufacturing, for extraction of food flavourings and decaffeination of coffee, as a blowing agent for polymer foams, in production of hydrofluorocarbon refrigerants, and in pesticides (OECD, 1994; IARC, 1999; NTP, 2011; EPA, 2012). Use of dichloromethane in Europe and the USA has been declining since the 1970s (Holbrook, 1993; WHO, 1996; EPA, 2012).
(a) Paint stripper
For use in paint strippers, dichloromethane is typically blended with other chemical components (Holbrook, 1993; WHO, 1996). Dichloromethane has been the major component of nearly all solvent-based paint stripper formulations for industrial, professional, and consumer use; the aircraft industry and military are important users (OECD, 1994). Alternative paint strippers have come onto the market (Joe et al., 2013), and paint-strippers containing dichloromethane are no longer permitted for professional or consumer use in Europe, although they remain available elsewhere (European Commission, 2009; Joe et al., 2013).
(b) Aerosols
Dichloromethane is used as propellant and solvent in aerosol products including paints, automotive products, adhesives, and hair sprays (WHO, 1996; ATSDR, 2000; NTP, 2011). The use of dichloromethane in consumer aerosol products has declined in the USA (ATSDR, 2000), and dichloromethane is no longer permitted for use in cosmetic products in the USA since 1989 (FDA, 1989).
(c) Process solvent
In chemical processing, dichloromethane is used in the manufacture of polycarbonate plastic, the manufacture of photoresist coatings, and as a solvent carrier for the manufacture of insecticides and herbicides. It is used by the pharmaceutical industry as a process solvent in the manufacture of steroids, antibiotics, vitamins and, to a lesser extent, as a solvent in the coating of tablets. Other uses include oil de-waxing, in inks and adhesives, and in plastics manufacture (Rossberg et al., 1986; Holbrook, 1993; IARC, 1999).
(d) Metal cleaning
In the metalworking industries, dichloromethane is used as a vapour degreasing solvent, or blended with petroleum and other hydrocarbons as a dip-type cleaner (IARC, 1999). In the manufacture of metal products, cleaning is needed before painting, plating, plastic coating, etc. Degreasing in the engineering industry is normally carried out with special equipment in which dichloromethane is used either in the liquid or vapour phase. Dichloromethane is also used in the electronics industry in the production of circuit boards and as a stripper for photoresists (OECD, 1994). In Japan and elsewhere, dichloromethane has widely been used for metal cleaning as an alternative solvent to 1,1,1-trichloroethane after the implementation of the Montreal Protocol on Substances that Deplete the Ozone Layer (OECD, 1994).
(e) Printing industry
Dichloromethane is a major ingredient of cleaning solvent used to remove printer ink during the offset printing process. For efficient manual wiping with a cloth, dichloromethane is often blended with other halogenated hydrocarbons or kerosene to adjust its evaporation rate. Almost all the dichloromethane in the solvent evaporates into the working environment. It is to be noted that offset printing is usually carried out indoors, sometimes with limited ventilation to ensure that temperature and humidity are kept constant (Kumagai et al., 2013). Offset proof printing requires frequent cleaning interventions, and offset web printing sometimes includes manual wiping under the machine, both of which lead to high concentrations of vapour in the breathing zone.
Ink for a three-dimensional printing process has been developed using a fast-drying thermoplastic solution comprising polylactic acid dissolved in dichloromethane (Guo et al., 2013).
(f) Other uses
Dichloromethane is used as feedstock in the production of hydrofluorocarbon-32 (HFC-32) refrigerant (difluoromethane). The demand for HFC-32 as a replacement chemical for HFC-22 (chlorodifluoromethane) may increase the use of dichloromethane in the USA (EPA, 2012).
1.3. Occurrence and exposure
1.3.1. Environmental occurrence
(a) Natural occurrence
Dichloromethane is not known to occur naturally.
(b) Outdoor air
Background levels from remote monitors in the USA in operation since 2003 have shown that the concentration of dichloromethane in air in isolated locations is very low (mean, 0.1 μg/m3) (McCarthy et al., 2006).
Levels of dichloromethane are higher in urban areas than in rural areas. For example, at 13 urban monitoring centres in the USA in 1996, the geometric mean concentration of dichloromethane varied from 0.05 to 0.24 ppb by volume (0.28 to 0.85 μg/m3) (Mohamed et al., 2002). In the 1990s, the concentration of dichloromethane at 22 urban sites in Canada was reported as being between 0.5 μg/m3 and 10 μg/m3 (Government of Canada, 1993).
There is also seasonal variation. In China, dichloromethane was one of the five most abundant volatile organic compounds measured in air at 14 sites in 9 cities in the south-eastern coastal region. The average concentration of dichloromethane in air was 50.2 μg/m3 in winter (range, 12.4–113 μg/m3) and 10.1 μg/m3 in summer (range, 6.3–22.8 μg/m3) (Tong et al., 2013).
Generally, the concentrations of dichloromethane in industrial areas tend to be much higher than those in residential and administrative areas. In a study of six different areas within Haicang, China, the mean levels of dichloromethane in two industrial areas were 102.0 μg/m3 and 219.1 μg/m3, in the harbour was 69.80 μg/m3, in surrounding residential and administration areas were 119.60 μg/m3 and 112.00 μg/m3, while in the background site in forests at a distance of 20 km, the level was 8.2 μg/m3 (Niu et al., 2012). Similarly, mean concentrations of dichloromethane were 42.5 μg/m3 in a biopharmaceutical plant in China and 3.5 μg/m3 in a residential area nearby (Pan et al., 2011).
(c) Indoor air
Eight-hour average concentrations of dichloromethane were measured in a range of indoor environments in China as follows: home, 1.0–1.3 μg/m3; office, 0.03 μg/m3; school, 0.1 μg/m3; restaurant, 3.3 μg/m3; shopping mall, 0.7 μg/m3; city train, 0.8 μg/m3; and bus, 0.4 μg/m3 (Guo et al., 2004).
A report from Canada quoted a study from 1988 that found that the mean concentration of dichloromethane in 757 homes was 16.3 μg/m3 (Government of Canada, 1993).
(d) Water
Dichloromethane has been detected in surface water and groundwater samples taken at hazardous waste sites and in drinking-water in Europe, the USA, Canada, and Japan. Concentrations in many water samples are below the limit of detection (ATSDR, 2000). Dichloromethane was measured in more than 5000 wells in the USA between 1985 and 2002; in 97% of samples, concentrations of dichloromethane were below maximum contaminant levels (MCLs). Dichloromethane was detected in 3% of samples, with concentrations ranging from 0.02 to 100 μg/L. These positive samples were mainly collected in agricultural areas, which may be a result of transformation of carbon tetrachloride used as a grain fumigant (Moran et al., 2007).
A report on dichloromethane in Canada summarized a range of measurements, and found that mean concentrations of dichloromethane in municipal drinking-water supplies in Canada during the 1980s ranged from 0.2 μg/L to 2.6 μg/L (Government of Canada, 1993). Measurements in groundwater near known spills were extremely high. For example, 25 years after the rupture of a storage tank near Toronto, the measured dichloromethane in groundwater was 25 × 106 μg/L. Mean concentrations in surface water were low (generally < 1 μg/L).
(e) Food
In the 1970s, dichloromethane was detected in decaffeinated coffee and tea, with levels ranging from < 0.05 to 4.04 mg/kg in coffee, and < 0.05 to 15.9 mg/kg in tea (Page & Charbonneau, 1984). Because of concern over residual solvent, most decaffeinators no longer use dichloromethane (ATSDR, 2000).
In an investigation of several halocarbons in table-ready foods, 8 of the 19 foods examined contained dichloromethane at concentrations above the quantification limit (0.008 ppb), with the following ranges reported: butter, 1.1–280 μg/kg; margarine, 1.2–81 μg/kg; ready-to-eat cereal, 1.6–300 μg/kg; cheese, 3.9–98 μg/kg; peanut butter, 26–49 μg/kg; and highly processed foods (frozen chicken dinner, fish sticks, pot pie), 5–310 μg/kg (Heikes, 1987).
1.3.2. Occupational exposure
The principal route of exposure in occupational settings is inhalation (ATSDR, 2000).
Occupational exposure to dichloromethane may occur in several industries. Workers may be exposed during the production and processing of dichloromethane, or during use of products containing dichloromethane, particularly when the end product is sprayed or otherwise aerosolized (ATSDR, 2000).
Monitoring data for dichloromethane up to 1999 have been reviewed previously (IARC, 1999). More than 1.4 million workers in the USA and approximately 250 000 workers in Europe were estimated to be potentially exposed to dichloromethane in the 1980s and 1990s (IARC, 1999; NIOSH, 2013). Exposure occurred across a range of industries, levels varying widely by operation and within operation. Concentrations of dichloromethane exceeding 1000 mg/m3 were recorded in paint stripping, in the printing industry, and in the manufacture of plastics and synthetic fibres. Full-shift exposures to dichloromethane at concentrations exceeding 100 mg/m3 were thought to have occurred in furniture-stripping shops, and in certain jobs in the aeronautical, pharmaceutical, plastic, and footwear industries (IARC, 1999).
In 2012, the United States Environmental Protection Agency (EPA) reviewed available historical studies that had monitored dichloromethane concentrations in workers stripping paint (EPA, 2012). Many of the studies included a very small numbers of exposed workers, and the results may not be generalizable. Exposure levels varied widely. For example, aircraft refinishing was reported to result in 8 hour time-weighted average (TWA) exposures of 86–3802 mg/m3 (25–1096 ppm) in different studies between 1994 and 2002. Workers stripping paint from metal, wood, or aircraft and furniture refinishing were all potentially exposed to 8 hour TWA exposures exceeding 1000 mg/m3.
Many of the industries in the EPA report do not now use dichloromethane (see Section 1.4). Data published since 2000 are summarized in Table 1.2. Levels now tend to be lower than earlier reports, with measured values in printing, polyurethane manufacture, and automotive and aircraft maintenance tending to be lower than 150 ppm. Studies in furniture-stripping plants showed that the installation of exposure surveillance was effective in reducing exposures to below 10 ppm (Estill et al., 2002; Fairfax & Grevenkamp, 2007).
Table 1.2
Measured occupational exposures to dichloromethane.
A new concern has been identified in connection with bathtub refinishing. In 2012, the United States Occupational Safety and Health Administration identified 13 fatalities associated with stripping agents containing dichloromethane that had been investigated in nine states during 2000–2011. These deaths occurred when products containing between 60% and 100% of dichloromethane were used to refinish bathtubs in bathrooms with inadequate ventilation and without use of respiratory protective equipment. Autopsy specimens showed blood concentrations of dichloromethane ranging from 18 to 223 mg/L in the six decedents for whom values were recorded; a concentration of < 2 mg/L is expected in a person working within the allowable air standard for the USA. Air concentrations of dichloromethane associated with such work were estimated to exceed 100 000 ppm (Chester et al., 2012).
Levels of exposure to dichloromethane were estimated in a printing company in Osaka, Japan, after the identification of a cluster of cancers of the biliary tract among workers at the plant (Kumagai et al., 2013). The circumstances of exposure were quite specific in that the workers removed ink from rollers using volatile solvents between 300 and 800 times per day, and the room was poorly ventilated. There was co-exposure for several years to both dichloromethane and 1,2-dichloropropane (see the Monograph on 1,2-Dichloropropane in the present volume). No monitoring was undertaken at the time, so the Japanese National Institute of Occupational Safety and Health undertook a reconstruction experiment to estimate exposure concentrations on the assumption that the exposure was proportional to the amount of chemical used. Estimated values of exposure to dichloromethane in the room where proofs were printed ranged from 80 to 210 ppm (278–728 mg/m3], with a mean of 140 ppm (486 mg/m3) in 1991–1992 and were higher in later years (mean, 360 ppm, equal to 1249 mg/m3) (Table 1.3). The estimated exposures in the front room were estimated to be 50 ppm (173 mg/m3) in 1991–1993, and 130 ppm (451 mg/m3) in 1992–1998 (Kumagai et al., 2013).
Table 1.3
Estimated exposure to dichloromethane and 1,2-dichloropropane among printers associated with clusters of cholangiocarcinoma in Japana.
In another case series of printing workers with cholangiocarcinoma in Japan, estimated concentrations of dichloromethane were modelled for the jobs in which the cases had worked (Yamada et al., 2014). The estimated shift TWA for two of the six workers was below 1 ppm. The other four workers were exposed to estimated shift TWAs of between 28 ppm (97 mg/m3) and 180 ppm (620 mg/m3). The highest levels were estimated for years before 1995. Additional details of the Japanese case-series studies are given in Section 2 of the Monograph on 1,2-Dichloropropane in the present volume.
1.3.3. Exposure of the general population
There are few data on exposure levels to dichloromethane of the general population. People may be exposed to dichloromethane from air, water, food, or during the use of consumer products containing dichloromethane (ATSDR, 2000). Exposure of the general population to dichloromethane may be much higher from indoor air than from outdoor air, especially from spray painting or use of other aerosols or consumer products containing dichloromethane as a solvent (ATSDR, 2000).
In the United States National Health and Nutrition Examination Survey (NHANES) study in 2003–2004, only 7 of the 1165 blood samples (0.6%) collected showed detectable levels of dichloromethane (CDC, 2009).
1.4. Regulations and guidelines
Several jurisdictions have acted to reduce the use and release of various volatile organic compounds, including dichloromethane. The California Air Resources Board was one of the first jurisdictions to regulate dichloromethane; in 1995, it limited the levels of total volatile organic compounds (VOCs) contained in aerosol coating products. Subsequent regulations prevented manufacture, sale, supply, or application of any aerosol coating product containing dichloromethane (Air Resources Board, 2001). California has also prohibited the manufacture, sale, or use of automotive cleaning and degreasing products containing dichloromethane.
In Japan, the environmental quality standards for dichloromethane state that outdoor air levels shall not exceed 0.15 mg/m3 (Ministry of the Environment Government of Japan, 2014).
A guideline value of 3 mg/m3 for 24-hour exposure is recommended by WHO. In addition, the weekly average concentration should not exceed one seventh (0.45 mg/m3) of this 24-hour guideline (WHO, 2000).
In the European Union, the VOC Solvent Emissions Directive (Directive 1999/13/EC) was implemented for new and existing installations on 31 October 2007 (European Commission, 1999). The Directive aims to reduce industrial emissions of VOCs from solvent-using activities, such as printing, surface cleaning, vehicle coating, dry cleaning, and manufacture of footwear and pharmaceutical products. Installations conducting such activities are required to comply either with emission limit values or with a reduction scheme. Reduction schemes allow the operator to reduce emissions by alternative means, such as by substituting products with a lower solvent content or changing to solvent-free production processes. The Solvents Directive was implemented in 2010 into the Industrial Emission Directive 2010/75/EU (IED).
The European Union has also restricted the use of paint strippers containing dichloromethane as of 2009 (Decision 455/2009/EC of the European Parliament amending Council Directive 76/769/EEC) as regards restrictions on the marketing and use of dichloromethane (European Commission, 2009). As noted above, dichloromethane-based paint strippers are banned for consumer and professional use. They may still be used in certain industrial applications with improved labelling and safety measures.
In the USA, the EPA National Emission Standards for Hazardous Air Pollutants (NESHAP) in 2008 adopted specific management practices to minimize emissions of dichloromethane in area sources that engage in paint stripping and spray application of coatings (EPA, 2008).
Occupational exposure limits for dichloromethane in air tend to be 50 ppm [176.5 mg/m3] over 8 hours, with United Kingdom permitting up to 100 ppm [353 mg/m3] (Table 1.4).
Table 1.4
International limit values for occupational exposure.
Biological monitoring regulations and recommendations
SCOEL (2009) recommended a biological monitoring limit value for dichloromethane in blood of 1 mg/L, and for dichloromethane in urine of 0.3 mg/L, both for samples collected at the end of a working shift. These figures were considered comparable to an 8-hour limit value of 100 ppm (353 mg/m3) for dichloromethane in air. The ACGIH recommended a Biological Exposure Index of 0.3 mg/L in urine at the end of a shift (ACGIH, 2012).
The Swiss authorities recommended a limit of 0.5 mg/L in blood (Suva, 2014). The Deutsche Forschungsgemeinschaft has provided the correspondence between concentrations in air and dichloromethane in blood (DFG, 2012).
- Exposure Data - Some Chemicals Used as Solvents and in Polymer ManufactureExposure Data - Some Chemicals Used as Solvents and in Polymer Manufacture
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