Dense Nonaqueous Phase Liquids (DNAPLs)

Halogenated Alkanes

Methylene Chloride

Human Health Toxicity

Methylene chloride (MC, or dichloromethane) does not occur naturally. It is present in the environment as a result of fugitive emissions originating from plants that manufacture the compound, as well as from its industrial uses as a degreasing agent, a feedstock for the production of other chemicals, and an extraction solvent by laboratories, the pharmaceutical industry, and food processors. MC has widespread domestic use as a paint stripper and thinner, a component of wood stains and varnishes, and a propellant in many aerosol spray products (NTP 2005).

The general population is exposed to MC via inhalation of both ambient air and vapors from consumer products, such as paint strippers and spot cleaners. In addition, populations can be exposed by the ingestion of drinking water, as MC has been found in finished drinking water, commercially bottled artesian well water, and groundwater (NTP 2005). If water containing MC is used for showering, there is a possibility of exposure via dermal absorption and inhalation. Little information is available relating to the presence of residual MC in foodstuffs.

MC is readily absorbed into the bloodstream from the lungs and gastrointestinal tract, and more slowly through the skin. Once absorbed into the bloodstream, MC is taken up by adipose tissues and the liver, and to a lesser extent, the brain, kidney, and adrenals. It is unlikely that MC bioaccumulates in tissues. The compound is metabolized by two pathways: oxidative transformation by cytochrome P-450 enzymes and glutathione conjugation. The metabolic end products are carbon dioxide and carbon monoxide, but both pathways can give rise to reactive, toxic intermediary metabolites. It has been suggested that the reactive metabolites might be responsible for the genotoxic effects of the compound in some cell types (ATSDR 2000).

High concentrations of MC are acutely toxic and can be lethal. Deaths from workplace exposure have been reported and appear to be due to the anesthetic action of MC on the central nervous system, causing narcosis and respiratory depression. The mechanism of action on the central nervous system is unknown. Both acute and long-term studies in laboratory animals suggest MC is hepatotoxic, i.e., it damages the liver of the animals under test; however, little information is available on the effects of MC on the human liver (ATSDR 2000).

Long-term laboratory rodent studies suggest that MC is carcinogenic, resulting in liver tumors in exposed subjects, but there is no clear evidence for the carcinogenicity or non-carcinogenicity of MC in humans (IRIS).

Few human reproductive or developmental studies of MC are available. Some evidence suggests an increase in the rate of spontaneous abortion in workers exposed to MC via inhalation, but this effect did not reach statistical significance. A group of workers reported testicular atrophy, low sperm counts, and recent infertility after direct contact with liquid MC for more than a year.

Inhalation exposure of two generations of laboratory rodents to MC did not suggest adverse effects on fertility and litter size. Few investigations have assessed the developmental effects of MC in humans, but one study of 90,000 births found no significant effect on birth weight from environmental concentrations of the compound. Adverse developmental effects of MC can be seen in laboratory rodent studies, but only at doses causing maternal toxicity (ATSDR 2000).

MC gives mixed results in tests for genotoxicity. Both positive and negative results are reported for in vitro tests using bacterial and mammalian cell lines. In mammalian systems, MC might act as a weak mutagen (ATSDR 2000).

EPA's Integrated Risk Information System (IRIS) classifies MC (as dichloromethane) as a "probable human carcinogen, based on inadequate human data and sufficient evidence of carcinogenicity in animals." In addition, NTP's Report on Carcinogens (2005) states the compound is "reasonably anticipated to be a human carcinogen." The overall evaluation provided by IARC says "Dichloromethane is possibly carcinogenic to humans (Group 2B)."

Human Health References

Dichloromethane (CASRN 75-09-2)
U.S. EPA, Integrated Risk Information System (IRIS).

Scope of the Risk Evaluation for Methylene Chloride (Dichloromethane, DCM)
EPA 740-R-17-006, 72 pp, 2017

EPA presents the occupational scenarios in which workers and occupational non-users might be exposed to methylene chloride during conditions of use, such as manufacturing, processing, repackaging, and recycling. EPA believes that workers and bystanders as well as certain other groups of individuals may experience greater exposures to methylene chloride than the general population. The report is accompanied online by a separate extensive bibliography of literature concerning the chemical's fate, exposure, and environmental and human health hazards.

Toxicological Profile Methylene Chloride
Agency for Toxic Substances and Disease Registry (ATSDR), U.S. Department of Health and Human Services, 313 pp, 2000

Dichloromethane
IARC Monographs on the Evaluation of Carcinogenic Risks to Humans. International Agency for Research on Cancer (IARC), Vol. 71, pt. 1, 66 pp, 1999

Dichloromethane (Methylene Chloride) CAS No 75-09-2
Report on Carcinogens, Fourteenth Edition. U.S. Department of Health and Human Services, Public Health Service, National Toxicology Program (NTP), 2016

Ecological Toxicity

Little information is available regarding the toxicity of MC to ecological receptors, particularly terrestrial species. The few available studies detail the responses of aquatic species to MC. Toxicological studies of plants, amphibians, and insects are cited in the Pesticide Action Network (PAN) Pesticides Database, but summary information is not readily available. LC₅₀ studies have found that MC is not acutely toxic (LC₅₀ values >100 mg/L) to opossum, brine, and daggerblade shrimps; a rotifer species; and a species of water flea. An evaluation of the compound's toxicity to eight species of fish indicated that it was not acutely toxic in seven species and was slightly toxic (LC₅₀ in the range of 10 to 100 mg/L) to the eighth species. An ecological screening value for MC to aid in the evaluation of risk to burrowing animals is presented in Roy et al. (2009).

Ecological References

Methylene Chloride: Identification, Toxicity, Use, Water Pollution Potential, Ecological Toxicity and Regulatory Information
Kegley, S.E., B.R. Hill, S. Orme, and A.H. Choi.
PAN Pesticide Database. Pesticide Action Network, San Francisco, CA, 2010