The widespread use of methyl chloroform as an industrial solvent and early interest in the chemical as an anesthetic have resulted in extensive study of its health effects. Research on the carcinogenic effects of the chemical on rats and mice has produced no significant adverse findings, nor have studies of possible teratogenic or reproductive effects in animals. Similarly, a study of the non-carcinogenic health effects of methyl chloroform on an exposed human population found no differences between the study and control groups.

After reviewing the available data, the Environmental Protection Agency (EPA) Health Assessment Document for methyl chloroform concluded that "[t]he weight of available evidence obtained from both human and animal data suggest that long-term exposure to environmental levels of methyl chloroform poses no serious health concern to the general population." While no definitive assessment of the compound's carcinogenic potential for humans is possible, EPA concluded that "[t]he likelihood of adverse health effects resulting from chronic exposure to the ambient air levels commonly encountered appears to be extremely low based on presently available data." Nevertheless, methyl chloroform is included, along with 188 other substances, in a list of hazardous air pollutants in Section 112 of the Clean Air Act, as amended in 1990. EPA will be regulating certain major and area sources of these substances, such as degreasing operations, in the coming years.

More significantly, the production of methyl chloroform is being phased out under the Montreal Protocol on Substances That Deplete the Ozone Layer, as amended in June 1990 and November 1992 and under Title VI of the federal Clean Air Act. Title VI of the Clean Air Act also establishes controls on certain uses of methyl chloroform and imposes labeling requirements for certain products containing or made with the substance. In addition, methyl chloroform produced or imported after January 1, 1991 is subject to a federal excise tax under the Omnibus Budget Reconciliation Act of 1990, as amended in 1992.

Methyl chloroform is a versatile, all purpose solvent, popular with industry because of its powerful cleaning properties, low flammability, and low relative toxicity. It was introduced in the mid-1950's as a cold cleaning solvent substitute for carbon tetrachloride. Today, methyl chloroform is used primarily for vapor degreasing and cold cleaning of fabricated metal parts and other materials. The chemical also is used in fluoropolymer synthesis, as a solvent in adhesive and aerosol formulations, for the production of certain coatings and inks, for a variety of textile applications, and for dry cleaning leather and suede garments.

Methyl chloroform is a member of a family of saturated aliphatic halogenated hydrocarbons. The colorless, volatile liquid is produced in the United States by Dow Chemical U.S.A., PPG Industries, Inc., and Vulcan Materials Company. Total U.S. demand for the solvent in 1989 was 308,000 metric tons (680 million pounds) of which a small percentage was imported.

The health effects of methyl chloroform are well characterized after many years of industrial use. The primary effects of acute overexposure to the chemical are to the central nervous system (e.g., light-headedness, drowsiness, headache). Lethal effects of excessive concentrations (at 1 0,000 ppm) of the chemical have been reported after accidental exposure or abuse. In these cases, death resulted from respiratory and cardiac failure caused by depression of the central nervous system. Cardiac sensitization also may have contributed. Prolonged exposure (15 minutes or more) to high concentrations (1000 to 2000 ppm) of the solvent in the air results in mild irritation of the eyes and respiratory tract. In its liquid form, methyl chloroform causes irritation to the skin and eyes upon contact.

Animal Studies

Several bioassays have been conducted in laboratory animals to test the carcinogenic potential of commercial formulations of methyl chloroform administered by inhalation or by oral intubation (gavage). None of these studies has produced positive results for carcinogenicity.

In 1977 the National Cancer Institute performed a gavage study of Osborne-Mendel rats and B6C3Fl mice. In this study, treatments of 750 and 1500 milligrams/kilogram (mg/kg) body weight five times a week for 78 weeks did not induce tumors. However, the relatively high mortality of the treated animals limited the study's sensitivity. A second gavage study, conducted by the National Toxicology Program (NTP), was judged inadequate by NTP to evaluate the carcinogenicity of methyl chloroform.

A more conclusive study was performed by The Dow Chemical Company, exposing Sprague-Dawley rats by inhalation to 875 and 1750 ppm of methyl chloroform under conditions similar to those of exposed workers (6 hours/day, 5 days/week, for 12 months). After observing the animals until the age of 31 months, no significant difference in tumor incidence was found between the treatment and control animals.

A second study by Dow Chemical exposed B6C3Fl mice and Fischer 344 rats by inhalation to levels between 150 and 1500 ppm for 6 hours/day, 5 days/week over the animals' lifetime (24 months). Again, no observable difference in the incidence of benign or malignant tumors between the test animals and their respective controls was found.

Methyl chloroform has been evaluated in several studies for its potential to adversely affect reproductive outcome in laboratory animals. Inhalation studies in rats and mice at levels of 875 and 2100 ppm did not produce abnormal birth ef f ects.

In a multigenerational study of ICR Swiss mice exposed to concentrations of between 100 and 1000 mg/kg body weight per day in their drinking water, no adverse effects on fertility, gestation, or pup survival were observed.

Pursuant to an EPA test rule, HSIA sponsored developmental toxicity studies of inhaled methyl chloroform in Sprague Dawley rats and New Zealand white rabbits at exposure concentrations of 0, 1000, 3000, and 6000 ppm. The chemical did not produce developmental effects in the offspring of either species following exposure of pregnant females to doses that were high enough to elicit toxicity in the females. Some reduction in maternal weight gain was noted in the rats at concentrations above 1000 ppm and in the rabbits above 3000 ppm.

Industrial usage of methyl chloroform does not contribute significantly to an increased ambient ozone (smog) concentration in the troposphere (lower atmosphere) or to the resulting public health risk. Rather than reacting in the lower atmosphere to form smog, methyl chloroform is removed through a reaction with readily available hydroxyl radicals (*OH). As a result, methyl chloroform is not regulated by EPA or by most states as a volatile organic compound.

Under EPA's accelerated phase-out schedule adopted pursuant to the Clean Air Act, production (and import) of methyl chloroform for other than feedstock uses will be phased out in accordance with the following schedule (allowable production expressed as a percentage of 1989 production)

        1994 - 50 %

        1995 - 30 %

        1996 - essential uses only

The accelerated phase-out schedule contains somewhat more stringent production limits than the amended Montreal Protocol, allowing only 30 percent of 1989 production (instead of 50 percent) in 1995. Both the accelerated phase-out schedule and the amended Protocol, however, require a 50 percent reduction in 1994, and phase out production of methyl chloroform in 1996.

Replacement of methyl chloroform in various uses, including metal cleaning, adhesives and coatings, and aerosols, will be directly affected by the significant new alternatives policy (SNAP) program, which EPA is in the process of implementing. A number of acceptable and unacceptable alternatives to methyl chloroform have been identified in these use sectors. EPA has provisionally designated perchloroethylene, trichloroethylene, and methylene chloride as acceptable alternatives for most methyl chloroform applications.

Effective May 15, 1993, containers of and products containing methyl chloroform introduced into commerce must bear a "clearly legible and conspicious" label stating that they contain a substance which harms public health and environment by destroying ozone in the upper atmosphere. Products made using methyl chloroform must bear a label so stating unless: (i) they were manufactured solely for export and are clearly identified as such, (ii) their manufacturer has achieved a total reduction of solvent use of methyl chloroform and/or CFC-113 of 95 percent or more, or (iii) EPA determines that no acceptable substitute product or processes exists.

Methyl chloroform is generally unaffected by the bans on non-essential products adopted by EPA in 1993 and 1994.

As part of the 1990 budget reconciliation, Congress extended the excise tax on substances with ozone depletion potential to methyl chloroform. The change conformed the list of taxable substances to the list of substances controlled under the amended Montreal Protocol. The tax applies to methyl chloroform sold or used by a producer or importer after December 31, 1990.

In 1992, the excise tax was amended to increase the tax for all substances, including methyl chloroform. Effective January 1, 1993, the increased tax applies to methyl chloroform according to the following schedule:

                Year                                  Excise Tax (cents/lb.)

                1993                                          21.1

                1994                                          43.5

                1995                                          53.5

After 1995, the excise tax on methyl chloroform will increase by 4.5 cents/lb each year.

Individual methyl chloroform users and distributors are not directly responsible for paying the tax to the Internal Revenue Service (IRS) on quantities of the virgin chemical purchased after 1990. They are responsible, however, for paying tax on quantities of the chemical in stock on January 1 of 1991, 1993, and the first of the year for each year thereafter (tax-increase dates). This "floor-stocks" tax generally applies to users and distributors who, on a tax-increase date, hold more than 400 pounds of methyl chloroform and other taxable chemicals for use or sale. Proposed regulations would modify this rule beginning January 1, 1995. The floor stocks tax must be paid on or before June 30. In addition, a tax is applied to imported products made with more than a de minimis amount of methyl chloroform.