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Emergency and Continuous Exposure Limits for Selected Airborne Contaminants, Volume 1 METHANE BACKGROUND INFORMATION PHYSICAL AND CHEMICAL PROPERTIES Structural formula: CH4 Molecular weight: 16.04 CAS number: 74–82–8 Boiling point: −161.49°C Freezing point: −182.48°C Vapor pressure: 40 mm Hg (−86.3°C) Flash point: −187.78°C Flammability limits: 5.3–14% Physical state: A colorless, odorless, flammable gas and the major component of natural gas. It forms explosive mixtures with air and is moderately soluble in water. SUMMARY OF TOXICITY INFORMATION Little information is available on the toxicity of methane. It appears that toxic effects of methane, considered biologically inert, are related to the oxygen deprivation that occurs when the simple alkane is present in air at a high concentration. Hunter (1978) stated that miners evacuate coal pits when the methane concentration in air reaches 2.5% by volume; it is not clear whether evacuation is prompted by the threat of a health hazard or by the danger of explosion. Kamens and Stern (1973) referred to a literature survey that indicated that methane is biologically inert and that exposure to methane at 10,000 ppm had no toxic effect; conditions of exposure and identification of the test animal were not given, but a U.S. Department of Health, Education and Welfare report was cited (1970). A report by Pennington and Fuerst (1971) supported the biologic inertness of methane. Suspensions of rabbit erythrocytes were exposed for 18 h to methane by bubbling through the suspensions at 150 ml/min. Such exposure had little or no effect on the color or morphology of the red cells, on the pH of the medium, on the ATP content of the cells, or on the electrophoretic pattern and UV/VIS absorption spectra of the hemoglobin obtained from exposed cells. Forney and Harger (1972), however, offered evidence that methane has mild anesthetic properties that cannot be explained by oxygen deficiency alone, although such deficiency does seem to be the most important factor. Two of six mice exposed to 70% methane in air died in 18 min, whereas mice exposed to 70% nitrogen in air developed only ataxia. Animals exposed to 50–90% methane in oxygen showed mild depression and a marked decrease in locomotion, but no ataxia. Thus, the toxic effect of methane is much greater than that of nitrogen when available oxygen is low, but methane has little effect when oxygen is readily available. It seems that the toxicity of methane should be
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Emergency and Continuous Exposure Limits for Selected Airborne Contaminants, Volume 1 discussed not alone, but rather with respect to the partial pressure of oxygen in the atmosphere in question. Carpenter (1954) demonstrated an anesthetic effect of methane under hyperbaric conditions in mice; 50% of a group of mice exposed to 2.9 atm of methane did not develop convulsions in response to electroshock treatment. EXPLOSION HAZARD OF METHANE Methane forms explosive mixtures with air and the loudest explosions occur when one volume of methane is mixed with 10 volumes of air (or 2 volumes of oxygen) (Windholz et al., 1976). Air containing less than 5.5% methane no longer explodes. The CRC Handbook of Chemistry and Physics (Weast, 1978–1979) gave the limits of flammability of methane as 5% and 15% by volume in air at room temperature. INHALATION EXPOSURE LIMITS ACGIH (1982) lists methane in its category of simple asphyxiants. This is described as being gases and vapors, which when present in high concentrations in air, act as simple asphyxiants without other significant physiologic effects. TLVs are not recommended because the limiting factor is the available oxygen. COMMITTEE RECOMMENDATIONS EXPOSURE LIMITS In 1966, the Committee on Toxicology set an EEL and a CEL for methane: 24-h EEL: 5,000 ppm 90-d CEL: 5,000 ppm No rationale accompanied these limits. It is obvious that an exposure limit that presents an explosion hazard cannot be recommended, even if it is well below a concentration that would produce toxicity; thus, exposure limits should not exceed 5% by volume in air. Animals exposed to methane at 10,000 ppm showed no toxic efects; an uncertainty factor of 2 is suggested to derive an EEL—5,000 ppm. There is no evidence that duration of exposure is important in methane toxicity. Therefore, no change in the previously recommended exposure limits seems necessary.
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Emergency and Continuous Exposure Limits for Selected Airborne Contaminants, Volume 1 REFERENCES American Conference of Governmental Industrial Hygienists. 1982. Threshold Limit Values for Chemical Substances and Physical Agents in the Work Environment with Intended Changes for 1982. Cincinnati, OH.: ACGIH. [93 p.] Carpenter, F.G. 1954. Anesthetic action of inert and unreactive gases on intact animals and isolated tissues. Amer. J. Physiol. 178:505–509. Forney, R.B., Jr., and Harger, R.N. 1972. Reaction of mice from acute exposure to various concentrations of methane, ethane, propane and butane in air, or in oxygen. Edinburgh, Scotland: Sixth International Meeting of Forensic Sciences. [12 p.] Hunter, D. 1978. The Diseases of Occupations. 6th ed. London: Hodder and Stoughton. p. 630–632. Kamens, R.M. and Stern, A.C. 1973. Methane in air quality and automobile exhaust emission standards. J. Air Pollution Control Assoc. 23:592–596. Pennington, K., and Fuerst, R. 1971. Biochemical and morphological effects of various gases on rabbit erythrocytes. Arch. Environ. Health 22:476–481. U.S. Dept. Health, Education and Welfare. 1970. Air Quality Criteria for Hydrocarbon. National Air Pollution Control Admin. Publ. No. AP-64. Washington, DC: U.S. Government Printing Office, p. 7–1, 7–3. Weast, R.C. ed. 1978–1979. CRC Handbook of Chemistry and Physics. 59th ed. West Palm Beach, FL: CRC Press, Inc. Windholz, M., Budavari, S., Stroumtsos, L.Y., and Fertig, M.N. 1976. The Merck Index: An Encyclopedia of Chemicals and Drugs. 9th ed. Rahway, NJ: Merck and Co. p. 776.
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