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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants B7 Methane Hector D. Garcia, Ph.D., and John T. James, Ph.D. Johnson Space Center Toxicology Group Biomedical Operations and Research Branch Houston, Texas PHYSICAL AND CHEMICAL PROPERTIES Methane is a colorless, odorless, nonpoisonous, flammable gas that burns with a pale, faintly luminous flame. Methane is soluble in alcohol, ether, and other organic solvents and is slightly soluble in water (Sax, 1984; Budavari, 1976). It forms explosive mixtures with air and reacts violently with halogens, interhalogens, and oxidizers when exposed to heat or flame (Sax, 1984). Formula: CH4 CAS number: 74-82-8 Synonyms: Natural gas, marsh gas, firedamp, methyl hydride Molecular weight: 16.04 Boiling point: −161.4°C Melting point: −182.6°C Density: 0.554 (air = 1) or 0.7168 g/L Vapor pressure: 40 mm Hg at −86.3°C Explosion limits: 5.3-14% in air (Sax, 1984) Atmospheric concentration: 0.00022% by volume (Budavari, 1976) Autoignition temperature: 650°C
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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants OCCURRENCE AND USE Methane occurs naturally as a product of decaying vegetable matter. In population studies, 35-61% of healthy human subjects have detectable methane in their breath (Segal et al., 1988). It is used as a fuel for illuminating and cooking; in organic syntheses; and in the manufacture of hydrogen, hydrogen cyanide, ammonia, acetylene, and formaldehyde. PHARMACOKINETICS Methane installed into the human colon is exhaled (Bjorneklett and Jenssen, 1982). Inhalation of commercial natural gas scented with 16.8 mg/m3 tert-butylmercaptan and 7.2 mg/m3 of methyl acrylate by mice in a gas chamber for 2 h produced isopropanol, acetone, sec-butyl alcohol, and methyl ethyl ketone in the blood and liver (Tsukamoto et al., 1985). Even though the authors considered these substances to be metabolites of the natural gas, the study did not distinguish between metabolism of the methane and metabolism of the added fragrance compounds. The authors also stated that methane (and propane) “can exert an anesthetic effect on animals even when sufficient oxygen exists, the results sometimes proving lethal” (Tsukamoto et al., 1985); however, they gave no citation for that report. No studies supporting this assertion were found. TOXICITY SUMMARY Methane is a simple asphyxiant and does not produce general systemic effects (Klaassen et al., 1986). Studies of the morphology of lung tissues of rats exposed to 8% methane, 20% oxygen for 1 h, followed by 100% methane found no effects attributable to the chemical specificity of methane but only to the decrease of oxygen (Morita and Tabata, 1988). Although patients with large bowel cancer have been found to have high levels of methane in their breath, no relationship has been found between in vivo production of methane and the risk of developing large bowel cancer (Segal et al., 1988; Flick et al., 1990). No studies on the effects of chronic inhalation of methane were found in the literature. The bowel cancer studies, however, found that outwardly healthy subjects had breath methane
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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants levels of up to 56 ppm, and in some people, methane (presumably generated by fecal microorganisms) is present in colonic gases but is not excreted in the breath (McKay et al., 1985). TABLE 7-1 Exposure Limits Set by Other Organizations Organization Concentration ACGIH's TLV None set for simple asphyxiants OSHA's PEL None set NIOSH's REL None set NIOSH's IDLH None set TLV = threshold limit value. PEL = permissible exposure limit. REL = recommended exposure limit. IDLH = immediately dangerous to life and health. TABLE 7-2 Spacecraft Maximum Allowable Concentrations Duration ppm mg/m3 Target Toxicity 1 h 5300 3800 0.1 × lower explosive limit 24 h 5300 3800 0.1 × lower explosive limit 7 d 5300 3800 0.1 × lower explosive limit 30 d 5300 3800 0.1 × lower explosive limit 180 d 5300 3800 0.1 × lower explosive limit RATIONALE SMACs were set on the basis of methane's explosive properties rather than its toxicity with the following considerations. First, methane has no demonstrable toxicity and does not produce general systemic effects other than being a simple asphyxiant. Air at sea level contains about 21% oxygen. Humans at rest are not significantly affected until the oxygen concentration falls to 14%, but with exercise, anoxia develops quickly owing to the limited rate of diffusion of oxygen through the lung's alveolar walls (Rogan, 1972). Table 7-3 shows the relationship between exercise, oxygen saturation of the blood, and oxygen concentration in the atmosphere (Rogan, 1972).
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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants TABLE 7-3 The Relationship Between Exercise and Oxygen Concentrations Oxygen in Air at 760 mm Hg, % Amount of Exercise 21 20 19 18 17 16 15 14 Resting Walking • Hurrying, moderate work • • • Heavy work • • • • = adequate oxygen saturation. • = desaturation likely (danger of rapid loss of consciousness). Even if an astronaut performed heavy work in an atmosphere containing a simple asphyxiant, Table 7-3 shows that a level of oxygen of about 18% would be adequate to prevent loss of consciousness. For methane to dilute the oxygen in the atmosphere to 18% would require the methane concentration to reach 14.3%: Second, as shown in Figure 7-1, 14% methane and 18% oxygen is capable of forming explosive mixtures if diluted slightly with air. If the atmosphere contains an appreciable amount of hydrogen, the Coward's triangle will have a different shape, and explosions can occur with a lower percentage of oxygen. The shape of the triangle can also be varied by high concentrations of carbon monoxide (Rogan, 1972). Thus, before increasing methane concentrations could reach asphyxiating levels, methane would form explosive mixtures with air. Also, if provision is made for maintaining 21% oxygen in the spacecraft by injection of pure oxygen, the assumption, based on Coward's triangle, is made that the lower-explosive-limit concentration of methane would not change appreciably. Therefore, SMACs were set at one-tenth the lower explosive limit of 5.3% methane. A safety factor of 10-fold below the explosive limit was used to allow for some inhomogeneities in the local distribution of methane near the site of leakage or generation of the methane, assuming that air circulation within the spacecraft produces moderately effective mixing.
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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants FIGURE 7-1 The explosive limits of methane—Coward's Triangle. REFERENCES Bjorneklett, A. and E. Jenssen. 1982. Relationships between hydrogen (H2) and methane (CH4) production in man. Scand. J. Gastroenterol. 17:985-992. Budavari, S. , ed. 1976. P. 938 in The Merck Index , 11th Edition. Merck & Co., Rahway, N.J. Flick, J.A., S.R. Hamilton, F.J. Rosales, and J.A. Perman. 1990.
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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants Methane excretion and experimental colonic carcinogenesis. Digestive Dis. Sci. 35:221-224. Klaassen, C.D., M.O. Amdur, and J. Doull, eds. 1986. P. 658 in Casarett and Doull's Toxicology , 3rd Edition. Macmillan, New York. McKay, L.F., M.A. Eastwood, and W.G. Brydon. 1985. Methane excretion in man—A study of breath, flatus, and faeces. Gut 26:69-74. Morita, M. and N. Tabata. 1988. Studies on asphyxia: On the changes of the alveolar walls of rats in the hypoxic state. II. The hypoxic state produced by carbon dioxide and methane gases. Forensic Sci. Int. 39:257-262. Sax, N.I. , ed. 1984. Pp. 1762-1763 in Dangerous Properties of Industrial Materials , 6th Edition. Van Nostrand Reinhold, New York. Segal, I., A.R.P. Walker, S. Lord, and J.H. Cummings. 1988. Breath methane and large bowel cancer risk in contrasting African populations Gut 29:608-613. Tsukamoto, S., S. Chiba, T. Ishikawa, and M. Shimamura. 1985. Experimental study on the metabolism of volatile hydrocarbons by inhalation of natural gas (13 A-Tokyo Gas). Nihon Univ. J. Med. 27:33-38. Rogan, J.M. , ed. 1972. Pp. 224-227 in Medicine in the Mining Industries. William Heinemann Medical Books, London.
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