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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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