Methane and Natural Gas Exposure Limits

PP-29-124 Background/Aims: A threshold limit value (TLV) of 1000 ppm for both hydrocarbons (C1–C4) and natural gas has been proposed by the American Conference of Governmental Industrial Hygienists (ACGIH), but the rationale and the science behind this has not been sufficiently explored. Methane and ethane (the major components of natural gas) are simple asphyxiants and the most toxic components (propane and butane) in the mixture are present in small amounts. Therefore, the rationale for the TLV for light hydrocarbons and natural gas needs to be explored on a scientific basis. Methods: A systematic search of literature was conducted using the following databases: PubMed, Scopus, Toxicology Abstracts, Health and Safety Science Abstracts, Compendex, OSHLINE, and TOXLINE. Search terms such as methane, natural gas, toxicity, aliphatic hydrocarbons, human health effects, source, exposure, TLV, occupational, environmental, hazard, and toxicology were included in the search string. Results: Having examined all the relevant databases and literature on the toxicity, epidemiology, exposure, and explosive limits of light hydrocarbons and natural gas, we confirm that methane behaves more as a simple asphyxiant than as a systemic or target organ toxin. Fatalities in workers exposed to methane in confined spaces are believed to be caused by a lack of oxygen or explosions having crossed the lower explosive limit, rather than the toxicity of methane. Conclusion: Methane appears to be a simple asphyxiant with no systemic toxicity and we propose that it should be assigned no occupational exposure limit; the same reasoning would apply to natural gas which contains 92% methane. However, considering the explosive characteristics, the other option would be to assign the occupational exposure limits for methane (5300 ppm), ethane (3000 ppm), and natural gas (5300 ppm) at 10% of their lower explosive limits. The current exposure guidelines for propane and butane would remain unchanged in consideration of their chemical and biological properties.

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