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Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 5
approach taken in SMAC development for methane, individual SMACs for these compounds were based on 10% of the lower limits of explosivity for each of these gases. By maintaining concentrations below each chemical’s limit, health concerns (including the potential for them to act as simple asphyxiants) are minimized. The C5-C9 alkanes are treated as a group, with the exception of n-hexane. Significant research and epidemiologic findings have established that n-hexane differs from the other alkanes (and from the other hexane isomers) in its potential to cause peripheral neuropathy (Spencer et al. 1980, Takeuchi et al. 1980, Frontali et al. 1981, Filser et al. 1996, ACGIH 2008). Because consideration of this specific effect for n-hexane would likely result in a group SMAC toxicologically irrelevant for most of members of the group, n-hexane is reserved for separate SMAC development and is not fully addressed in this document from a toxicologic standpoint.
PHYSICAL AND CHEMICAL PROPERTIES
Physical and chemical properties for the C2-C9 alkanes are presented in Table 6-1. Ethane (C2) through butane (C4) exist as gases at standard temperature and pressure, whereas pentane (C5) to nonane (C9) are liquids. Various branched isomers exist for many of the n-alkanes, and the physical and chemical properties for these isomers may differ from those presented in Table 6-1, which are specific to the n-alkanes.
OCCURRENCE AND USE
The group of compounds that comprise the C2-C9 saturated aliphatic alkanes (the linear and branched alkanes from ethane through nonane) are present in earth gases and crude oil and have a variety of commercial and industrial applications. The gaseous alkanes within this group (ethane, propane, and butane) are principal components of natural gas and are used widely as fuels and propellants (Sandmeyer 1981). They are also greenhouse gases and contribute to the formation of ground-level ozone (Katzenstein et al. 2003). The liquid alkanes (pentane, hexane, heptane, octane, and nonane) also may be found at low concentrations in natural gas but are known more for their applications as solvents and as important components of crude oil, diesel, and gasoline. n-Hexane is a widely used industrial solvent, as are pentane and heptane (Finkel 1983). Octane is used extensively as a preignition additive for high-compression engine fuels; other alkanes are either found in or are added to gasoline and other fuels. They are also formed and released after combustion of the fuels in automobiles, boilers, and other machinery (Sandmeyer 1981). The human body produces some of the more volatile alkanes (e.g., ethane and pentane) endogenously as a result of the breakdown of polyunsaturated fatty acids (Galvin and Marashi 1999); they can be measured at low concentrations in human breath (Frank et al. 1980).