HEXANE ISOMERS

OSHA comments from the January 19, 1989 Final Rule on Air Contaminants Project extracted from 54FR2332 et. seq. This rule was remanded by the U.S. Circuit Court of Appeals and the limits are not currently in force.

CAS: None; Chemical Formula: C₆H₁₄

Previously, OSHA had no limit for the hexane isomers. The ACGIH TLVs for the hexane isomers are 500 ppm as an 8-hour TWA and 1000 ppm as a 15-minute STEL. NIOSH has a recommended TWA limit for these isomers of 100 ppm, supplemented with a 510-ppm 15-minute ceiling. The proposed and final rule PELs are an 8-hour TWA of 500 ppm and a 15-minute STEL of 1000 ppm. The hexanes are clear, highly volatile liquids with a mild gasoline-like odor.

A study by Drinker, Yaglou, and Warren (1943/Ex. 1-730) shows that humans exposed to 1400 to 1500 ppm of hexane experienced nausea and headache. Patty and Yant (1929, as cited in ACGIH 1986/Ex. 1-3, p. 307) found that a 10-minute exposure to 5000 ppm caused giddiness and dizziness in exposed subjects. A study by Nelson, Enge, Ross et al. (1943/Ex. 1-66) showed no effects in unacclimated subjects exposed to hexane isomers in concentrations of 500 ppm, but narcotic effects have often been seen in subjects exposed at levels above 1000 ppm (Elkins 1959d, as cited in ACGIH 1986, Ex. 1-3, p. 307). The ACGIH based its limit primarily on the Nelson, Enge, Ross et al. (1943/Ex. 1-66) study.

NIOSH recommends limits for the hexane isomers of 100 ppm as a 10-hour TWA and 510 ppm as a 15-minute short-term limit. These recommendations are based on human and animal evidence showing that exposure to n-hexane below concentrations of 500 ppm is associated with the development of polyneuropathy (Inoue, Takeuchi, Takeuchi et al. 1970/Ex. 1-75; Miyagaki 1967/Ex. 1-198); NIOSH (1977a/Ex. 1-233) did not distinguish between n-hexane and other hexane isomers when making its recommendation for an exposure limit. NIOSH concluded that all of the C5-C8 alkanes are potential neuropathic agents and should have the same PELs as those established for n-hexane.

OSHA disagrees with NIOSH that all C(5)-C(8) alkanes are potential neuropathic agents. As discussed in Section V of the preamble, OSHA believes that a metabolite of n-hexane (2,5-hexanedione) is responsible for the unique neurotoxic properties of n-hexane (see also the discussion of n-hexane in Section VI.C.1 of the Preamble). Thus OSHA agrees with the ACGIH that “it seems unlikely that all the hexanes would follow the same metabolic route in the body [as n-hexane], in view of the marked variations in structure of the molecule” (ACGIH 1986/Ex. 1-3, p. 307). The majority of commenters supported OSHA’s conclusion that n-hexane is uniquely toxic because of the presence of 2,5-hexanedione and that the other alkanes are not toxic in this way (Exs. 3-593, 3-896, and 3-1246). However, the AFL-CIO (Ex. 194, p. A-7) argued for a lower limit for the hexane isomers and all petroleum solvents (see the discussion for heptane, above), and the UAW (Ex. 197) noted that controls are available to reduce exposures (see Section VII for a discussion of feasibility).

After reviewing the evidence cited by the ACGIH (1986/Ex. 1-3), NIOSH (1977a/Ex. 1-233), and commenters to the record, OSHA finds that workers exposed to hexane isomers are at significant risk of experiencing narcosis and of developing neuropathy at exposure levels above the new PELs. The Agency concludes that establishing an 8-hour TWA of 500 ppm and a 15-minute STEL of 1000 ppm will substantially reduce these risks. OSHA finds that both narcosis and neuropathy constitute material health impairments.