n-BUTYL MERCAPTAN

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: 109-79-5; Chemical Formula: CH₃CH₂CH₂CH₂SH

n-Butyl mercaptan is a colorless, flammable liquid and has a strong, obnoxious, garlic-like odor. It is used as a solvent, a chemical intermediate, and an odorant for natural gas. OSHA’s previous limit for n-butyl mercaptan was 10 ppm as an 8-hour TWA. OSHA proposed a lower limit of 0.5 ppm TWA, based on the ACGIH recommendation, and the final rule establishes this limit.

Humans exposed to concentrations of n-butyl mercaptan report that the “readily noticeable” odor level for this substance is between 0.1 and 1 ppm, although the odor threshold is significantly below this level (ranging from 0.001 to 0.0001 ppm). Gobbato and Terribile (1968/Ex. 1-178) have reported that symptoms of CNS toxicity occurred in humans exposed for one hour to concentrations of n-butyl mercaptan believed to lie in the range of 50 to 500 ppm. These same authors reported that mucosal irritation occurred in human volunteers exposed to 4 ppm of ethyl mercaptan, a closely related substance. Irritation did not occur at exposures to 0.4 ppm. The ACGIH established the TLV for n-butyl mercaptan at 0.5 ppm, to protect against the intolerable odor effects, mucosal irritation, and CNS toxicity that occur on exposure to higher concentrations of this substance.

The current PEL of 10 ppm is between 10 and 100 times higher than the concentration of n-butyl mercaptan that is readily detected by smell and is more than twice the concentration reported as causing mucosal irritation for a closely related substance. OSHA finds that workers are at risk of significant acute effects in the absence of a more stringent limit.

In its pre hearing comments, NIOSH (Ex. 8-47, Table N7) pointed out that it has recommended a 0.5-ppm ceiling limit for n-butyl mercaptan, rather than a TWA limit, for this substance. No other comments were submitted to the record. In accordance with the criteria in its June 7, 1988 NPRM (53 FR 20977), OSHA is establishing the 0.5-ppm TWA limit for n-butyl mercaptan. The Agency concludes that this PEL will substantially reduce the risks of irritation, CNS toxicity, and intolerable odor effects, which together constitute material health impairments.