ETHYL 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: 75-08-1; Chemical Formula: C₂H₅SH

OSHA previously had a ceiling limit of 10 ppm for ethyl mercaptan. An 8-hour TWA limit of 0.5 ppm was proposed for this substance, based on the ACGIH (1986/Ex. 1-3) recommendation; NIOSH (Ex. 8-47, Table N1) concurred with OSHA’s proposal. The final rule establishes a PEL of 0.5 ppm as an 8-hour TWA for ethyl mercaptan. Ethyl mercaptan is a colorless liquid with a persistent and penetrating leek-like odor.

Acute animal toxicity data concerning ethyl mercaptan are taken from a single study that reports the following findings. The 4-hour inhalation LC(50) values in rats and mice are 2770 ppm and 4420 ppm, respectively. In the rat, the intraperitoneal LD(50) is reported to be approximately 450 mg/kg. One drop applied to rabbit eyes caused only slight irritation, but high concentrations of vapor caused considerable irritation within 15 minutes. Maximal sublethal intraperitoneal doses have been reported to induce deep sedation, with higher exposures causing restlessness, muscular incoordination, skeletal muscular paralysis, cyanosis, respiratory depression, coma, and death. Although inhalation tests showed no noteworthy pathology in rats, intraperitoneal injection caused lymphatic infiltration of the liver with occasional necrosis (Fairchild and Stokinger 1958/Ex. 1-415). In chronic inhalation studies of rabbits, rats, and mice, a five-month exposure to 40 ppm caused minimal cardiovascular and other systemic effects (Blinova 1965/Ex. 1-603).

Studies of human volunteers, exposed at 4 ppm for three hours daily for 5 to 10 days, have reported adverse effects. At this level, all subjects experienced altered taste and olfactory reactions, periodic nausea, mucous membrane irritation, and fatigue. Exposure to 0.4 ppm produced no unpleasant symptoms (ACGIH 1986/Ex. 1-3, p. 262).

The Workers Institute for Safety and Health (WISH) (Ex. 116) was critical of OSHA’s proposal to establish an 8-hour TWA limit rather than a STEL or ceiling for ethyl mercaptan. OSHA believes that the health evidence on ethyl mercaptan shows that a 0.5 ppm TWA limit will be sufficient to reduce the adverse acute effects associated with exposure to this substance; for example, a 3-hour exposure to 4 ppm, which caused adverse acute effects in human volunteers (ACGIH 1986/Ex. 1-3, p. 262), would exceed 0.5 ppm as an 8-hour TWA. The health evidence discussed above demonstrates that, at the previous PEL of 10 ppm (ceiling), employees were at risk of nausea, fatigue, and irritation; these effects have been demonstrated to occur on exposure to 4-ppm concentrations of this substance for just a few days. OSHA considers these exposure-related effects of nausea, fatigue, and irritation to be material impairments of health. The Agency concludes that the revised limit of 0.5 ppm will substantially reduce this significant risk. Therefore, OSHA is lowering its limit for ethyl mercaptan to 0.5 ppm as an 8-hour TWA.