PHENYL 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: 108-98-5; Chemical Formula: C₆H₅SH

OSHA previously had no exposure limit for phenyl mercaptan. The ACGIH has a TLV-TWA of 0.5 ppm. NIOSH recommends a 15-minute ceiling limit of 0.1 ppm for phenyl mercaptan (benzenethiol). The Agency proposed a permissible exposure limit of 0.5 ppm as an 8-hour TWA, and the final rule establishes this limit. Phenyl mercaptan is a colorless liquid with an offensive, garlic-like odor.

The primary acute hazards of exposure to phenyl mercaptan are central nervous system stimulation followed by post-convulsive CNS depression, severe eye and skin irritation, systemic toxicity to spleen, kidney, lung, and liver tissues, and narcotic effects (ACGIH 1986/Ex. 1-3, p. 478).

Phenyl mercaptan has been reported to have 4-hour inhalation LC(50) values of 33 and 28 ppm for rats and mice, respectively (Doull and Plzak 1962, as cited in ACGIH 1986/Ex. 1-3, p. 478; Fairchild and Stokinger 1958/Ex. 1-415). The oral LD(50) for the rat is reported to be 46 mg/kg (McCord and Witheridge 1949/Ex. 1-882; Robles 1975, as cited in ACGIH 1986/Ex. 1-3, p. 478). For the rabbit and rat, the dermal LD(50) values are 134 mg/kg and 300 mg/kg, respectively (Doull and Plzak 1962, as cited in ACGIH 1986/Ex. 1-3, p. 478; Fairchild and Stokinger 1958/Ex. 1-415); Schafer 1972/Ex. 1-362). The responses of animals to phenyl mercaptan exposure were uniform regardless of species, and progressed from CNS stimulation to incoordination, skeletal and muscular paralysis, and respiratory depression, followed at high concentrations by coma and death. High doses (not further specified) administered via inhalation produced lung, liver, and kidney changes in mice (Doull and Plzak 1962, as cited in ACGIH 1986/Ex. 1-3, p. 478; Fairchild and Stokinger 1958/ Ex. 1-415); Schafer 1972/Ex. 1-362). In rabbits, phenyl mercaptan is a severe eye and skin irritant (McCord and Witheridge 1949/Ex. 1-882; Robles 1975, as cited in ACGIH 1986/ Ex. 1-3, p. 478; Schafer 1972/Ex. 1-362).

In humans, phenyl mercaptan is a moderately toxic skin irritant and causes severe dermatitis, headaches, and dizziness at unspecified levels (Fairchild and Stokinger 1958/Ex. 1-415; McCord and Witheridge 1949/Ex. 1-882). NIOSH (Ex. 8-47, Table N7; Tr. p. 3-99) believes that the limit for phenyl mercaptan is better expressed as a ceiling than as a time-weighted average; however, OSHA believes that a TWA limit set at 0.5 ppm will protect against phenyl mercaptan’s toxic effects. No other comments on the health effects of phenyl mercaptan were submitted to the rulemaking record.

OSHA is establishing an 8-hour TWA limit of 0.5 ppm for phenyl mercaptan. The Agency concludes that this limit will protect workers from the significant risks of CNS effects, skin irritation, and systemic injury, all material impairments of health that are potentially associated with exposure to phenyl mercaptan at the uncontrolled levels formerly permitted by the absence of any OSHA limit.