OSHA had no former limit for hexachlorobutadiene (HCBD); the proposal included a PEL of 0.02 ppm and a skin notation for this substance. NIOSH (Ex. 8-47, Table N6A) supported the selection of this limit. The ACGIH recommends a TLV-TWA of 0.02 ppm with a skin notation and classifies this substance as a suspected human carcinogen (A2). The final rule establishes an 8-hour TWA of 0.02 ppm but does not include a skin notation. Hexachlorobutadiene is a heavy, clear liquid.
Hexachlorobutadiene has a moderate-to-high acute oral toxicity. The LD(50)s reported for mice, rats, and guinea pigs are 87, 350, and 90 mg/kg, respectively (Murzakev 1963, as cited in ACGIH 1986/Ex. 1-3, p. 298). Gul'ko and co-workers (1964/Ex. 1-1082) reported LD(50) values of 116 mg/kg for mice and 270 mg/kg for rats (Gul'ko, Zimina, and Shroit 1964/Ex. 1-1082). The dermal LD(50) in rabbits is 1211 kg/mg (Dangerous Properties of Industrial Materials, 6th ed., p. 2145, Sax 1984). A single exposure of 133 to 150 ppm via inhalation has been fatal in rats when the exposure lasts for four to seven hours. All rats survived exposures at 161 ppm for 0.88 hour or 34 ppm for 3.3 hours; similar exposure of guinea pigs and cats to the same concentrations resulted in the death of most animals (Kociba, Schwetz, Keyes et al. 1977/Ex. 1-494). Another inhalation study in rats showed eye and nose irritation, respiratory difficulty, and damage to kidney tissue and the adrenal cortex after two 4-hour exposures at 250 ppm; twelve 6-hour exposures to 100 ppm caused eye and nose irritation, respiratory difficulty, weight loss, anemia in the female animals, and kidney and adrenal damage; fifteen 6-hour exposures at 25 ppm caused retarded weight gain in females, respiratory difficulty, and kidney damage; fifteen 6-hour exposures at 10 ppm caused retarded weight gain in females but no systemic injury; and fifteen 6-hour exposures at 5 ppm resulted in no adverse effects (Gage 1970/Ex. 1-318).
Reproductive studies in male and female rats demonstrated multiple toxicological effects, including kidney damage in both sexes and increased liver weight in males, at the high-dose level of 20 mg/kg/day. Dietary administration of 20, 2, or 0.2 mg/kg daily had no effect on conception percentages, gestational survival, neonatal survival, neonatal sex ratios, neonatal morphology, or neonatal body weights (except for the high-dose neonates) (Schwetz, Smith, Humiston et al. 1977/ Ex. 1-368). Results of lifetime dietary studies suggest that the no-effect level for hexachlorobutadiene in rats is 0.2 mg/kg/day, that a clear dose-response relationship exists for HCBD-induced toxicity affecting primarily the kidney, and that carcinogenic effects (i.e., renal neoplasms) result from ingestion of 20 mg/kg/day (Kociba, Schwetz, Keyes et al. 1977/Ex. 1-494). These authors also reported that HCBD-induced neoplasms occur only at HCBD doses higher than those causing discernible renal injury. The ACGIH states that "HCBD would seem to qualify as a carcinogen of intermediate potency" (ACGIH 1986/Ex. 1-3, p. 299). NIOSH (Ex. 8-47, Table N6A) concurs with the limit being established by OSHA and notes that this substance could be classified as a potential occupational carcinogen.
OSHA is not including a skin notation in the final rule. This decision is based on the Agency's policy in the matter of skin notations (see Section VI.C.18 of the preamble for a discussion of this issue). OSHA is establishing an 8-hour TWA limit of 0.02 ppm for this hazardous substance. Assuming a 10-m 3 per day breathing volume per 8-hour workshift and a 70-kg body weight for humans, this limit corresponds to a daily hexachlorobutadiene intake of approximately 0.03 mg/kg, which is about 10 times below the observed no-effect level in rats fed hexachlorobutadiene. The Agency concludes that this 0.02-ppm limit will protect workers exposed to HCBD from the significant risks of kidney damage; eye, skin, and pulmonary irritation; and renal neoplasms, all of which constitute material health impairments that are associated with exposure to HCBD at levels above the new limit.