OSHA's former PEL for dioxane was 100 ppm as an 8-hour TWA, with a skin notation. The Agency proposed a 25-ppm 8-hour TWA PEL for this substance, with retention of the skin notation; these limits, which are consistent with those of the ACGIH, are established in the final rule. NIOSH (Ex. 8-47, Table N6A) agreed with the selection of this PEL. Dioxane is a colorless liquid with an ethereal odor.
A two-year drinking water study conducted by the Dow Chemical Company (1972b, as cited in ACGIH 1986/Ex. 1-3, p. 217), in which male and female rats were given water containing 1.0, 0.1, or 0.01 percent dioxane, showed that animals given the highest dose developed liver and nasal tumors, in addition to pathological changes in the liver and kidney. Rats in the 0.1-percent group showed renal tubular sloughing and hepatocellular degeneration but no significant increase in neoplasms. Because this study demonstrated hepatoand nephrotoxic effects at doses 10 times lower than the dose causing cancer in animals, the permissible exposure limit has been set at a level that will prevent dioxane's liver and kidney effects. A study by Torkelson et al. (1974/Ex. 1-111) in four species of animals exposed to multiple daily airborne exposures of dioxane at 50 ppm showed no gross or histopatho-logic organ changes; this study demonstrates that the 25-ppm level should protect against the risk of liver and kidney effects in exposed workers. Dioxane has been shown in several studies to readily penetrate the skin of humans and animals and cause liver and kidney damage (NIOSH 1977n, p. 151, as cited in ACGIH 1986/Ex. 1-3, p. 218).
NIOSH (Ex. 8-47, Table N6A; Tr. 3-96 to 3-97) concurs with OSHA's exposure limit for dioxane, but notes its cancer potential. The AFL-CIO (Ex. 194, p. A12) also urged OSHA to designate dioxane as a carcinogen, as did the International Chemical Workers Union (Tr. 9-217 to 9-218). IARC (1987) has classified dioxane as a Group 2B (possible human) carcinogen based on a finding of sufficient evidence in animals. OSHA is aware of the emerging literature on dioxane's carcinogenic potential and intends to monitor this substance in the future.
Thomas Robinson, representing Vulcan Chemicals, stated that it was "most appropriate" for OSHA to adopt a TWA limit of 25 ppm for dioxane (Ex. 3-677), and the Halogenated Solvents Industry Alliance also supported OSHA's proposed PELs.
OSHA finds that the evidence for dioxane indicates that it is a liver and kidney toxin at levels substantially lower than those at which it produces a carcinogenic response. The Agency concludes that an 8-hour TWA of 25 ppm for dioxane, with a skin notation, is necessary to protect exposed workers against the significant risks of kidney and liver damage and cancer, all material health impairments that are associated with exposure at levels above the new PELs. OSHA has determined that the 25-ppm TWA limit will substantially reduce this risk.