OSHA formerly had an 8-hour TWA limit of 75 ppm for p-dichlorobenzene. The ACGIH has a limit of 75 ppm TWA and a STEL of 110 ppm for this white crystalline material, which has a camphor-like odor. The ACGIH's limit recognizes that the para isomer is somewhat less toxic than the ortho isomer, for which the ACGIH has established a ceiling limit of 50 ppm. The proposed PEL retained the 75-ppm TWA limit and added a STEL of 110 ppm; the final rule establishes these limits.
In animal studies, an injection of 0.005 gram of p-dichlorobenzene in rats caused slight liver necrosis (Cameron, Thomas, Ashmore et al. 1937/Ex. 1-471). The intraperitoneal injection LD(50) for rats has been reported as 2562 mg/kg (Zupko and Edwards 1949/Ex. 1-878). The oral LD(50) in mice is 2950 mg/kg (Domenjoz 1946, as cited in ACGIH 1986/Ex. 1-3, p. 179); for rats, the oral LD(50) is 2512 mg/kg (Varshavskaya 1970, as cited in ACGIH 1986/Ex. 1-3, p. 179). Rabbits fed a daily dietary exposure of 5 grams developed opacity of the lens in 3 weeks (Berliner 1939/Ex. 1-715); this finding was not confirmed, however, in repeated studies (Pike 1944/Ex. 1-656).
Reports of a human inhalation exposure to unspecified levels of p-dichlorobenzene describe swelling of the feet, ankles, and hands after day-long use of a mothproofing agent consisting of this substance (Clayton 1935/Ex. 1-306). Other reports describe cataracts caused by exposure to unspecified concentrations of the vapor of p-dichlorobenzene (Berliner 1939/Ex. 1-715). Petit and Champaix (1948, as cited in ACGIH 1986/Ex. 1-3, p. 179) report the case of a woman who experienced tingling of the hands, vertigo, and loss of weight from working for 18 months with a mixture of 90 parts p-dichlorobenzene and 10 parts hexachloroethane (airborne concentration not specified).
OSHA received three comments on p-dichlorobenzene: from NIOSH (Ex. 8-47), the Workers Institute of Safety and Health (WISH) (Ex. 116), and the Halogenated Solvents Industry Alliance (HSIA) (Ex. 186). WISH simply pointed out that the ACGIH Documentation (1986/Ex. 1-3) entry for this substance includes fewer, and different, references from those relied on by the Agency for Toxic Substances and Disease Registry (Ex. 116, Table 1), without further comment. The HSIA (Ex. 186, App. D) submitted a letter from EPA's Science Advisory Board to Lee Thomas, Administrator of EPA (3/9/88). The letter points out that there is a scientific hypothesis to the effect that, for many halogenated organics (including p-dichlorobenzene), the mechanism causing tumors in rats exposed to these substances may not be operative in humans (Ex. 186D). According to the HSIA, this hypothesis may have "important implications for human health risk assessment" (Ex. 186D, p. 2). On the other hand, NIOSH (Ex. 8-47, Table N6B) interprets the evidence for p-dichlorobenzene to mean that it is a potential human carcinogen that deserves full Section 6(b) rulemaking. OSHA will consider NIOSH's recommendation in light of the Agency's rulemaking priorities.
In the final rule, OSHA is retaining the 8-hour TWA PEL of 75 ppm TWA and adding a STEL of 110 ppm for p-dichlorobenzene. The Agency concludes that both a TWA and a STEL are necessary to protect workers from the significant risk of eye damage, vertigo, and neuropathic effects, which constitute material impairments of health that are associated with occupational exposure to p-dichlorobenzene at levels above the 8-hour TWA PEL.