OSHA formerly had no individual limit for calcium hydroxide consequently the nuisance dust limit applied, the ACGIH has a TLV-TWA of 5 mg/m3. In the proposal, the PEL was 5 mg/m3 as an 8-hour TWA, and NIOSH (Ex. 8-47, Table N1) concurred with this limit. The final rule promulgates an 8-hour TWA PEL of 5 mg/m3 for calcium hydroxide. Calcium hydroxide is a soft, white, odorless, crystalline powder with an alkaline, bitter taste.
Calcium hydroxide is a moderate to severe caustic irritant when it comes in contact with the skin, eyes, or mucous membranes of the upper respiratory tract (ACGIH 1986/Ex. 1-3, p. 92; Sax and Lewis 1989, p. 682). The oral LD(50) in rats is reported to be 7.34 g/kg (Smyth, Carpenter, Weil et al. 1969/Ex. 1-442). Industrial experience with this substance has not shown a high incidence of adverse health effects, although Sax (Dangerous Properties of Industrial Materials, 6th ed., 1984) reports that it is known to cause dermatitis (p. 621). Calcium hydroxide is also mutagenic (Dangerous Properties of Industrial Materials, 7th ed., Sax and Lewis 1989, p. 682). Calcium hydroxide has less alkalinity than the hydroxides of the alkali series, and the ACGIH has suggested that limits for exposures to calcium hydroxide should be based on its total alkalinity.
OSHA received only one comment other than NIOSH's on calcium hydroxide. The National Lime Association (NLA) (Ex. 3-890) raised several issues related to this substance. First, the NLA argues that the health evidence for calcium hydroxide does not support a PEL of 5 mg/m3 for this substance. According to the NLA, the oral toxicity study described by OSHA in the proposal has no relevance to airborne lime exposures (Ex. 3-890, p. 13). In addition, the NLA believes that calcium hydroxide should be regulated as a nuisance dust, with a PEL of 10 mg/m3. OSHA does not agree that calcium hydroxide is a biologically inert substance. The Agency agrees with Sax (1984), who reports that, "in the form of dust, it [calcium hydroxide] is considered to be an important industrial hazard." OSHA finds that a PEL of 5 mg/m3, half that of the proposed inert particulate limit, is appropriate for this well-known eye, skin, and upper respiratory tract irritant.
The NLA's second point is that the monitoring methods available for measuring workplace exposures to lime are inadequate to distinguish between "different compounds of calcium" because they are "element- not compound-specific" (Ex. 3-890). OSHA's decision in the final rule to establish 5-mg/m3 limits for both calcium oxide and calcium hydroxide (see the discussion below for calcium oxide) should eliminate this problem for affected employers.
In the final rule, OSHA is establishing an 8-hour TWA limit for calcium hydroxide of 5 mg/m3 to protect against the significant risk of skin, eye, and mucous membrane irritation, which are material impairments of health that are caused by exposure to this substance at levels above the new PEL. The Agency concludes that this limit will reduce these risks substantially.