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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: 1305-78-8; Chemical Formula: CaO

OSHA's former 8-hour TWA permissible exposure limit for calcium oxide was 5 mg/m3, and the proposal contained a revised 8-hour TWA PEL of 2 mg/m3 for this substance. NIOSH (Ex. 8-47, Table N1) concurred with this proprosal. This revised limit was consistent with the ACGIH TLV for calcium oxide, which was set on the basis of analogy with sodium hydroxide, a widely recognized sensory irritant. The final rule retains OSHA's former 5-mg/m3 8-hour TWA PEL for calcium oxide, for the reasons discussed below.

Calcium oxide (lime) is produced when limestone is calcined to drive off carbon dioxide. Calcium oxide is used as a refractory material; as a flux in steelmaking; as a binding agent in building, pulp and paper manufacture, sugar refining, and leather tanning; as the raw material for chlorinated lime bleaching powder, and as a soil treatment in agriculture (Encyclopedia of Occupational Health and Safety, Vol. 2, p. 1234, International Labour Office 1983).

The amount of information that has been published specifically about calcium oxide's toxicological effects in animals or humans is limited, which accounts for the ACGIH's reliance on the similarity in action between calcium oxide and sodium hydroxide in establishing a TLV of 2 mg/m3 for calcium oxide. The National Lime Association (NLA)(Ex. 3-890) and the American Iron and Steel Institute (Tr. p. 11-130 to 11-131; Ex. 188) objected to the comparison of calcium oxide's properties with those of sodium hydroxide; according to the NLA, "no qualitative or quantitative analysis is offered [in the proposal] to support the use of this analogy." OSHA's analysis of this issue is discussed below.

In direct contact with tissues, calcium oxide can result in burns and severe irritation because of its high reactivity and alkalinity. The major complaints of workers exposed to lime consist of irritation of the skin and eyes, although inflammation of the respiratory passages, ulceration and perforation of the nasal septum, and even pneumonia have been attributed to inhalation of the dust (ACGIH 1986/Ex. 1-3, p. 92). The Pennsylvania Department of Health reported that strong nasal irritation occurred as a consequence of exposure to a mixture of calcium-oxide-containing dusts at a concentration of approximately 25 mg/m3, but that exposure to concentrations of 9 to 10 mg/m3 produced no observable irritation (Wands 1981a, in Patty's Industrial Hygiene and Toxicology, 3rd rev. ed., Vol. 2B, p. 3054). By comparison, exposure to airborne sodium hydroxide at a concentration of between 0.005 and 0.7 mg/m3 produced burning/redness of the nose, throat, or eyes in workers engaged in cleaning operations (Hervin and Cohen 1973/Ex. 1-945, as cited in NIOSH 1976k/Ex. 1-965). Thus, the demonstrated effect level for sensory irritation caused by exposure to sodium hydroxide is below 1 mg/m3, while that for calcium oxide is above 9 mg/m3.

OSHA finds that analogy with sodium hydroxide is not an appropriate basis for establishing a PEL for calcium oxide, because there is nearly a tenfold difference in the no-effect levels for these two substances. Based on evidence that exposure to calcium oxide at levels above 9 mg/m3 may cause eye-tearing and mucous membrane irritation, OSHA concludes that the Agency's former limit of 5 mg/m3 as an 8-hour TWA continues to be appropriate for this substance. The Agency concludes that this limit protects exposed workers from the significant risk of sensory irritation known to occur at concentrations somewhat above 9 to 10 mg/m3.