SILICON CARBIDE

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: 409-21-2; Chemical Formula: SiC

OSHA formerly regulated silicon carbide under its generic 15-mg/m³ total particulate limit. The ACGIH has a 10-mg/m³ 8-hour TWA limit, measured as total dust. The proposed total particulate PEL for silicon carbide was 10 mg/m³, and the final rule promulgates this limit and retains the 5-mg/m³ respirable fraction limit for silicon carbide, which is a green to blue-black irridescent crystal.

An animal study (Gardner 1923/Ex. 1-737) showed that, although exposure to silicon carbide alone produced no fibrosis of the lungs, exposure of guinea pigs infected with tuberculosis to silicon carbide (six hours/day, five days/week for one year) aggravated pulmonary tuberculosis to the extent that extensive fibrosis occurred. Guinea pigs exposed to silicon carbide dust and infected with the tubercle bacteria developed tuberculopneumoconiotic lesions (Gross, Westrick, and McNerney 1959/Ex. 1-697). Miller and Sayers (1941/Ex. 1-595) observed that silicon carbide dust administered by intraperitoneal injection to guinea pigs produced no reaction.

Bruusgaard (1949/Ex. 1-1143) found that X-rays of 10 out of 32 workers exposed to average levels of 34 mppcf of silicon carbide for 15 years or more demonstrated pulmonary changes; these 10 workers were also tuberculin-positive. Miller, Davis, Goldman, and Wyatts (1953/Ex. 1-40) described three cases of pulmonary reactions and hyperglobinemia in tungsten carbide industry workers; these authors concluded that exposure to silicon carbide was not a hazard unless the exposed workers already had pulmonary tuberculosis. NIOSH (Ex. 8-47, Table N4) has not reviewed the health effects literature for silicon carbide in depth. No other comments on this substance were submitted.

In the final rule, OSHA is establishing a 10-mg/m³ TWA total particulate limit for silicon carbide and retaining the 5-mg/m³ TWA respirable fraction limit. The Agency concludes that these limits will protect workers from the significant risk of material health impairment in the form of the physical irritation that is associated with exposure to this particulate.