OSHA's former Z tables had no specific limit for silicon; however, silicon was formerly regulated under OSHA's generic particulate limits of 15 mg/m3 TWA (total particulate) and 5 mg/m3 (respirable fraction). The ACGIH has a 10-mg/m3 8-hour TWA for silicon, measured as total dust. The proposed total particulate PEL for silicon was 10 mg/m3 as an 8-hour TWA, and this limit is established in the final rule; the 5-mg/m3 respirable fraction limit is retained. Silicon is a black to gray, lustrous, needle-like crystal that is used in the manufacture of semiconductors.
The evidence of silicon's toxicity in animals is conflicting. An early study by McCord, Fredrick, and Stolz (1937/ Ex. 1-640) reported no response in guinea pigs and rats injected intraperitoneally with silicon. A more recent study (Schepers 1971/Ex. 1-570) demonstrated pulmonary lesions in rabbits administered an intratracheal dose of 25 mg silicon dust. NIOSH (Ex. 8-47, Table N4) has not thoroughly reviewed the health effects evidence for silicon, and no other comments on silicon were submitted to the record.
In the final rule, OSHA is establishing an 8-hour TWA limit of 10 mg/m3 (total particulate) for silicon and retaining the 5-mg/m3 (respirable fraction) limit. The Agency concludes that these limits will reduce the significant health risks potentially associated with exposure to this substance at higher levels. These risks include eye, skin, mucous membrane and other forms of physical irritation and may include chronic respiratory effects. OSHA finds that these effects constitute material health impairments.