Fused silica is a colorless, odorless solid that is a form of quartz. As such, it was formerly covered by OSHA's limit for quartz (Table Z-3). Exposure to fused silica has long been known to cause the fibrogenic lung disease, silicosis. OSHA's former limit for quartz dust was the formula 10 mg/m3/ % SiO(2) + 2, measured as total respirable dust. This limit corresponds to a respirable quartz concentration ranging from 0.08 to 0.1 mg/m3, measured as free silica. The ACGIH recommends an 8-hour TWA limit of 0.1 mg/m3, measured as free silica; the ACGIH adopted this limit in 1985 to simplify the monitoring of quartz dust concentrations. Thus, this revision does not represent a re-evaluation of the toxicity data for fused silica. NIOSH (Ex. 8-47, Table N6B) does not concur with the final rule's limit and recommends a separate 6(b) rulemaking for fused silica, which NIOSH considers a potential occupational carcinogen.
OSHA is replacing its limit for fused silica, which is expressed as the formula presented above, with a numerically equivalent limit of 0.1 mg/m3 as total respirable silica dust; the Agency is establishing this limit to simplify employee exposure monitoring.