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SILICA, CRYSTAL

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: 1317-95-9; Chemical Formula: SiO2

Tripoli is a colorless microcrystalline form of quartz. Although OSHA's Table Z-2 did not specifically indicate a limit for tripoli, OSHA formerly specified a limit for crystalline quartz based on the formula measured as total respirable dust: 10 mg/m3/ % SiO(2)+2. Expressed as mg/m3, this limit corresponds to a limit in the range of 0.08 to 0.1 mg/m3 for respirable dust containing from 10 to 100 percent silica. The 8-hour TWA ACGIH limit for tripoli is 0.1 mg/m3, measured as respirable silica dust. This limit was adopted by the ACGIH in 1985 to simplify the monitoring of quartz dust concentrations. Thus, this revision does not represent a re-evaluation of toxicity data for tripoli. NIOSH (Ex. 8-47, Table N6B) does not concur with the final rule's limit and recommends a separate 6(b) rulemaking for tripoli, which NIOSH considers a potential occupational carcinogen. (see section above on Crystalline Quartz for OSHA's discussion of the record evidence on the carcinogenicity of silica). No other comments were received on tripoli.

OSHA is replacing its limit for quartz, which is expressed as the formula presented above, with a numerically equivalent limit of 0.1 mg/m3 TWA as respirable silica dust; the final rule establishes this limit for tripoli.

 

 
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