The former OSHA PEL for respirable tridymite was expressed as one-half the value of the mass formula for quartz dust. This formula corresponds to a range of 0.04 to 0.05 mg/m3, measured as silica, for dusts containing 10 to 100 percent tridymite. The Agency proposed, and the final rule establishes, a PEL of 0.05 mg/m3 TWA for tridymite. The ACGIH recommends an 8-hour TWA limit of 0.05 mg/m3, measured as silica dust. The ACGIH limit is based on a study conducted by King, Mohanty, Harrison, and Nagelschmidt (1953/Ex. 1-85) that found tridymite to be the most active of the free silica forms when injected intratracheally into rats. Tridymite is a transparent, tasteless form of free silica.
Although expressed in different units, the current ACGIH and former OSHA limits for tridymite are comparable. The ACGIH's mg/m3 limit, adopted in 1985, does not reflect a re-evaluation of tridymite's toxicity but was adopted merely to simplify the monitoring of tridymite dust concentrations. NIOSH (Ex. 8-47, Table N6A) concurs with the selection of this limit but recommends that tridymite be designated as a potential occupational carcinogen. No other comments were received on tridymite.
OSHA is replacing its former limit for tridymite, which is described above, with a numerically equivalent limit of 0.05 mg/m3, measured as respirable silica dust; the final rule establishes this change to simplify employee exposure monitoring.