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: 15468-32-3; Chemical Formula: SiO2

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.

Page last reviewed: September 28, 2011