The former OSHA PEL for respirable cristobalite was one-half the value calculated from the mass formula for quartz, measured as respirable dust. This limit corresponds to a range of 0.04 to 0.05 mg/m3, measured as silica, for dusts containing 10 to 100 percent quartz. The ACGIH recommends an 8-hour TWA limit of 0.05 mg/m3, measured as respirable silica dust. Although expressed differently, the current ACGIH and former OSHA limit for cristobalite are comparable. The ACGIH's mg/m3 limit, adopted in 1985, does not reflect a re-evaluation of cristobalite's toxicity but was adopted merely to simplify the monitoring of cristobalite dust concentrations. The ACGIH limit is based on a study by Gardner (1938, as cited in ACGIH 1986/Ex. 1-3, p. 522) that was confirmed by King, Mohanty, Harrison, and Nagelschmidt (1953/Ex. 1-85). Experimental animals injected with cristobalite showed a more severe response than that produced by quartz, and the fibrosis that followed was diffuse rather than nodular. OSHA proposed, and the final rule establishes, a permissible exposure limit of 0.05 mg/m3 TWA for cristobalite, measured as respirable silica dust. Cristobalite, one of the three major forms of silicon dioxide, is transparent, tasteless, and stable at high temperatures.
The final rule replaces OSHA's former limit for cristobalite, which is expressed, as described above, with a numerically equivalent limit of 0.05 mg/m3; the Agency is establishing this time-weighted average limit to simplify employee exposure monitoring. NIOSH (Ex. 8-47, Table N6A; Tr. pp. 3-96 to 3-97) concurred with the selection of this limit but recommended that cristobalite be designated as a potential human carcinogen. OSHA's discussion of this and other rulemaking issues appears in the following entry describing the record evidence on quartz dust.