SILICA, AMORPHOUS

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: None; Chemical Formula: SiO₂

OSHA currently has a limit of 20 mppcf (which is equivalent to a limit of 6 mg/m³) for amorphous silica. The ACGIH recommends a TLV-TWA of 10 mg/m³ measured as total dust containing less than 1 percent quartz. OSHA is retaining the current PEL in the final rule but is expressing this limit in milligrams per cubic meter; NIOSH (Ex. 8-47, Table N1) concurs with the Agency’s decision. There are numerous methods of producing precipitated silica; those that apply heat to siliceous products produce airborne dusts that are less toxic than quartz dust because the particles are generally sheathed in a molecular layer of amorphous silica (ACGIH 1986/Ex. 1-3, p. 521).

Studies of laboratory animals have shown no fibrosis after intratracheal and intraperitoneal injection of precipitated silica or silica gel (Klosterkotter 1954/Ex. 1-1156; Klosterkotter 1958/Ex. 1-1039). Schepers and colleagues reported in 1957 that rats exposed for one year and guinea pigs and rabbits exposed for two years to a concentration of 126 mg/m 3 of precipitated amorphous silica displayed no pulmonary fibrosis; the effects of exposure were limited to macrophage accumulations and mild proliferation of reticulin fibers (Schepers, Durkan, Delahant et al. 1957/Ex. 1-755).

In a study of human exposures to precipitated amorphous silica, Wilson and associates reported no ill effects in 165 workers exposed for an average of 8.6 years (Wilson, Stevens, Lovejoy et al. 1981/Ex. 1-1177).

The ACGIH considers the precipitated and gel forms of amorphous silica to have low biological activity, based on the evidence discussed above. PPG Industries (Ex. 3-1158) commented that an unpublished NIOSH study (Groth, Kommineni, Stettler et al. 1979, as cited by H.E. Stokinger in Patty’s Industrial Hygiene and Toxicology, 3rd rev. ed., Vol. 2B, pp. 3011-3014) showed that rats, guinea pigs, and monkeys developed accumulations of macrophages in the lungs following exposure to precipitated silica. In addition, the presence of collagen was seen in “very few” monkeys; by comparison, collagen was not seen in any animal exposed to silica gel but was seen in significant amounts in monkeys exposed to fumed silica. PPG remarked that the findings in animals exposed to precipitated silica showed “no evidence for effects…which are inconsistent with the ACGIH criteria for nuisance particulates” (Ex. 3-1158). PPG urged OSHA to adopt a 10 mg/m³ PEL for precipitated silica based on this observation. SASSI (Ex. 3-630) also requested that OSHA adopt either a 10 mg/m³ total dust limit or a 5-mg/m³ respirable dust limit for precipitated silica, based on the recommendation of ASTM’s E34.16 Committee.

After reviewing these comments, OSHA concludes that the available evidence does not meet the criteria described earlier in this section for determining that an increase in the present PEL is warranted. OSHA notes that, in the study cited by PPG, there was collagen formation only in a few animals exposed to precipitated silica. Furthermore, the report by Wilson et al. (1981/Ex. 1-1177) involved only a relatively small number of employees who had been exposed for fewer than 10 years. Accordingly, OSHA is retaining its current PEL of 6 mg/m³ (equivalent to 20 mppcf) at the present time. However, to facilitate the accurate monitoring of employee exposures, the Agency is changing the units in which its permissible exposure limit for amorphous silica is expressed.