SOAPSTONE

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: 3 MgO-4 SiO₂-H₂O

OSHA’s former exposure limit for soapstone, total dust, was 20 mppcf (6 mg/m³), and the Agency had no separate limit for the respirable fraction. The ACGIH has established individual TLV-TWAs for these two forms of soapstone: 6 mg/m³ for total dust, and 3 mg/m³ for the respirable fraction, both measured as total dust or respirable dust containing less then 1 percent quartz. Because the ratio of total dust mass to the mass of the respirable fraction is 2:1 (ACGIH 1984, p. 480), the 6 mg/m³ total dust limit automatically implies a 3 mg/m³ limit for the respirable fraction. OSHA proposed, and the final rule establishes, permissible exposure limits of 6 mg/m³ TWA (total dust) and 3 mg/m³ TWA (respirable dust) for soapstone. NIOSH (Ex. 8-47, Table N1) concurred with this determination.

A study by Dreessen and DallaValle (1935/Ex. 1-588) of mill workers exposed to soapstone showed lung changes in these workers, but it is believed that the dusts involved in these exposures were actually steatite talc, which had a tremolite content of 10 percent. Experiments by Miller and Sayers (1941/Ex. 1-595) showed no measurable toxic effects in guinea pigs injected intraperitoneally with various samples of soapstone. No comments were received on soapstone other than those submitted by NIOSH.

The final rule expresses the limit for soapstone as total dust in mg/m³, rather than mppcf, to simplify employee sampling and analysis. The total dust limit being established, 6 mg/m³, is equivalent to the previous limit of 20 mppcf, and the new limit of 3 mg/m³ for respirable dust is actually implicit in the total dust limit.