Amorphous-silica dust limit under misprinted formula

NOTICE: This is an OSHA Archive Document, and may no longer represent OSHA Policy. It is presented here as historical content, for research and review purposes only.

July 14, 1975

MEMORANDUM FOR: DAVID H. RHONE
Assistant Regional Director,
Philadelphia

FROM: BARRY J. WHITE
Associate Assistant Secretary for
Regional Programs

SUBJECT: Mass Concentration Standard for Amorphous Silica

This is in response to your memorandum of April 29, 1975.

The formula for calculating the mass concentration exposure limit for total dust containing amorphous silica was listed incorrectly when originally promulgated under the Walsh-Healey Public Contracts Act. The correct formula is:

80 mg/M³
--------
% Si0₂+ 5.33%

rather than

80 mg/M³
--------
% Si0₂

as listed in Table Z-3, 29 CFR 1910.1000 (formerly Table G-3), 29 CFR 1910.93 recodified May 28, 1975). The Solicitor's Office advises that since the formula was listed incorrectly when originally promulgated under Walsh-Healey, OSHA cannot enforce the proper formula until it has followed through with the requirements of section 6(b) of the Occupational Safety and Health Act of 1970.

Thus

80 mg/M³
--------
% Si0₂

is currently the lawful formula for calculating mass exposure limits for dust containing amorphous silica. Note that when the amorphous silica content of the dust falls below 5.33%, the exposure limit calculated from the formula exceeds the total dust exposure limit of 15 mg/M3 for inert (or nuisance) dust. Therefore at amorphous silica content below 5.33%, apply the total dust exposure limit for inert dust. Do not include the cyclone in the air sampling train since this is a total dust standard.

Any questions may be directed to Gail Brinkerhoff at 202/523-8096.