Subject: Mass concentration standard for amorphous silica originally listed incorrectly.
Information Date:07/14/1975
29 CFR 1910.1000 Table Z-3
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.