- Standard Number:
OSHA requirements are set by statute, standards and regulations. Our interpretation letters explain these requirements and how they apply to particular circumstances, but they cannot create additional employer obligations. This letter constitutes OSHA's interpretation of the requirements discussed. Note that our enforcement guidance may be affected by changes to OSHA rules. Also, from time to time we update our guidance in response to new information. To keep apprised of such developments, you can consult OSHA's website at https://www.osha.gov.
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:
% Si02+ 5.33% rather than
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.
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.