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Standard Interpretations - Table of Contents
• Standard Number: 1910.1000

March 22, 2007

Mr. Mark J. Pereira, CIH
Practical Safety Solutions, LLC
PO Box 792
50 Browns Lane
Old Lyme, CT 06371

Dear Mr. Pereira:

Thank you for your September 7, 2006 letter to the Occupational Safety and Health Administration (OSHA) requesting information on OSHA's amorphous silica permissible exposure limit (PEL) in the general industry air contaminants' standard at 29 CFR 1910.1000. This letter constitutes OSHA's interpretation only of the requirements discussed and may not be applicable to any situation not delineated within your original correspondence. Your questions are paraphrased below followed by our response.

Question 1: How was the (80 mg/m3)/(% silica) PEL for amorphous silica in Table Z-3 derived? No documentation in the American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Values (TLVs) exists.

Reply: The exposure limits in 29 CFR 1910.1000, Table Z-3, were not adopted from the ACGIH, but from a United States Department of Labor (DOL) regulation in existence at the time, 41 CFR Part 50-201 — "Safety and Health for Federal Supply Contracts." This Department standard was published in the Federal Register, Vol. 34, No. 96, May 20, 1969, pages 7946-7954 (copy enclosed). In 1971, under Section 6(a) of the OSH Act, 29 USC 655(a), OSHA adopted this edition of 41 CFR 50-204.

The Walsh-Healey Act of 1938, as amended, USC 38, required in part that companies contracting with the United States Government provide safety and health protection to their employees. To ensure compliance with this law, in 1969 the United States Department of Labor promulgated 41 CFR 50-204 — Safety and Health for Federal Supply Contracts, which delineated specific safety and health standards for contractors. In Part 50-204.50 of the standard, the Department set exposure limits for gases, vapors, fumes, dusts, and mists. Part 50-204.50 included the adoption of the 1968 TLVs from the ACGIH, which became OSHA's Table Z-1. Part 50-204.50 also contained "Table I," which listed several standards from the United States of America Standards Institute (USASI) [later the American National Standards Institute (ANSI)]. The information in this listing was adopted by OSHA under the 6(a) rulemaking process as OSHA's Table Z-2. Finally, Part 50-204.50 contained "Table II," which set exposure limits for mineral dusts, including amorphous silica. Table II's contents became OSHA's Table Z-3.

Question 2: Using the formula for calculating the amorphous silica PEL in Table Z-3, assuming 100% silica, gives a PEL of 0.8 mg/m3. Is this the correct application of the PEL?

Reply: Yes, that is the correct application.

Question 3: The PEL formula used by OSHA for amorphous silica gives a lower exposure limit (0.8 mg/m3) than either ACGIH (10 mg/m3) or the National Institutes for Occupational Safety and Health (6 mg/m3). Additionally, ACGIH has withdrawn its documentation "due to insufficient data." Given this, how does OSHA address situations where an exposure is below the NIOSH or ACGIH exposure limit but over the OSHA PEL?

Reply: If an employer exceeds the OSHA PEL, the employer may be cited. Neither the ACGIH nor the NIOSH exposure limits are enforced by OSHA. We would also note that ACGIH withdrew the TLV documentation for amorphous silica largely because the TLV was derived mostly from studies on diatomaceous earth (DA), and most of those studies did not take into account the crystalline silica content of the DA.

We hope you find this information helpful. 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 http://www.osha.gov. If you have any further questions, please feel free to contact the Office of Health Enforcement at (202) 693-2190.


Richard E. Fairfax, Director
Directorate of Enforcement Programs

Standard Interpretations - Table of Contents

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