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.

December 21, 1992

Mr. David L. Walker
Division of Environmental Quality
State of Missouri Department of Natural Resources
P.O. Box 176
Jefferson City, Missouri 65102

Dear Mr. Walker:

This is in response to your August 28 letter to Mr. James Foster, Director of the Office of Information and Consumer Affairs of the Occupational Safety and Health Administration. Your letter was forwarded to the Directorate of Compliance Programs for response. In your letter you sought interpretations on the Final Rule on Process Safety Management (PSM) of Highly Hazardous Chemicals published in Volume 57, Number 36 of the Federal Register on Monday, February 24, 1992. We apologize for the delay in responding. Your questions and our answers follow.

Question 1: Does the threshold quantity listed under Appendix A, 29 CFR 1910.119 apply to the quantity of the whole mixture or just the quantity of the component chemical, or to neither (that is, does the threshold quantity apply only to quantities of pure chemical unless otherwise specified in the appendix)?

Reply: The threshold quantities listed in Appendix A apply only to pure (or "chemical grade") chemicals unless otherwise specified, for example, Hydrogen Peroxide, 52% by weight or greater.

Question 2: Does the PSM standard apply to the EPA regulated and permitted RCRA hazardous waste treatment, storage and disposal (TSD) facilities, when such facilities keep on-site in one location a hazardous waste chemical in a concentration and quantity which exceeds the applicable threshold quantity of Appendix A. If so, why? If not, why not?

Reply: Employers of worksites with TSD facilities which contain covered processes must comply with the PSM standard. The requirements of the PSM standard are intended to eliminate or mitigate workplace catastrophic releases of highly hazardous chemicals and resulting employee exposure to explosion, fire and toxic hazards.


Roger A. Clark,
Directorate of Compliance Programs