Archive Notice - OSHA Archive

NOTICE: This is an OSHA Archive Document, and may no longer represent OSHA Policy. It is presented here as historical content, for research and review purposes only.

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

June 9, 1994

Mr. Gary F. Lindgreen
Vice President, Environmental Compliance
Heritage Environmental Services, Inc.
7901 West Morris Street
Indianapolis, Indiana 46231

Dear Mr. Lindgreen:

This is in response to your January 26 letter, requesting interpretation of the Process Safety Management (PSM) of Highly Hazardous Chemicals (HHCs) standard, 29 CFR 1910.119. Please accept our apology for the delay in responding. Your questions and our response follow:

Scenario: The following HHCs are listed in Appendix A of the PSM standard

1. Hydrochloric Acid, Anhydrous (Hydrogen Chloride), Chemical Abstract Service (CAS) number (#) 7647-01-0

2. Hydrofluoric Acid, Anhydrous (Hydrogen Fluoride), CAS #7667-39-3

3. Hydrogen Sulfide, CAS #7783-06-4

4. Hydrogen Cyanide, Anhydrous, CAS #74-90-8

Question 1:

Are the preceding HHCs regulated when they are gases at conditions of standard temperature and pressure including mixtures or solutions?

Question 2:

Are the preceding HHCs regulated when they are contained in mixtures or solutions, including aqueous solutions?

Question 3:

If the preceding HHCs are regulated when contained in mixtures or solutions, including aqueous solutions, to what extent are the mixtures or solutions regulated?


The PSM standard covers HHCs and HHC mixtures listed in Appendix A. With exceptions, other mixtures containing Appendix A listed HHCs are not covered by the PSM standard. Also with exception, the entire HHC mixture must be considered when determining the threshold quantity (TQ) of an Appendix A listed HHC substance. These clarification are delineated in the following paragraphs.

Most of the HHCs listed in Appendix A are "commercial grade" concentrations including those listed above which are gaseous at standard temperature and pressure conditions. The Occupational Safety and Health Administration (OSHA) defines "commercial grade" as a typical maximum concentration of the chemical that is commercially available and shipped. The term "commercial grade" includes reagent grades, which, in some cases, will be in different concentrations from the typical commercial grades. In cases where different concentration for commercial and reagent grades are typically shipped, the lower of the two maximum concentrations (and any concentration greater) is intended to be covered by the PSM standard. In order to determine the covered concentration, reference may be made to any published catalogue of chemicals for commercial sales. To determine the correct entry(s) in the catalogue, reference must also be made to the Chemical Abstract Service (CAS) number listed in Appendix A of the PSM standard.

Other Appendix A HHCs are listed specifically as percentage by weight or greater concentrations. For example, hydrogen peroxide (52 percent by weight or greater) mixtures are covered by the PSM standard. The entire mixture would be considered to determine the TQ.

Please note the 10,000 pounds (45.35 kg) or greater amount of flammable liquids or flammable gases including mixtures involving an HHC listed in Appendix A would be covered by the PSM standard. Also, the manufacturing activity of a process containing any amount an explosive, including a mixture involving an HHC listed in Appendix A would be covered by the PSM standard.

We appreciate your interest in occupational safety and health. If we can be of further assistance, please do not hesitate to contact us.


John B. Miles, Jr., Director
Directorate of Compliance Programs