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.

April 22, 1994

Mr. Luc Hamelin
Place Mercantile
777 Rue Sherbrooke Quest
Montreal (Quebec) Canada H3A 1G1

Dear Mr. Hamelin:

This is in response to your February 25 letter, requesting interpretation of the Process Safety Management (PSM) of Highly Hazardous Chemicals (HHCs) standard 29 CFR 1910.119. Specifically, you requested clarification on whether the threshold quantity (TQ) of HHCs listed in Appendix A of the PSM standard refers only to component hazardous substances of a mixture or to the whole mixture or solution (regardless of the concentration). Restating your question, you then asked: does the TQ consider the component of the mixture which is in Appendix A (e.g. concentration in weight percent multiplied by the mass (in pounds) of the mixture) or does the TQ consider the entire mixture regardless of the solution concentration? Please accept our apology for the delay in responding.

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

Most of the HHC's listed in Appendix A are "commercial grade" concentrations. 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 concentrations 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.

When an Appendix A listed HHC, for example, Chlorine Dioxide (ClO(2)), which is not commercially available and which is produced in a workplace process, the threshold quantity is determined not only by the free standing quantity of the HHC, for example, ClO(2) gas but also by the amount of HHC(s) in the mixture(s), for example, ClO(2) in a water solution. A process containing 150 pounds (68.00 kg) of ClO(2) gas and 900,000 pounds (408,231.00 kg) of ClO(2) water solution would be covered as follows.

Assuming that the ClO(2) is 0.1 percent by weight of the water solution, there is 900 pounds (408.23 kg) of ClO(2) in the water solution. This process contains 150 + 900 or 1050 pounds (476.27 kg) of ClO(2) which is greater than the threshold quantity of 1000 pounds.

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 as noted in the preceding example.

Please note that 10,000 pounds (4535.9 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 of 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.


H. Berrien Zettler, Deputy Director
Directorate of Compliance Programs