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 18, 1994

Ms. Susan Kunst
Boushell Law Offices of McKenna and Cuneo
1575 Eye Street, N.W.
Washington, D.C. 20005

Dear Ms. Boushel:

This is in response to your May 3 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 response. In your letter you asked for confirmation that a threshold quantity (TQ) is determined on the amount of HHC in the process at any one point in time and not aggregated over a period of time.

This is to confirm that a process must contain at any one point in time, a TQ or greater amount of a specified HHC to be covered by the PSM standard. For the purpose of determining whether there is a TQ or greater amount at any one point in time, flammable liquids contained in a process are considered aggregate. The same criteria applies to flammable gases. From an aggregate standpoint, flammable liquids and flammable gases are treated separately. Also, the HHCs, listed in Appendix A as toxics and reactives are treated separately and aggregated individually for coverage purposes. In cases where an Appendix A listed HHC is a flammable liquid or gas, the coverage threshold is the lower of the following amounts: the threshold quantity specified in Appendix A or 10,000 pounds (4535.9kg).

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