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.

June 1, 1994

Mr. Chris Kuiper
Environmental Health and Safety Inc.
6093 S. Quebec Street, Suite 203
Englewood, Colorado 80111

Dear Mr. Kuiper:

This is in response to your February 16 letter, requesting interpretation of the Process Safety Management (PSM) of Highly Hazardous Chemicals standard, 29 CFR 1910.119. Please accept our apology for the delay in responding. Your questions and our replies follow.

Question 1:

Does "threshold quantity" mean the amount that is in the system at any time or what the system is capable of holding? For example, a company's ammonia (NH(3)) tank is capable of storing 20,000 pounds (9071.8 kg), but they maintain an inventory of only 5000 pounds (2267.85 kg) and the distribution system can only hold 2000 pounds (907.18 kg).


The threshold quantity (TQ) is determined by the amount of covered HHC(s) in the process at any particular time. A process which has the capacity to contain a TQ or greater amount of a covered HHC and which is operated to contain, for example, less than TQ amounts of NH(3) and never contains more than the TQ amount, would not be covered by the PSM standard. It is important that controls be in place to assure the NH(3) remains below the TQ amount.

Question 2:

Will OSHA require assurance that the inventory of NH(3) is not allowed to reach the 10,000 pounds (4535.9 kg) TQ? In what form should the assurance be maintained?


Coverage by the standard is determined by the amount of HHC, for example, NH(3), in a process at any particular time and not on the inventory amount as described in question 2 above. If the worksite employer intends to limit to quantity of HHC in a process to avoid being subject to the PSM standard, he or she must have an effective control program and be prepared to demonstrate to OSHA that the process is at all times not covered. How the demonstration is formatted and maintained is considered a performance oriented requirement for the employer. Employer demonstration including documentation related to current and past operations is anticipated.

Thank you for your interest in occupational safety and health. If we can be of further assistance, please contact us.


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