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 2, 1994

Mr. Jim Dykes
Director of Safety Education
American Institute of Baking
1213 Bakers Way
Manhattan, Kansas 66502

Dear Mr. Dykes:

This is in response to your November 29, 1993 letter in which you referred to the material safety data sheet (MSDS) for pure ammonia and found what you believe is a discrepancy in the Process Safety Management (PSM) of Highly Hazardous Chemical's (HHC's), 29 CFR 1910.119, standard. Specifically, you questioned whether anhydrous ammonia (NH(3)) should be listed in Appendix A of the PSM when the MSDS on NH(3) does not identify it as a "toxic, reactive, flammable or explosive chemical."

In support of your belief you stated in your letter that a threshold quantity (TQ) of 10,000 pounds (4535.9 kg) of NH(3) appears to be a "purely arbitrary amount without any basis for selection." This statement was made with reference to only six other Appendix A listed HHC's which have a TQ of 10,000 pounds. Also, you stated your opinion that the nature of ammonia used in a refrigeration system does not meet the criteria for a Process Hazard Analysis (PHA) when all that occurs is a simple change of state from liquid to gas and back to liquid in a closed system.

The purpose of the PSM standard is to prevent or minimize the consequences of an uncontrolled release from a workplace process containing a threshold quantity or greater amount of highly hazardous chemicals such as 10,000 pounds or more of NH(3). As discussed in the background section on page 6356 of the preamble to the PSM Final Rule, there exists a potential for an accidental release if a HHC is not properly controlled, regardless of the industry. Thus, the PSM standard is aimed not only at the petrochemical industry as stated in your letter but also at other industries, irrespective of size, at which there are processes containing HHC's such as NH(3). A copy of the Final Rule is enclosed for your use.

As discussed in the preamble to the Final Rule on pages 6364 and 6365, the Environmental Protection Agency's (EPA's) Extremely Hazardous Substance (EHS) list, which includes NH(3), was a reference source for determining HHC's and associated TQ's covered by the PSM standard. NH(3) is one of the HHC's covered by the PSM standard. Please note on page 6365 that the TQ for anhydrous ammonia was increased from 5000 pounds (2267.95kg), which was the TQ amount identified in the proposed rules, to 10,000 pounds to better reflect its hazard. The National Fire Protection Association (NFPA) rates anhydrous ammonia as a 3 (on a scale of 4) - a most severe toxic health hazard and as a 1 (on a scale of 4) - a flammable gas. There is an extensive history of catastrophic releases of NH(3) in the nations workplaces resulting in severe injuries and death. Although NH(3) would not be flammable as defined in 1910.1200(c), it does have lower (16 percent by volume) and upper (25 percent by volume) flammable or explosive limits. As such, NH(3) is flammable or explosive when subjected to a flame or in a fire. In consideration of the hazards noted above, OSHA believes that the release of anhydrous ammonia from a workplace process containing a TQ or greater amount presents a potential catastrophic situation including potential exposure of employees and therefore is covered by the PSM standard.

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


H. Berrien Zettler, Deputy Director
Directorate of Compliance Programs