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Standard Interpretations - Table of Contents
• Standard Number: 1910.119; 1910.119(a)(1)(ii)(B); 1910.119(b); 1910.119(o)

February 1, 2002

Donald L. Olesen, P.E., C.S.P.
Vice President
J&H Marsh & McLennan
Risk Control Consulting
200 Clarendon Street
Boston, MA 02116

Dear Mr. Olesen:

Thank you for your September 17, 1998 letter to the Occupational Safety and Health Administration's (OSHA's) Directorate of Compliance Programs (DCP). You have questions regarding OSHA's Process Safety Management of Highly Hazardous Chemicals; Explosives and Blasting Agents Standard (PSM), 29 CFR 1910.119. Please be aware that this response may not be applicable to any question or situation not delineated within your original correspondence. Your specific question is related to the application of the PSM standard to a warehouse operation that stores and distributes chemicals. We apologize for the delay in our response.

Scenario: A company has a warehouse for the storage and distribution of laboratory grade chemicals such as flammable liquids and formaldehyde. The companys storage and distribution operation does not include any processing, dispensing, mixing, stirring, agitation, blending, filtering, evaporating, or drying. Chemical containers can range in size from 1 liter bottles in multi-container boxes to 4 liter bottles in 6 container boxes. Additionally, they store five (5) gallon metal containers of chemicals. The flammables and oxidizers are stored separately in engineered flammable storage vaults. Incompatibles are segregated so that, should a bottle be broken, incompatibles will not mix.

Question 1: Does PSM apply to the warehouse operation described in the above scenario?

Reply: There is insufficient information in your request to provide a definitive response. However, the following information should assist you in your determination of whether the process you described is a PSM-covered process. The warehouse operation you described is a PSM-covered process when:
  1. PSM-covered highly hazardous chemical(s) (HHC) exists in your warehouse/process. You listed two HHCs that are covered by PSM: formaldehyde and flammable liquids. If other HHCs exist in the warehouse/process, they too would be a basis for establishing a PSM-covered process;

  2. Greater than a threshold quantity (TQ) of formaldehyde (1000 pounds), flammable liquids (10,000 pounds), or other specified HHC at or above their listed TQ exists in the process. The containers of the chemicals you have specified each contain only a small quantity of HHC. However, when an HHC is stored in one or more cans, packages, containers, tanks, vessels, etc., the amount is aggregated if a single event such as a fire, explosion, incompatible chemical reaction, etc. could result in a release of HHC. For example, if a fire could consume an entire warehouse all the HHCs would be aggregated as either flammable liquids, flammable gasses, or by specific Appendix A chemical listing, where applicable.

    An employer may elect to segregate HHCs to a point that a TQ does not exist. If this is the case, the HHCs must be so removed from each other that a single event could not possibly result in a release of a TQ amount of HHC.

    You mentioned that flammable liquids are stored in an engineered vault. First, an employer is exempt from the requirements of PSM when a threshold quantity of flammable liquids is stored in atmospheric tanks or transferred without the benefit of chilling or refrigeration (29 C.F.R. 1910.119(a)(1)(ii)(B)). This exemption would also apply to your storage of flammable liquids in small containers, even if an aggregate TQ or greater amount is stored in the engineered storage vaults. Second, if the oxidizers you mentioned are Appendix A HHCs but not flammable liquids and Appendix A HHCs are stored inside the engineered vaults in aggregate amounts equal to or greater than TQ, then these HHCs would not be exempt from PSM requirements.

    To assist you in determining when the storage and handling of HHCs are segregated to the point their amounts do not need to be aggregated, OSHA has stated1 that engineering controls (i.e. sprinkler systems, self-closing fire doors. etc.), and administrative controls, i.e., operating procedures, used to prevent and mitigate a catastrophic release of a covered HHC may not be used to determine the extent of a process as defined in paragraph 1910.119(b). Note: for non-interconnected equipment, passive mitigation2 may be used to limit the extent or the boundaries of a covered process. Further, the employer must determine if other aspects of the stipulated covered process are also part of the covered process. If an aspect of the covered process could affect a release of an HHC or interfere with the mitigation of the consequences of a release from the covered process, then the aspect being considered would be part of the covered process. For example, if a fire in a part of the warehouse (aspect under consideration) which stores non-HHCs could spread to the stipulated covered process and affect a release of an HHC, then the aspect under consideration would be part of the covered process.

  3. Requisite activities are conducted which define a covered process including storage of an HHC, and on-site movement. Your scenario specifies that the chemical storage at the facility, “does not include any processing, dispensing, mixing, stirring, agitation, blending, filtering, evaporating, or drying.”   Please note, that while each of these activities by themselves can invoke PSM coverage, the mere activity of storage (except the storage of flammable liquids discussed below), itself, can invoke coverage by the standard; and

  4. Except for possibly 29 CFR 1910.119(a)(1)(ii)(B) [exemption for the storage of flammable liquids under the prescribed conditions], none of the PSM exemptions apply.
Question 2: If the PSM standard applies in this situation, what elements would apply and what actions should the employer take to ensure compliance?

Response: When the PSM standard does apply, the employer must comply with all elements of the standard. To ensure compliance with PSM, OSHA requires an employer to perform a compliance audit at least every three years as per 29 CFR 1910.119(o). Further, OSHA provides employers with compliance assistance on our website (
http://www.osha.gov) related to all of our safety matters. This information includes the PSM standard, PSM compliance directive, and all letters of interpretation on the subject. Further, Appendix C to §1910.119 - Nonmandatory Compliance Guidelines and Recommendations for Process Safety Management is a good tool for employers wishing to obtain background information on the standard. This document is also available on our website.

Thank you for your interest in occupational safety and health. We hope you find this information helpful. 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
http://www.osha.gov. If you have any further questions, please feel free to contact the Office of General Industry Compliance Assistance at (202) 693-1850.


Richard E. Fairfax, Director
Directorate of Compliance Programs Assistance

1OSHA Standards Interpretations and Compliance Letters - Akzo-Nobel Chemicals - Limits of a Process, February 28, 1997 [back to text]

2Passive mitigation protection for separate equipment includes protections where actions/interventions are not required. Passive engineering controls include, but are not limited to: 1) separation by distance; 2) blast resistant construction; etc.  When determining if separate equipment is part of a covered process, the employers evaluation may take credit for well designed and reliable passive mitigation systems. In other words, if the employers evaluation determines that a storage tank is adequately separate from a process under consideration based on its separation distance or the construction of a blast resistant wall, the storage tank would not be considered part of the process under consideration.[back to text]

Standard Interpretations - Table of Contents

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