Archive Notice - OSHA Archive

NOTICE: This is an OSHA Archive Document, and may no longer represent OSHA Policy. It is presented here as historical content, for research and review purposes only.

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

May 10, 1996

Brian D. Hardin, Ph.D.
Acting Deputy Director
National Institute for
Occupational Safety and Health
Department of Health and Human Services
Atlanta, Georgia 30333

Dear Dr. Hardin:

Thank you for your letter dated April 17, addressed to Joseph Dear, the Assistant Secretary for the Occupational Safety and Health Administration (OSHA), concerning the immediately dangerous to life or health concentrations (IDLH) values listed in the National Institute for Occupational Safety and Health's (NIOSH's) Pocket Guide. Your letter was referred to the Directorate of Technical Support (DTS), Office of Science and Technology Assessment (OSTA) for response.

I appreciate your efforts in reaching out to interested parties to establish protocol to identify, prioritize, and update IDLH values. Since the appropriate IDLH value is critical in many health related applications, and specifically in the selection of respirators, OSHA is especially supportive of and interested in the development of these values.

Please keep us advised on the progress of this project. Mr. Al Abadir, Acting Director of OSTA, can be reached at (202) 219-7056 extension 117, and is available to discuss this project with you.


Stephen J. Mallinger
Acting Director
Directorate of Technical Support

April 17, 1996

Joseph A. Dear
Assistant Secretary for
Occupational Safety and Health
U.S. Department of Labor
Room S-2315
200 Constitution Avenue, NW
Washington, DC 20210

Dear Mr. Dear:

I received a copy of the March 18, 1996 letter to you from Mr. Frank White of Organizational Resources Counselors (ORC). In that letter, Mr. White urges OSHA to issue compliance guidance on the use of NIOSH IDLH values. I would like to add my own comments and clarify some of the points made in reference to past and current NIOSH activities.

Mr. White's review of the events leading to the June 1994 Pocket Guide is generally accurate. NIOSH was in fact developing new IDLH values even as the Federal Register notice was seeking comment on the uses of IDLH values and suggestions on how their derivation might be improved. Although that parallel process may not have been optimal, there were considerations that made NIOSH feel obliged to act on IDLHs without delay.

The Pocket Guide is one of the most popular and widely used publications produced by the federal government. The previous edition was out of print and demand for more was high but because that edition was very dated NIOSH was unwilling to spend its limited resources to print additional copies. NIOSH has prepared a new edition that was expanded to include hundreds of additional chemicals, but the adequacy of several existing IDLH values had been questioned by NIOSH staff or by experts outside NIOSH. For example, the IDLH for chlorine was 30 ppm but the National Research Council had set an Emergency Exposure Guidance Level (EEGL) for chlorine of 3 ppm for 1 hour. Given the urgent need for a new Pocket Guide edition and the concerns for the adequacy of some existing IDLHs, NIOSH determined it would be best to reevaluate all IDLHs by the best process we could manage with limited time and resources. In several instances that review led to reductions, e.g., to an interim value of 10 ppm for chlorine. The new Pocket Guide (pages xi-xiii) made it clear that the 1994 IDLH values would require another revision after the process for developing IDLHs was refined.

Interim though they may be, the IDLHs contained in the 1994 Pocket Guide are the best and most extensive listing of the kind. Out of over 380 IDLH values, only three were seriously questioned by ORC. NIOSH continues to believe that some of the older IDLH values were not adequately protective. Therefore, it would be inadvisable to disregard the 1994 IDLHs on procedural grounds and to revert to the previous values without weighing the merits on a case-by-case basis.

The Pocket Guide briefly explains the process by which NIOSH derived the 1994 IDLHs and advises the reader how to obtain a detailed explanation plus documentation for each IDLH (Documentation for Immediately Dangerous to Life and Health Concentrations (IDLHs), NTIS Publication No. PB-94-195047). That documentation will soon be available on the NIOSH World Wide Web home page (

NIOSH has had discussions with ORC and others on how best to establish a process for the on-going revision of existing and development of the new IDLHs. We expect to publish a Federal register notice this summer in which comments received in response to the previous notice will be summarized and comments will be invited on a proposed new process. That new process will include a method for identifying and prioritizing IDLHs that stakeholders believe should be reevaluated.

NIOSH is committed to developing, in partnership with all interested parties, an improved IDLH process. Please don't hesitate to call me at 404-639-3773 if you or members of your staff would like to discuss our thoughts and plans in this regard.

Sincerely yours,

Bryan D. Hardin, Ph.D.
Acting Deputy Director