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, 2000

Mr. Larry Janssen, CIH
Technical Service Specialist
3M Occupational Health and Environmental Safety Division
3M Center Building 260-3B-09
St. Paul, MN 55144-1000

Dear Mr. Janssen:

Thank you for your December 10, 1999 letter to the Occupational Safety and Health Administration's (OSHA's) Directorate of Compliance Programs (DCP). You requested clarification regarding OSHA's position on the selection of air purifying respirators (APRs) for gases and vapors with poor warning properties, particularly the common diisocyanates such as Toluene-2,4-diisocyanate (TDI), Hexamethylene-1,6-diisocyanate (HDI) and Methylene bisphenyl isocyanate (MDI). We apologize for the delay in this response.

The Preamble to the revised standard points out that there is a great variation between individuals in their sensory detection of chemicals and most toxic substances do not have appropriate sensory warning properties. The standard no longer permits the use of warning properties as the sole basis for determining that an air-purifying respirator affords adequate protection against exposures to vapors and gases. For atmospheres which are not Immediately Dangerous to Life or Health (IDLH), APRs are now considered acceptable as long as appropriate precautions and change out schedules are in place, even for use against substances with poor warning properties.

Paragraph 1910.134(d)(1)(i) of the revised Respiratory Protection Standard states "the employer shall select and provide an appropriate respirator based on the respiratory hazard(s) to which the worker is exposed and the workplace and user factors that affect respirator performance and reliability."

Paragraph(d)(3)(iii) further states that for protection against gases and vapors an air-purifying respirator may be used provided that:

(1) The respirator is equipped with an end-of-service life indicator (ESLI) certified by NIOSH for the contaminant; or

(2) If there is no ESLI appropriate for conditions in the workplace, the employer implements a change schedule for canisters and cartridges that is based on objective data that will ensure that canisters and cartridges are changed before the end of their service life. The employer shall describe in the respirator program the information and data relied upon and the basis for the change schedule and the basis for reliance on the data.

Currently, there are few respirators available on the market with end-of-service life indicators (ESLI), and none for MDI. An employer must select a cartridge or canister recommended for that chemical by the manufacturer. The employer must then implement a change schedule for the canister or cartridges that is based on objective information or data that will ensure that the canister and cartridges are changed before the end of their service life. The data relied upon and the information forming the basis of the determination must be included in the written respirator program. If more information becomes available, an employer would be expected to review and if necessary, revise the change out schedule.

The employer is required to select the appropriate respirator for each situation. Some factors that should be included in the evaluation by the employer are:









  1. The concentration of contaminant in the air to which employees will be exposed. Most substances which have permissible exposure limits, have 8-hour time-weighted-averages. Other substances (including TDI and MDI) have Ceiling Limits. For these substances, employee exposure may at no time exceed the exposure limit for the substance. 29 CFR 1910.134(d)(3)(i) requires the employer to "provide a respirator that is adequate to protect the health of the employee and ensure compliance with all other OSHA statutory and regulatory requirements under routine and reasonably foreseeable emergency situations." Therefore, when selecting a respirator for protection against substances that have Ceiling Limits, the employer must not only consider if exposure levels may be reached or exceeded during routine operations but also if they may be exceeded during reasonably foreseeable emergency situations. The employer must then select a respirator that would provide adequate protection against these levels.
  2. Negative pressure respirators carry a greater risk of leakage than positive pressure respirators. If the face-to-facepiece seal is compromised, more contaminated air is likely to leak in than would be the case with positive pressure respirators.
  3. Vapors of diisocyanates are corrosive and severely damaging to the eyes. Contact may cause permanent eye damage. If a half mask respirator is selected, an employer would also be required under 29 CFR 1910.133(a)(1) to ensure that the employee uses appropriate eye or face protection.
  4. Vapors of isocyanates may cause skin irritation and sensitization. The employer is required under 29 CFR 1910.132(d) to assess the workplace and select appropriate personal protective equipment. Additional personal protective equipment to protect the skin of the face and neck may be required if an employer elects respirators which leave these areas exposed.
  5. Exposure to diisocyanates can cause various respiratory ailments. If an employee using an APR reports any medical signs or symptoms which could be attributed to isocyanate exposure, the employer must take appropriate action. If the symptoms may affect an employee's ability to use a respirator, 1910.134(e)(7)(i) requires additional medical monitoring.

We anticipate that some employers who perform the required evaluation will determine that APRs are appropriate for their circumstances. Others may prefer to provide powered-air purifying respirators (PAPRs). Under some circumstances, other employers may determine that SARs may be the only appropriate type of respirator for these hazards. As with any other situation requiring the use of respirators, an effective written respiratory protection program must be developed and implemented. Key provisions include fit testing, medical evaluations, proper use of respirators, training and information (including the need for a user seal check each time the respirator is donned), maintenance and care of respirators, program evaluation and recordkeeping.

You also noted that the OSHA website contains six letters of interpretation which predate the revision to the respiratory protection standard as well as statements on the "Isocyanates" Technical Links page which reflect that diisocyanates have poor warning properties, are sensitizers, or are too hazardous to allow air purifying respirators. OSHA is currently reviewing all letters on our website for inconsistencies with OSHA policy and appropriate action for these letters will be taken.

Thank you for your interest in occupational safety and health. We hope you find this information helpful. Please be aware that OSHA's enforcement guidance is subject to periodic review and clarification, amplification, or correction. Such guidance could also be affected by subsequent rulemaking. In the future, should you wish to verify that the guidance provided herein remains current, you may consult OSHA's website at http://www.osha.gov. If you have further questions please feel free to call OSHA's Office of Health Compliance Assistance at (202) 693-2190.


Richard E. Fairfax, Director
Directorate of Compliance Programs