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.


January 10, 1990

SUBJECT: Use of Air-Purifying Respirators in Dangerous Concentrations of Gases or Vapors


This is in response to your memorandum of September 27, 1989, commenting on the letter dated August 23, 1989, from Acting Assistant Secretary Alan C. McMillan to Duane A. Siler, Counsel for International Fabricare Institute (IFI).

The letter relates that if appropriate procedures are followed, it is acceptable to use air-purifying respirators in the dry cleaning industry for protecting employees from perchloroethylene. The letter then identifies the procedures that must be followed. I will repeat each of them receiving your comments, repeat your comments, and then provide some clarification.

Procedure: The respirator cartridge must be changed periodically to assure that its capacity has not been exceeded.

Your comment: This is difficult to do because concentrations of the chemicals change.

Clarification: Employers must choose intervals between changing cartridges that are short enough so that the cartridge capacity will not be exceeded at the highest air contaminant concentration that occurs. For example, if airborne perchloroethylene concentrations range from 25 to 250 ppm, the employer must choose an interval between changing cartridges that is based on a perchloroethylene concentration of 250 ppm.

Procedure: The filter must be changed immediately if there is evidence of filter breakthrough because the employee detects the odor of perchloroethylene while wearing the respirator.

Your comment: The exposure limit is 25 ppm. The odor threshold is 5 to 75 ppm. Therefore, for some individuals when they notice an odor, they may already be exposed to three times the permissible exposure limit.

Clarification: Periodic changing of respirator cartridges is the basic means of protecting employees. Therefore, filter breakthrough should not occur. However, in an accidental case where it happens, it is important to stress the need for immediate remedial action. Moreover, the permissible exposure limit concentration is an 8-hour time-weighted average value. Therefore, if an employee's odor threshold for perchloroethylene were only 75 ppm, in order for the employee to become overexposured to perchloroethylene before being able to sense that it had broken through the cartridge, more than two hours and forty minutes of time would have to elapse prior to the concentration inside the respirator increasing to 75 ppm.

Procedure: The employer must abide by appropriate respirator protection factors.

Your comment: Unless air samples are taken frequently, the employer does not know the concentration and would not know how much of a protection factor is needed.

Clarification: Your comment applies for any type respirator. One way to reduce the required frequency of sampling is to provide respirators with protection factors high enough for all occasions. For the majority of dry cleaning establishments, there are indications that a protection factor of 10 is high enough for all perchloroethylene exposure situations. Of course, each dry cleaning establishment must make its own determination whether respirators with a protection factor of 10 are adequate for all exposure situations.

Where an inspection reveals inadequacies in the employers cartridge replacement program or in determining the efficacy of a certain respirator type, appropriate violations of [1910.134(c), 1910.134(d), 1910.134(g), 1910.134(h), and 1910.134(k)] shall be considered.

[Corrected 3/23/2009]