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 27, 1989



Regional Administrator
Office of Field Programs
[Directorate of Science, Technology, and Medicine]
SUBJECT: Sampling Procedures for Employees Using Air-Supplied Respirators


This is in response to your memo of February 27 to Thomas Shepich, Director of the [Directorate of Enforcement Programs], transmitting a January 23 memo from William A. Thomas, Area Director, concerning sampling procedures for employees using air-supplied respirators.

We are providing answers to his questions in the order that they were raised.

Question 1. The Industrial Hygiene Technical Manual states that the filter cassette should be placed inside the employee's hood when monitoring during work operations where air-supplied hoods are used. Does this apply to any operation where this type of equipment is used?

Answer: As you may know, the Industrial Hygiene Technical Manual (IHTM) is being revised in light of the latest Review Commission decisions, industry practice and new OSHA standards. Currently Section II. F.3. of the Industrial Hygiene Technical Manual conflicts with OSHA Instruction CPL 2.45A which requires that sampling be performed without regard to the use of respirators. In other words, it is OSHA policy to not allow sampling in or beneath any protective device classified and approved as a respirator. This conflict will be eliminated when the revised manual is completed.

The primary purpose of sampling is to determine the extent of employee exposures and the adequacy of protection. Sampling outside the respirator informs the employer whether the employer meets the obligation to keep employee exposures below the 8-hour TWA exposure limit as well as any applicable short-term exposure limit. Sampling outside the respirator also permits the employer to evaluate the effectiveness of engineering and work practice controls and informs the employer whether additional controls need to be installed. Furthermore, sampling outside the respirator is necessary in order to determine whether the action level has been reached or exceeded, and if so, the need to provide medical surveillance, training, and any other provisions triggered by the action level.

Question 2. Does it apply to situations where employees are exposed to lead? If so, if we measure beneath the hood and the exposure beneath the hood is below the action level, does this mean that the employer does not have to comply with the medical and training provisions of the lead standard?

Answer: If the hood is classified as a respirator, then sampling must be conducted outside.

Question 3. We have a situation where foundry chippers and grinders were exposed to silica above the PEL. In response to the citation, the company provided air-supplied hoods and is in the process of installing ventilation to reduce the amount of silica in the air. However, if monitoring is conducted in accordance with procedures specified in the Technical Manual exposure would be below the PEL and no additional engineering modifications are necessary. Is this correct?

Answer: The use of air supplied hoods provides, as an interim protective measure, effective respiratory protection to the exposed employees while feasible engineering controls are being installed. Air sampling within these respirators would not be allowed according to FOM policy or in the revised manual.

After feasible engineering controls have been installed, the ultimate acceptance of these controls in achieving compliance must be based on air sampling conducted outside of the air supplied hoods if they are still being worn.

Question 4. If employers are allowed to provide air-supplied hoods instead of installing engineering controls, considering listing this as a feasible abatement method on citations. Please let me know your thoughts in this area.

Answer: OSHA does not plan to deviate from its traditional policy regarding the hierarchy of controls. This hierarchy as expressed in the preambles to most OSHA health standards specifies that engineering and work practice controls be used in preference to respiratory protection.

However, the use of air-supplied respirators is permitted in the following circumstances:



  1. During the interval necessary to install or implement feasible engineering and work practice controls;
  2. In work operations such as maintenance and repair activities or other activities for which the employer establishes that engineering and work practice controls are not feasible;
  3. In work situations where feasible engineering and work practice controls are not yet sufficient to reduce exposure to or below the PEL; and
  4. In emergencies.

The above limitations on the use of the respirators are consistent with the requirements of past OSHA health standards and with good industrial hygiene practice.

[Corrected 10/22/2004]