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.

February 3, 1999

Mr. Randy A. Milbrath
HSE Consulting and Sampling
2211 South 156th Circle
Omaha, Nebraska 68130

Dear Mr. Milbrath:

This is in response to your letter dated November 3, 1998, addressed to the Occupational Safety and Health Administration's (OSHA's) Directorate of Compliance Programs (DCP), regarding the proper air sampling methodology for welding operations. In your letter you have asked for a formal response regarding the correct placement of an air sampling cassette on employees performing welding operations while wearing a supplied air breathing ensemble.

The correct placement for air sampling cassettes is near the breathing zone of the employee. It should be as close as possible to the nose and mouth of the employee, i.e., in a hemisphere forward of the shoulders with a radius of 6 to 9 inches. If the employee is wearing a welding helmet and either no respirator or a negative pressure respirator, sampling should be done inside the helmet.

Since we are concerned with what an employee's potential exposure would be, sampling must always be located outside of any respiratory protective equipment. Sampling inside the ensemble that you have described would be measuring an unrepresentative atmosphere composed of a mixture of the air passing out of the respirator hood and the air outside the hood. This area would be an artificially low concentration. Therefore, sampling should be done as normally would be done for any other contaminant (i.e., in the breathing zone outside the ensemble).

I hope this answers any questions you have on OSHA's sampling policy for welding fumes. If you have any further questions, you may contact OSHA's Office of Health Compliance Assistance at (202) 693-2190.


Richard E. Fairfax
Directorate of Compliance Programs