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.

October 20, 1988

Col. William D. Christensen
Chief, Analytical Services Division
Occupational and Environmental Health Laboratory
(AFSC) Brooks Air Force Base, Texas 78235

Dear Col. Christensen,

I have received your request for interpretation of the requirement for laboratory blanks. It is necessary to understand that compliance to OSHA analytical requirements for asbestos is based in the OSHA Reference Method (ORM) which is Appendix A in either the General Industry standard 29CFR1910.1001 or the Construction Industry standard 29CFR1926.58. The ORM gives a set of instructions and procedures that must be included in any asbestos method for determination of fibers in air. The NIOSH method 7400 is one method which complies with the ORM. I have included the OSHA method ID160 which is the method used here at the Salt Lake City Analytical Laboratory (SLCAL is the OSHA national laboratory).

The ORM requires 10% blanks with a minimum of 2 blanks for any set. These are interpreted to be field blanks. Field blanks are cassettes which are taken to allow an estimation of any contamination that may occur as a result of handling apart from actual air flow through the filter. They are to be opened and held open for about 30 seconds face down at the same place where the personal samples are mounted on the sampled employees. They are then closed, stored and sent to the analysis laboratory with the rest of the sampling set. There is no provision in the ORM for laboratory blanks. Point 1 of Appendix A requires that the filters that are used for this analysis are designated by the manufacturer as suitable for asbestos. This would of necessity require that manufacturers prescreen the cassettes for fibers. At this level we accept this prescreening as adequate for laboratory blanks.

Point 11 of the ORM requires that any set represented by a blank concentration in excess 7 fibers per 100 fields be rejected. Thus, any samples taken on filters with an excess of fibers would be eliminated from determinations of personal exposure. It is therefore unnecessary to analyze laboratory blanks when properly assembled and checked filter cassettes are used.

In conclusion, since the filters are prescreened by the manufacturer and since the blank is subtracted out of the result and since there is a cap on how much fiber may be on a filter, it is unnecessary for the user to submit laboratory blanks on a regular basis. Further, the only blanks referred to in the ORM are field blanks and as a result, only field blanks are required.

Should you have any questions feel free to contact me.


Daniel T. Crane
Supervisory Physical Scientist