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 http://www.osha.gov.

April 26, 1989

MEMORANDUM TO:          LEO CAREY, Director
                        Office of Field Programs

THRU:                   EDWARD J. BAIER, Director
                        Directorate of Technical Support

THRU:                   FLOYD A. MADSEN, Director
                        SLC Analytical Laboratory

FROM:                   DANIEL T. CRANE, Supervisory Physical Scientist
                        SLC Analytical Laboratory

SUBJECT:                Asbestos Monitoring

The difference in collection efficiency for the asbestos collection devices has been thrown into the spotlight recently due to a set of paired samples taken in Region 3. We are aware of sampling differences between the 37 mm cassette and the 25 mm cassette. Most of the disparity between the two cassettes was eliminated by requiring electrically conductive cassettes. We are also aware that there are differences that have not been fully evaluated. In the case of these paired samples, the reason for the difference in results may be attributed to a loss of counting efficiency due to obscuring dust. In a note to AIHJ Vol. 47, April 1986, we reported a reduction of counting efficiency traceable to the load of non-asbestos dust on the filter. The samples taken by the Allentown Area Office were paired at the same flow rate for the same sampling times. This will lead to a 2.26x heavier loading on the 25 mm filters. Since the loss of efficiency appears exponential, this has a dramatic effect on the apparent count of the filters. Nearly all the difference between the paired sets can be traced back to this effect.

This fiber obscuration effect is not limited to the 25 mm cassette. It has been present on the 37 mm cassette as well. We have reopened an investigation of the cassette to determine a permanent course of action. In the meantime the following recommendations should be followed for occupational samples.

1. The 37 mm cassette may only be used with written justification. This is limited to situations which cannot be replicated by some use of the 25 mm cassette. The only situation currently accepted is where a long sampling time is required and the available pumps will not reliably sample lower than 1 Lpm.

2. With the exception above, use only new 25 mm dia. 0.8 µm pore size MCEF loaded cassettes.

3. Except in atmospheres expected to be very clean such as in office environments, do not sample higher than 1.0 Lpm as the suggested rate.

4. Sample for no longer than 2 to 4 hours per cassette to avoid overloading.