EPA memo re: wipe sampling and cancer risk at WTC,

NYC 9/11 Public Portal Document

PCM is still used to determine whether workers are under the permissible exposure limit (PEL) by OSHA. The OSHA method requires reporting only of single fibers, not structures of asbestos (bundles and clumps of asbestos). The OSHA method also only alloys the counting of fibers 5.0 gm in length or longer. The OSHA method uses TEM in conjunction with PCM, but only to mle out those fibers found by PCM which were not asbestos. The TEM part of the OSHA method is not used to detect the smaller fibers that were not found by PCM.

PLM measurements for percent asbestos In bulk dust only

Polarized light microscopy (PLM) is used for bulk building materials, dust and debris samples to determine the percentage of asbestos.

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For thin settled dust layers, there is not enough to scoop up or place in ajar or bag. Thus, the PLM method above cannot be used. The settled dust must be sampled by either wiping or with a microvacuum. A microvacuum is a modified personal air sampling pump.

There are no EPA methods for either wipe samples or micro-vacuum sanpies. ASTM methods D-6480 (wipe sampling) and D-5755 (microvacuum) will most commonly give the results in units of structures/square centimeter. This is not a mass concentration - percentage.

Generally speaking, there is no readily available method to determine a mass percentage of asbestos in these settled dust layers, as you can using the PLM method above. There is one method, ASTM D-5756, a microvacuum method, which is designed to give results as mass concentration - percentage, but this method is rardy used, and very tedious and expensive in terms of laboratory time and effort.

TEM method for air samples and settled dust samples detects smaller fibers and

The best method of measuring asbestos in air is by transmission electron microscopy (TEM). TEM is a newer, more costly technique that can detect fibers smaller than 0.5 micrometers (0.5 or 0.5 um), which are missed by PCM. Thus, TEM measurements will give higher readings of asbestos fibers than PCM measurements if most of the fibers are actually asbestos, and if the asbestos fibers themselve^ have a significant fraction that are smaller than 0.5 gm. In WTC dusts, all the data I have seen indicates that TEM measurements are higher than PCM measurements of the same sample.

Dlffer^yce between “fibers” of asbestos and “structures** of asbestos

Asb-stos levels are given as “structures” or “fibers” or “structures + fibers.” Asbestos structures include both individual fibers as well as bundles of fibers, clumps of fibers, and things like particles of plaster with fibers sticking out. Since around the mid 198O's, methods have included counting all structures, since in the environment and/or body, these can easily break down into individual fibers.

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