NYC-WTC_000230857

NYC 9/11 Public Portal Document

Submitted to American Industrial Hygiene Association. October 2, 2001

taken to understand each method’s limitations as well as its reliabihty and suitability for a specific measurement set of conditions (Yamate, 1984; Chang, 1987)). Failure to correctly select the proper method, or failure to properly weigh the significance of resulting data, based on method limitations, may result in incorrect estimation of actual worker exposures and corresponding health risk. In this specific case it is particularly important since the particle size of the overloading non-asbestos material is of the same size as the fiber length of the thin asbestos fibers. The increased particulate load requires thicker carbon films for support which in turn compromises proper identification of the thinnest fibers by selected area electron diffraction (SAED).

The data presented in this report suggest that some surface residues deposited as a result of the fire and collapse of the WTC towers contain asbestos in the range of 0.25 - 0.75%. Additionally, the particle size of the surface residue is very fine (and dry), a condition not typically encountered during routine asbestos removal projects. Furthermore, the asbestos present at a concentration of approximately 1 % is only a small fraction of the total mass of the surface residue, a residue that is a fine and dry powder. There is every indication, and it is reasonable to assume, that under commonly encountered conditions the components of the fine surface residue will become airborne and thus available for inhalation by workers. Under conditions where rapid dilution of the air does not occur, such as within building structures, it should be anticipated that very small asbestos fibers may become airborne and remain in the breathing zone of workers for extended periods. This condition will most likely occur inside buildings that have been contaminated with surface residues generated by the collapsed office buildings. This is evident in the data produced by the TEM method that detected asbestos fibers (asbestos structures) less than or equal to (<) 5 pm in length at elevated concentrations in indoor air.

Airborne asbestos fiber concentrations estimated by Phase Contrast Microscopy (PCM) (NIOSH method 7400) are useful when particle size is large, non-fiber particle loading is light, and measured atmospheres are assumed to be dominated by asbestos fibers. In cases when fiber size is small and the concentration is low, PCM analysis may underestimate the asbestos fiber concentration. In contrast, when non-asbestos fibers dominate the atmosphere PCM analysis may overestimate the asbestos fiber concentration. For these reasons, application of PCM analysis is best suited for routine compliance screening of asbestos abatement practices and may not be applicable to investigations such as those conducted at the WTC site. A review of the comparison between PCM/TEM data presented in Table 6 illustrates the limitation of PCM analysis for investigations of atmospheres such as those encountered at the WTC site. Generally, health hazard assessors look at both TEM and PCM data when performing assessments with the understanding that TEM is recognized as providing the most complete data about exposure to asbestos fibers.

During the performance of testing for this preliminary health hazard evaluation, care was taken to perform all methods under strict adherence to published procedures. These procedures were appropriately modified when sample backgrounds exceeded the tolerance of the method. This level of care was a recognition of the unique condition presented by the composition and particle size profile of the surface residue as

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