NYC-WTC_000230857

NYC 9/11 Public Portal Document

Submitted to American Industrial Hygiene Association. October 2, 2001

determined prior to measuring airborne concentrations of asbestos. Similar care is advised for other analysts and investigators who are measuring and reporting worker and general population exposures to airborne asbestos fibers at or near the WTC disaster area.

The very short (typically <2 pm) and thin («0.25 pm) airborne asbestos fibers presented on a background of fine non-asbestos particulate (fibrous and non-fibrous) is a problem sample when conducting any airborne asbestos analytical method. Under the best of circumstances, a sample presenting with the combination of conditions, small thin fibers with a fine particulate background, requires the full attention of the analyst to produce and report reliable, unbiased data. It is critically important for the analyst to fully communicate all difficulties encountered when performing the analysis and to describe the potential influence any difficulty could have on the validity of the reported results.

It is equally important for investigators in the field to convey to the laboratory special conditions observed in the field when collecting air samples that could present an interference to the requested analytical method. If detection and measurement of short, thin asbestos fibers are important for the health hazard evaluation, or for compliance with regulatory standards, TEM analysis should be requested^. In addition, when short thin asbestos fibers are known to be present in the atmosphere, the influence of air sampling protocols on recovery of these fibers should be considered. Air filters submitted for transmission electron microscopy (TEM) should be polycarbonate or mixed cellulose ester filters with a 0.45 pm pore size, air sampling flow rates should be reduced to limit deep impaction of short thin fibers in filter media producing low fiber recovery, and laboratories should be instructed to employ method modifications if particle loading outside the tolerance limits of the method is encountered.

The set of data presented were developed as part of a prel iminary health hazard evaluation conducted at the site of the collapsed WTC towers and buildings. The data represent the type of data required to make proper health hazard assessments. Preliminary site surveillance suggested that asbestos-containing residue produced during the collapse of the WTC structures would have particle dimensions not typically encountered in routine environmental hygiene practice. This hypothesis was tested and ultimately supported after conducting particle characterization with size differentiation on the surface residue. Subsequently, it was deduced from the character of the surface residue that aerosols within the damaged but standing building structures could be dominated by short, thin asbestos fibers. For this reason, special attention to air sample collection protocols, analytical method selection, together with informing analysts of known particle characteristics, produced the best available estimate of asbestos aerosol concentrations. It is recommended that investigators be alert to the uniquely fine particle component in surface residue that contaminates some sites near the WTC and take appropriate measures to insure that sample collection and analytical methods account for this condition.

* Fibers less than 0.25 pm are not observed by NIOSH method 7400 (PCM) or reported by NIOSH method 7402 (TEM).

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