VOC sampling locations table for Ground Zero area, Jan 2002

NYC 9/11 Public Portal Document

accomplished by multiplying ±e S/mm^ term by a conversion factor defined as the area of the filter paper, mm^, divided by the volume of air, liters or cubic centimeters, that is drawn into the air monitor. The AHERA Final Rule establishing a 70 S/mm^ standard for asbestos in schools (EPA, 1987; see next section for more details on this benchmark), one of the standards used in this report to evaluate the WTC data, specifies that a volume at least 1199 L (liters; 1.199*10^ cc) must be drawn into a monitor with a 25 mm filter (this filter size corresponds to an effective area of 385 mm^) or that a volume of at least 2799 L (2.799*10® cc) must be drawn into a monitor with a 37 mm filter (effective area of 855 mm^) when applying the standard. Therefore, the conversion factors for both filters, to convert S/mm^ to S/cc, are 0.000321 mm^/cc for the 25 mm filter and 0.000305 mm^/cc for the 37 mm filter. As a reasonable approximation, all results in S/mm^ can be converted to a volumetric S/cc basis by multiplying by 3*10'^ [S/cc]/[S/mm2], assuming 1200 L or 2800 L is drawn through an appropriate filter. Using this conversion factor, the AHERA standard of 70 S/mm^ is equivalent to 0.021 S/cc.

However, the conversion to S/cc still does not put the data on an equal footing with PCM data expressed on a f/cc basis, because “fibers” are almost always different from “structures”. Structures are bundles of fibers and many more “structures” can be identified by TEM than “fibers” by PCM because TEM can identify much smaller structures. Therefore, a TEM result for a given air sample (expressed on a volume basis, S/cc) will generally be greater than a PCM result for that same air sample (expressed as free). In general there is not a good correlation between PCM and TEM measurements, and the ratios of the fiber counts seen with these two methods will vary according to the types of asbestos involved and the nature of the exposure setting. As noted below in making some rough comparisons, ATSDR has assumed a ratio of 60 of volumetric TEM data to volumetric PCM data; that is, PCM data was multiplied by 60 to convert the data to TEM units by ATSDR (ATSDR, 1999). The was done for the purpose of comparing different data from around the country to make observations about background asbestos concentrations.

As much of the health effects data on asbestos are expressed in terms of PCM free, a useful variant of the TEM technique is to include counts of structures that would be expected to be visible under PCM and meet PCM criteria for counting as fibers. Specifically, structures meeting a minimum diameter of >0.3 pm with length >5 pm are counted as PCM equivalent (“PCME”) fibers.

IV.e.2. Risk Assessment Benchmarks for Evaluation of Asbestos Air Data The principal benchmark used in this assessment for evaluation of asbestos in air data from the WTC site is the Asbestos Hazard Emergency Response Action (AHERA) standard of 70 S/mm^. This standard is determined by TEM analysis. This standard is described in the Final Rule and Notice for Asbestos-Containing Materials in Schools (40 CFR Part 763, October 30, 1987; cited in this report as EPA, 1987), and that rule also provides details on the monitoring apparatus and the structure counting procedure. This counting procedure includes discussions on the amount of filter area to examine with different volumes of air, and also the requirement to count fibers with an aspect ratio of > 5:1 (aspect ratio = length to width ratio) and a length >0.5 pm. Briefly, this count of 70 S/mm^ is specific to a minimum volume of air requirement (1200 L if the filter size is 25 mm, and 2800 L if the filter size is 37 mm), and with this volume, a reading

DRAFT-DO NOT QUOTE OR CITE 88 October 2002

NYC-WTC_000149032