NYC-WTC_000199812

NYC 9/11 Public Portal Document

bundles, leading to reported chrysotile fiber concentrations higher than would be the case if the material were examined directly on the sampled surface. The magnitude of the increase is a function of fiber length, with long fibers and bundles being less affected than short fibers and bundles. It is also important to consider that if the surface dust and debris are disturbed in such a manner that they become airborne, then elevated airborne asbestos levels will be generated.

5.3 Measurements of Asbestos in Exterior Dust and Debris

5.3.1 Sample Collection

Each sample of exterior dust and debris was collected by removing the material down to the substrate, and transferring the collected material to a clean polyethylene bag. On the north side.of the World Trade Center site, one sample was collected from the roof of an automobile parked on Church Street, south of Duane Street, and a second sample was collected from the apartment roof outside the Sth Floor loft at 45 Warren Street. On the southwest side of the World Trade Center site, two samples were collected at 250 South End Avenue. One sample was collected from the exterior window ledge of Apartment 1 ID, on which material originating from the collapse of buildings had collected to the maximum thickness possible on the ledge. Another sample was collected from the top of a low-level wall in the ground-level courtyard at the back of the building.

5.3.2 Sample Analysis

A gravimetric matrix reduction procedure (ASTM STP 1342) was used to analyze the samples. To ensure that the analyses were representative of the collected materials, a sub-sample of several grams was analyzed for each sample. In the gravimetric matrix reduction procedure, organic constituents were removed by ashing, acid-soluble constituents were removed using hydrochloric acid, and large particles were separated by sedimentation. The chrysotile in the material remaining suspended after sedimentation was quantified by optical microscopy using size-selective point counting of prepared filters. Large fiber bundles of chiysotile were hand-picked from the sediment and weighed. In order to quantify any amphibole asbestos present, the sediment remaining from the gravimetric matrix reduction was further separated by heavy liquid centrifugation. Large amphibole asbestos fibers were

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NYC-WTC_000199854