NYC-WTC_000232959

NYC 9/11 Public Portal Document

Air sampling was conducted to determine the actual levels of fibers, dust, and crystalline minerals in the air at the time of settled surface dust sampling. Where feasible, these results are correlated to the settled surface dust results.

1. Fibers in Air

Samples for airborne fibers were first analyzed by phase contrast microscopy (PCM) to count all fibers present according to the National Institute for Occupational Safety and Health (NIOSH) Method 7400 counting rules. Phase contrast microscopy identifies fibers based on size and shape, but not the specific type of fiber. Therefore, the fibers seen by PCM include asbestos fibers as well as other fibers, such as textiles, fiberglass, and cellulose that might be present. Any sample with airborne fibers above the highest PCM levels found in the comparison locations was re-analyzed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The TEM methodology can distinguish what types of fibers are in a sample, particularly asbestos fibers. The SEM method identifies SVF. Some fibers present in the settled dust of the area may become suspended in the air and be present in the air sample.

a. Indoor Air

Fiber levels in air are expressed as the number of fibers in each cubic centimeter (cc) of air or fibers per cc (f/cc). Low levels of fibers were found in the comparison areas above 59th Street homes, where fiber levels ranged from less than 0.001 f/cc (<0.001 f/cc) to 0.003 f/cc. Because no asbestos was detected in the settled surface dust of the compaiison areas above 59th Street, these fibers are not likely to have been asbestos. SEM analysis showed they were not SVF. Figure 7 shows all airborne fiber levels determined by PCM in the lower Manhattan area with respect to the fiber levels found in the comparison areas above 59th Street. These data represent all fibers found according to the analytical method used (NIOSH 7400) and are not specific to asbestos or SVF.

The majority of the lower Manhattan air samples from indoor areas without asbestos in the dust were within the range of fibers found in the comparison areas above 59th Street (<0.001 to 0.003 f/cc). Six of these air samples (from 3 common areas and 3 residences) had higher levels of fibers in air than the comparison areas above 59th Street (Table 8). Four of these areas had measurable levels of SVF in the settled surface dust. All six-air samples were re-analyzed by TEM to determine if the fibers present included asbestos fibers. Asbestos fibers were not found in these six air samples (Table 8). A third analysis was done by SEM to determine if SVF fibers were present in those air samples from areas with SVF in the settled surface dust. SVF fibers were found in three of these air samples at concentrations from 0.000037 f/cc to 0.000255 f/cc. Although SEM is a more sensitive method, detecting thinner fibers than PCM, reported SVF levels were well below the PCM method detection limit of O.OOlf/cc. SVF fiber concentrations in the comparison areas above 59th Street ranged from none detected to 0.000087 f/cc. Although air samples from comparison areas above 59th Street were not re-analyzed by TEM, no asbestos fibers were noted on the subsequent SEM analysis for SVF. Therefore the six air samples that had total

23

CITY_21MC100_011171460

NYC-WTC_000232991