NYC-WTC_000232959

NYC 9/11 Public Portal Document

The XRD settled surface dust results are also considered semiquantitative (“J”) due to particle sizes used for standard curve/calibration (as described for the air samples) and due to the lack of standard analytical error (SAE) information from the laboratory.

E. Data Analysis

The data were entered into an Excel spreadsheet, and descriptive statistics were developed for the sample results. Basic trend analysis on the analytical data was conducted using the SAS statistical program (version 8.01). The SAS System is an integrated system of software providing complete control over data access, management, analysis, and presentations. Examples of some of the SAS procedures (“Proc”) used were the “Proc Mean” and “Proc t-Test.”

F. Limitations and Uncertainties

The intent of the sampling was not to characterize the total extent of contamination in lower Manhattan or determine the extent of exposures for all people in lower Manhattan that may have resulted from the collapse of the WTC towers. Rather, it was a preliminary investigation to determine if building collapse-related materials could be present at levels of possible health concern in lower Manhattan. Results from this investigation do not necessarily reflect conditions that would be found in other buildings, at other times just following the collapse, or after the samphng period. The measurements reflect conditions present at the time of the sampling (November 4 December 12, 2001) in the buildings and areas sampled.

Some uncertainties tend to underestimate the risk of exposure while other uncertainties tend to overestimate the risk of exposure.

• The locations selected for sampling within each building may not be representative of the rest of the building; the building-specific results may either overestimate or underestimate the levels of WTC-related materials in other areas of the building.

• Sampling was conducted in November and December of 2001. Levels of particulate matter and airborne irritants were likely to have been higher in the days and weeks immediately after the WTC collapse. Levels of settled surface dust were likely higher as well. Outdoor dust contamination would have been reduced by wind, rain, and cleaning (HEPA trucks to vacuum the streets and sidewalks). Indoor settled surface dust may have been reduced if appropriate cleaning had been conducted prior to this sampling event. Therefore, these results serve as estimates, and are not necessarily reflective of conditions at earlier time points or of cunent conditions.

• Likewise, if continued cleaning has occurred since the November-December 2001 sampling event, levels of contaminants should be lower today. Therefore, the results of this sampling effort would tend to overestimate the current levels of WTC-related materials in the buildings.

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