VOC sampling locations table for Ground Zero area, Jan 2002

NYC 9/11 Public Portal Document

A simple comparison of an air measurement and a health benchmark can be thought of as a “screening” exercise; the risk assessor is screening for possible problems. If the majority of samples are much less than a benchmark, then in most cases it would be appropriate to conclude that a health impact is imlikely. On the other hand, if most samples exceed the benchmark, then it may be appropriate to consider the possibility that a health impact may have occurred, or could occur, depending on the circumstances.

For dioxin toxic equivalent (TEQ) exposures, the air monitoring data are additionally used to conduct an assessment on cancer and noncancer risk. This involved defining the exposure scenario in greater detail, quantifying exposure within these scenarios for purposes of cancer risk estimation, and modeling the change in body burden over the exposure period for non-cancer assessment. Simple cancer screening exercises are also conducted for PCBs and asbestos. Insufficient information about the other chemicals precluded similar modeling. Where the data allowed, the best possible screening approach appropriate to each chemical class is used to evaluate potential health consequence from measured air concentrations.

In order to characterize exposure and risks, it is necessary to characterize the duration of exposure. Immediately following the collapse of the WTC towers, the NYC Mayor’s Office of Emergency Management restricted access to the WTC and surrounding sites. From September 11 through 14, this restricted zone included lower Manhattan south of 14* Street. Figure 1 shows the zones restricted after September 14. The areas in Figure 1 that are identified by a date show when those areas became accessible to the general public. Further details and additional maps can be found at, http;//www.nyc.gov/html/oem/html/other/restricted_zones/frozen_zone_history_pdf_page.html . When a zone was restricted, all pedestrian and vehicular traffic was limited to emergency management and rescue personnel and other credentialed people. Residents were not allowed to occupy homes located in the restricted zones. Although some people in certain areas might have come and gone quickly (for example, to collect pets), noone was living or spending a significant amount of time in these areas unless they were part of the rescue, recovery and cleanup operations. As of mid-May, 2002, there were 15 residential buildings in the restricted zone.

As described below, the collapse of the WTC towers resulted in exceedences of screening benchmarks. However, most of these exceedences were in restricted zones and persons who live and routinely work in these areas were unlikely to have been exposed. Regardless of the level of contamination that may be present, if there is no exposure there is no human health risk. In discussions below, the timing and location of all exceedences are identified, and it is noted whether the location was in a restricted zone at the time of exceedence. These restricted zones also influenced the development of “exposure scenarios” that were used in some of the contaminant-specific evaluations.

DRAFT-DO NOT QUOTE OR CITE 16 October 2002

NYC-WTC_000149097