EPA interim final report on WTC background study, April 2003

NYC 9/11 Public Portal Document

Toxicity criteria for asbestos;

Inhalation Unit Risk = 2.3 E-01 (f'cc)’’ [IRIS, 2002]

Risk-based criteria were used to develop the Tiered benchmark levels for asbestos in air. Conservative assumptions of continuous exposure to a constant level of airborne fibers for either 1 year (Tier I) or 30 years (Tier HI) were combined with the IRIS Slope Factor to establish benchmarks representative of a lx 10-4 estimate of excess lifetime cancer risk for each scenario. This approach makes several assumptions, chief among those is the quantification of asbestos fibers in air based on the PCM definition of a fiber (greater than Sum in length with an aspect ratio of 3:1 or greater) and the use of the IRIS Slope Factor which was designed to apply to fibers so defined. Although there is some concern regarding shorter fibers, the approach used here represents the current consensus by the US EPA for quantifying risk of airbome asbestos fibers. It should be noted there is ongoing debate regarding the nature of health effects which may be attributed to shorter asbestos fibers. Both EPA and ATSDR are currently pursuing meetings to discuss and further refine these issues. However for the purposes of this response, addressing PCM equivalent fibers is considered protective.

The Tier I level for asbestos in settled dust was based on K factors [Millette and Hays, 1994], which are empirical relationships between concentrations of asbestos fibers in settled dust and indoor air. Millette developed the K factors by studying matched air and settled dust samples taken from various homes, at varying levels of activity in the home. The Tier I level was based on the K factor for a worst case scenario of a high level of activity. Although K factors are not rigorous enough to rely upon for a firm health decision, they are used here only to define what might constitute elevated fiber coimts, that would need to be addressed as a near-term concern. This approach may overestimate potential exposures, but as such, is protective.

It should be noted that the airborne Tier I level of 0.028 l/cc PCM equivalents roughly equates to the AHERA TEM standard of 70 f/mm^ (0.022 f/cc) in total fiber counts. Based on empirical information from the WTC ambient air monitoring program [EPA, 2002c] that recorded less than 20% of total AHERA TEM fiber counts to be >5 um in length, the AHERA TEM standard, of 70 f/mm^ (0.022 free) and the alternate AHERA PCM standard of 0.01 free both meet the risk­ based criteria established for the Tier I screening level.

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