EPA interim final report on WTC background study, April 2003

NYC 9/11 Public Portal Document

carcinogenic endpoint, a separate wipe assessment analysis should be performed taking into account childhood exposure parameters. A more complete discussion of the uncertainties associated with the methodology for surface contact assessment is presented in Appendix D.

With the exception of asbestos, the extremely limited air sampling data for specific building constituents in indoor air dictated that an assessment approach had to be based on historical data relating the concentration of fibers in air (fibers per cubic centimeters or f/cc) to the load in settled dust (fibers per square centimeters or fZcm^) [Millette and Hays 1994]. Although the empirical relationships (called K factors) are not rigorous enough to rely on for a firm health decision, they are used in this report to define what might constitute elevated building material counts that would need to be addressed on a near-term basis. This approach may overestimate potential exposures but, as such, is protective.

Developing Benchmark Levels Based on Occupational Health Standards

For fibrous glass and silica, risk-based methods were not available for indoor air due to lack of established environmental toxicity criteria. Therefore, benchmark levels were set based on occupational health standards established by the American Conference of Governmental Industrial Hygienists [ACGIH] (called “threshold limit values” or TLVs) and the Occupational Safety and Health Administration [OSHA] (called “permissible exposure limits” or PELs). TLVs/PELs are established to protect employees who might have been exposed to substances during an eight-hour day, for five days a week. For Tier I levels, TLVs/PELs were divided by ten to be more protective of residents who might be exposed to substances twenty-four hours a day, seven days a week. In addition, the TLVs/PELs were established to protect a healthy worker. For Tier HI levels, TLVs or PELs were divided by 100 to be protective of a more diverse population that includes sensitive individuals such as children, older adults, and the infirm.

The following tables show the benchmark levels developed for each contaminant of potential concern. Each table is accompanied by risk equations, toxicity criteria and a summary of any contaminant-specific assumptions that were made in developing the benchmark values.

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