EPA interim final report on WTC background study, April 2003

NYC 9/11 Public Portal Document

In cases where appropriate standards did not exist (e.g., asbestos), risk-based criteria were developed using established EPA risk assessment methods: for indoor air, methods described in EPA’s “Risk Assessment Guidance for Superfund” [RAGS, 1989] were used; for settled dust, EPA Region Hl’s “Wipe Sample Assessment” guidance was utilized with modifications (see Appendix D). The risk-based criteria reflect the most current toxicity criteria (Cancer Slope Factors and RfDs/RfCs) on EPA’s Integrated Risk Information System (IRIS), which is a regularly updated (quarterly), online database that reports chemical toxicity reference values and information on human health effects that may result fi-om exposure to chemicals in the environment.

For contaminants that lacked IRIS verified toxicity criteria, occupational standards were employed. Additional safety factors were added to account for higher exposure and greater sensitivity within the general population.

The clearance criteria paradigm described above is pathway and chemical specific. Accordingly, the benchmarks for each COPC do not account for multimedia and multiple chemical exposure.

Individual sampling results that exceed benchmark values should not be interpreted to represent the occurrence of an adverse health effect. Because benchmark levels assume continuous exposure for an extended duration, the average of the measured concentrations is more appropriate for evaluating risk than individual measurements. Consequently, isolated individual values above the benchmark level may not necessarily be indicative of a hazard.

Developing Risk-Based Criteriafor Indoor Air

For carcinogenic compoimds, the benchmarks were set so that a local resident’s lifetime risk of developing cancer fi-om exposure to WTC-related contaminants would not exceed a one-in-ten- thousand probability (1x10"^) above the resident’s background risk without this exposure. The amount of time that residents and office workers were exposed to WTC-related contaminants is unknown. To be conservative, the Tier I screening level was chosen to be protective of a resident who may have been exposed to WTC-related contaminants for one year. The Tier IH clearance level was chosen to be protective of a resident who is exposed to WTC-related contaminants for 30 years, which is the upper-bound estimate for residency in one dwelling [EPA, 1989]. Cancer risk fi'om less-than-lifetime inhalation exposure is given as:

Risk = LAC* UR

where LAC is the air concentration averaged over a lifetime, calculated as: AC * [EF*ED/LT], where AC is the average air concentration during the period of exposure (pg/m^), EF is the exposure fi-equency (days/year), ED is the exposure duration (years), LT is lifetime (days), and UR is the unit risk factor, expressed in units of 1/concentration.

The following table lists the input parameters and numerical values (along with a brief explanation) used in this procedure. (See Appendix D for equations and a more detailed discussion).

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