WTC health criteria appendix, Dec 2001

NYC 9/11 Public Portal Document

Appendix D

Wipe Sample Assessment Methodology

Introduction

There is no consistent, established national guidance for the estimation of risks from wipe samples. Indeed, even the literature is scant on this issue. Besides literature searches, the expertise of USEPA scientists throughout the country (specifically NCEA and Superfund Regional risk assessors) was sought. A discussion of the various methods that were considered follows.

Because of the uncertainties involved when estimating risks from wipe samples, this assessment was not performed in the usual manner; that is, with data assessment, exposure assessment, toxicity assessment, risk characterization, uncertainty analysis, in that order. Instead, the risk estimation methods are discussed, the toxicity of the chemicals of potential concern are addressed, the various possible exposure scenarios are discussed, and concentrations that are associated with certain levels of risk are derived. The site data are to be compared with these levels. Risk characterization and uncertainty analysis follow.

This approach (hereafter referred to as the "multiple risk-based level approach") was suggested during the consultations with NCEA and the USEPA Superfund risk assessors throughout the nation. It was adopted because it is felt that the risks and the uncertainties associated with the risks are most clearly and appropriately described in this fashion. All the elements of a standard risk assessment are incorporated, but are arranged to best represent the site-specific situation. This approach is also consistent with many wipe sample evaluations in the literature, which usually involve the derivation of "re-entry guidelines" (for buildings after spills or fires) or "cleanup levels," and which are followed by comparison of those levels to the site-specific levels.

The most commonly used estimation method involves treating wipe samples as indicative of the contaminant source, estimating exposure parameters to determine dose, and using this information to estimate risk. This is based on traditional risk assessment approaches seen in USEPA, 1989a.

More recently, the above method has been discussed as being over-conservative. A time­ dependent transfer coefficient method, which assumes that duration of skin contact is more truly indicative than activity level of dermal uptake, has been proposed (Durkin et al, 1995). This

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CONFIDENTIAL, CITY_21MC100_003423735 SUBJECT TO PROTECTIVE ORDER NO. 2, SEPT. 2008

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