WTC health criteria appendix, Dec 2001

NYC 9/11 Public Portal Document

method is typically applied to pesticide-sprayed turf, but it is intended to relate dislodgeable residues of contaminated surfaces to dermal exposure. The main drawback to using this method for contaminated building surfaces is that infoiTnation about duration of skin contact is no more available than information about frequency of skin contact for this scenario.

In at least one case, the traditional method was supplemented by consideration of conservation of mass using a mass-balance model (Michaud et al, 1994). The rationale to this approach was to avoid the overestimation of exposure by acknowledging that the reservoir of contaminants is necessarily limited. This approach may be more appropriate for short-term spills or fires than for surface contaminations that have built up over the long term and penetrated deeply into the surface. However, uncertainties related to this aspect of the risk assessment should be characterized.

In at least one case, an attempt was made to deal with the uncertainties involved in the contact rate and fraction transferred to skin by assuming that a certain percentage of contaminants present in the building material would ultimately be transferred to the receptor over the exposure duration. The drawback to this method is that there is little information available that could be used to estimate this total transfer percentage, and this approach found little favor among USEPA risk assessors. This method has not been pursued further in this risk assessment.

Traditional Approach

Exposure to a contaminant on a building surface can be estimated as follows:

Ds = C X SA X CF X FTSS x EF x ED

Ds = Amount of chemical deposited on skin (mg) C = Concentration of chemical on contaminated surface (mgAm’) SA =Exposed skin surface area (cm^) CF =Contact frequency of skin against surface (1/day) FTSS= Fraction transferred from surface to skin EF =Exposure frequency (days/year) ED ^Exposure duration (years)

From Ds, oral and dermal exposure may be estimated:

Do =(Ds X FTSM x ABSo) / (BW x AT)

Do =Oral dose (mg/kg/day) FTSM= Fraction transferred from skin to mouth ABSo= Oral absorption fraction BW =Body weight (kilograms)

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