EPA interim final report on WTC background study, April 2003

NYC 9/11 Public Portal Document

Do = Oral dose (mg/kg/day) BW= Body weight (kilograms) FTSM= Fraction transferred fi-om skin to mouth AT = Averaging time (days) ABSo = Oral absorption fraction

The surface concentration term (C) was assumed to remain constant over the entire 30 year exposure period. This is probably not true for surfaces in the residences impacted by the WTC attack. Surface loadings will decline as a result of volatilization, chemical degradation, surface cleaning and transfers to skin/clothing. While some redeposition will also occur, the net long term effect should be a gradual decline. In this sense, the method will overestimate exposures. This same issue has been recognized in similar assessments involving building clean-ups and pesticide exposures:

Two scenarios were considered in developing the procedures used to develop re-entry guidelines for the Binghamton State Office Building (Kim and Hawley, 1985). One assumed a constant surface concentration and the other assumed a first order exponential decay. The OPP guidance (EPA, 1997a and EPA, 2001a) uses a “dissipation” factor to account for degradation and other loss mechanisms after pesticide application. Similarly, Durkin et al (1995) has proposed a time-dependent transfer coefficient method for lawn treatment pesticides. Michaud et al (1994) proposed a mass balance model which accounts for losses from surfaces associated with building clean-ups.

Thus, while methods have been proposed to deal with changes in surface residue strength over time, it is uncertain how well these apply to the situation in residences near WTC. Thus for purposes of this screening assessment, it was decided to assume constant levels.

4. Calculating Clearance Criteria In order to calculate a clearance criteria for wipe samples, the formulas identified above need to be rearranged to derive a concentration based on the target cancer risk or non-cancer hazard. The clearance criteria for settled dust, using a wipe sample, for each COPC can be calculated using the formula listed below and the exposure parameters listed in Table 3 in the body of the document.

Clearance criteria (mg/m2) = (TRxBWxAT) / f((Ddx CSFd) + (Dox CSFo)) xEFxED]

The other terms in this procedure and associated uncertainties are discussed below:

Exposed Skin Surface Area (SAI The skin surface area of400 cm^ is a typical value for the palm side of adult hands (EPA, 1997b) which is the body part that is most likely to contact surfaces.

Contact Frequency (CFI Michaud et al (1994) assumed 8 contacts per day, apparently based on professional judgment. Low-end values of 1/day and "worst-case" rates of 24/day (3/hour) may be assumed as a matter

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