WTC health criteria appendix, Dec 2001

NYC 9/11 Public Portal Document

Contact Frequency (CF)

This parameter is one of the most difficult to estimate. Previous assessments have usually assumed 1 contact per day. Often, this assumption is made with little discussion, as if multiple contacts with the surface were not considered. 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 of professional judgment; such exposure would obviously involve a very physical job with frequent contact with the walls and floor. If available, site-specific activity data should take precedence.

Fraction Transferred from Surface to Skin (FTSS)

This parameter is also difficult to estimate. Obviously, the worst-case assumption would be an FTSS of 1, or 100% transfer. However, for some chemicals, the range may be narrowed even beyond that. FTSS is assumed to be chemical-specific.

USEPA has previously assumed transfer of 0.5 for PCBs (USEPA, 1987) based on an Office of Toxic Substances (OTS) assessment. Michaud et al (1994) assumed 0.5 for PCBs and dioxins, but stated that 0.1 might be more realistic. In developing re-entry guidelines for an office building after a fire, a panel of assessors used ranges of 0.1, 0.25, and 0.5 of the arm surface area for PCBs and dioxins (New York State Dept, of Health, 1985). However, that actually assumed 100% transfer over a smaller surface area (that is, these percentages were technically used to adjust the exposed skin surface area rather than attempting to estimate a transfer rate).

In a study of Malathion uptake from different surfaces, USEPA-EMSL found that FTSS of malathion from painted sheetrock to human hands was only 0.0003. (Mean transfer from vinyl flooring to hands was 0.0018, and from carpet to hands was 0.0152.) Malathion is a pesticide assumed to have lipophilicity more similar to PCBs than to volatiles or metals. However, the representativeness of such a number for PCBs and dioxins is unknown. PCBs are more lipophilic (have higher Rows) than malathion.

Exposure Frequency (EFI

The standard default factor for workplace EF is 250 days/year (USEPA, 1991). This represents days at work. The factor CF, discussed above, accounts for actual contacts with the contaminated surface during a working day.

Because central tendency estimations allow adjustment to 219 days/year (to account for all holidays, sick days, business trips, and vacation days), this factor may also be used for the low- end estimate for a full-time worker, in the absence of site-specific data (USEPA, 1993a).

Exposure Duration (EDI

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