VOC sampling locations table for Ground Zero area, Jan 2002

NYC 9/11 Public Portal Document

IV.d. Dioxin

Dioxin-like compounds (referred to also in discussions below simply as “dioxins”) are formed during combustion, and it is expected that production of these compounds would have resulted from the initial impact and ensuing fires at the WTC. Dioxin-like compounds may be formed in other ways as well, such as in the chlorine bleaching process for paper products or in the manufacturing process for certain chlorinated organic chemicals. However, uncontrolled or improperly controlled combustion appears to be the major source of new emissions (EPA, 2000). A total of 30 compounds are considered to be “dioxin-like”: 7 polychlorinated dibenzo-p-dioxins (abbreviated dioxins), 10 polychlorinated dibenzofurans (ftirans), and 13 coplanar PCBs. These individual compounds are called “congeners.” Measurements at the WTC included only the 17 polychlorinated dioxin and furan congeners, not the PCB congeners (total PCBs were measured, as discussed above). Because dioxin-like compounds are present at minute quantities, concentrations in this section will be described in terms of picograms per cubic meter (pg/m-’).

Concentrations of dioxin-like congeners are expressed on a toxic equivalent, or TEQ basis. A TEQ concentration is calculated as the sum of the toxically equivalent concentrations of each of the 17 congeners. A congener’s TEQ concentration is calculated as its concentration (Cj) times its toxicity equivalency factor, or TEFj. TEF values are equal to 1.0 or less, and relate the toxicity of 16 of the 17 congeners to the most toxic congener, 2,3,7,8-TCDD (the 17* congener naturally assigned a TEF of 1.0). An overall TEQ concentration is, therefore, 2(TEFj*Ci). When a congener was not detected in the sample, a value of one-half the detection limit was used for that congener in calculating the TEQ concentration. This is the traditional approach to calculating a TEQ concentration when some of the congeners are not detected and others are; alternate approaches include assigning either 0 or the detection limit to the congeners that are not detected.

The TEQ concentrations on the WTC web site were developed using the “International” set of TEF values (I-TEF; EPA, 1989). In 1998, the World Health Organization proposed a new set of TEF values (WHO-TEF; Van den Berg, 1998). The principal change relevant to quantifying the TEQ concentration of a mixture comprised of the 17 dioxin and furan congeners is that the TEF for the penta dioxin-like congener, 1,2,3,7,8-PCDD, has been increased from 0.5 to 1.0. The other change of less impact to calculating the TEQ concentration is that the TEF values of the two octa congeners, OCDD and OCDF, were reduced from 0.001 to 0.0001. Since the penta congener occurs in most samples, the net effect of this change is to increase the TEQ concentration slightly, in the range of 5-10%, depending on the prevalence of 1,2,3,7,8-PCDD in the sample. For example, on the sample taken from the WTC Building 5 monitor on September 23, when the I-TEQ concentration was 161 pg TEQ/m^, the 1,2,3,7,8-PCDD concentration was 40.8 pg/m’. In the I-TEF scheme, this contributes 20.4 pg/m’, but in the WHO-TEF scheme, this would add 40.8 pg/m’ to the final TEQ concentration. Therefore, the measurement of 161 pg TEQ/m’ would increase to 181 pg TEQ/m’ when switching to the WHO-TEF scheme, an increase of about 12%. An examination of the data shows that other I-TEQ concentrations would also increase, some by less than 5%.

This assessment uses the I-TEQ concentrations presented on the EPA WTC web site. Converting to WHO-TEQ concentrations would make a small and insignificant change to the

DRAFT-DO NOT QUOTE OR CITE 73 October 2002

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