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Table 7:
Calculated equivalent human exposure to TCDD and HexaCDD based on daily
animal dose levels from NTP (1980a, 1984a) carcinogenicity studies (CDHS, 1986)
a
Chemical Animal Daily Dose Level Airborne Concentration for
(µg/kg-day) Equivalent Human Exposure
(ng/m 3 )
TCDD female rat (0.45) a 0.0014 0.93
0.0071 4.6
0.071 46
male mice (0.048) 0.0014 0.44
0.0071 2.2
0.071 22
female mice (0.04) 0.0057 1.7
0.029 8.4
0.29 84
HexaCDD female rats (0.45) a 0.18 120
0.36 230
0.71 460
female mice (0.04) 0.36 100
0.71 210
1.43 420
Number in parentheses is animal body weight in kilograms.
Because the animal dose levels for TCDD were converted to human equivalent exposure from
inhalation, the 95% UCL, q 1 *, is a measure of the greatest potential excess cancer risk for humans. If
the lifetime daily exposure is expressed in µg/m 3 , then q 1 * is the excess risk associated with this
exposure. Since q 1 * for humans is a unit measure of excess lifetime cancer risk associated with
exposure to TCDD, it is termed the unit risk. With the unit risk, the 95% UCL of excess risk may be
calculated for any low-level exposure to TCDD by the equation R = unit risk × dose, where R is the
95% UCL of excess lifetime cancer risk. The cancer unit risks calculated by CDHS using the above
procedure for TCDD and HexaCDD were 38 (µg/m 3 ) -1 and 1 (µg/m 3 ) -1 , respectively.
CDHS recognized that total PCDD/PCDF in the air is composed of dozens of PCDD and PCDF
homologues and isomers. The chemicals in such a mixture are difficult to quantitate analytically. As a
result, usually only total PCDD and total PCDF are measured. In the Air Toxics Hot Spots program,
certain dioxin sources are required to perform stack testing and speciate the 2,3,7,8-congeners. Thus,
more data are becoming available to adequately characterize the risk from dioxin sources in California.
To estimate cancer risks from such mixtures requires information about: (1) the proportion of each
PCDD and PCDF in the mixture, and (2) the carcinogenic potency of each. However, these data are
not generally available. The proportion of isomers differs depending on the emission source, and only
three isomers had been tested for carcinogenic potency (2,3,7,8-TCDD and a mixture of 1,2,3,6,7,8-
and l,2,3,7,8,9-HexaCDD). It was also recognized that not all 2,3,7,8-isomer PCDDs and PCDFs are
equally carcinogenic. The results of the bioassays on TCDD and HexaCDD suggested that
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