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marginally signficant (p < 0.07). When analyzed, these data yield a human cancer potency factor of 2.0
× 10 3 to 5.8 × 10 3 (mg/kg-day) -1 .
In the Kociba et al. (1974) study, 60 male and female Sherman rats were exposed to concentrations of
dioxane of 0, 0.01, 0.1, and 1.0 % for 716 days. The tumor incidence for the combined male and
female data set was 1/120, 0/120, 1/120, and 10/120. Using the multistage procedure and an
interspecies scaling factor, an estimate for human cancer potency of dioxane was 5.7 × 10 -4
(mg/kg/day) -1 . This dataset was not used since tumor incidences for males and females were averaged.
The National Cancer Institute (1978) exposed male and female Osborne-Mendel rats to 0, 0.1, or 1.0
% dioxane in their drinking water for 110 or 90 weeks, respectively. The incidence of nasal tumors
were 0/33, 12/33, and 16/33 in the males, and 0/34, 10/35, and 8/35 in the females. From measured
water consumption and body weight data, the human cancer potency from a multistage polynomial fit of
these data was 9.5 × 10 -3 (mg/kg/day) -1 from male rat data, and 4.9 × 10 -3 (mg/kg/day) -1 from female
rat data. An adjustment for early mortality following the procedure of EPA (1988) yielded cancer
potencies of 1.1 × 10 -2 (mg/kg/day) -1 and 6.0 × 10 -3 (mg/kg/day) -1 from male and female rat data,
respectively. The NCI (1978) study using B6C3F1 mice was used as the basis for the cancer potency
for dioxane. This study contained the best data on the most sensitive species and sex, and the most
sensitive target tissue. In this study, 50 male or female mice were exposed to 0, 0.5, or 1.0% dioxane
for 90 weeks. Average doses were determined from weekly measurements of water consumption. The
estimated doses were 0, 720, and 830 mg/kg/day for the males and 0, 380, and 860 mg/kg/day for the
females. The incidence of hepatocarcinomas were 2/49, 18/50, and 24/47 for males, and 0/50, 12/48,
and 29/37 for the females. The incidence of hepatocarcinomas or adenomas were 8/49, 19/50, and
28/47 in males, and 0/50, 21/48, and 35/37 in females.
Methodology
A linearized multistage procedure (CDHS, 1985) was applied to the female mouse combined
hepatocellular carcinoma and adenoma incidence from the NCI (1978) study. The animal cancer
potencies were 8.3 × 10 -4 and 1.4 × 10 -3 (mg/kg/day) -1 , for the males and females, respectively. The
animal cancer potency, q animal , was calculated from the linear slope using the lifetime scaling factor q animal
= q 1 * × (T/T e ) 3 , where T/T e is the ratio of the experimental duration to the lifetime of the animal. The
animal cancer potencies were therefore adjusted for the short duration of the experiment, using the
factor (104/90) 3 . A value for the human cancer potency was determined using the relationship q human =
q animal × (bw h /bw a ) 1/3 , where bw is the default body weight of human or animal (mouse). Body weights
for interspecies scaling were assumed to be 0.04 and 0.035 kg for males and females, respectively.
The combined incidence of hepatocarcinomas and adenomas in males and females gave human cancer
potencies of 1.5 × 10 -2 , and 2.7 × 10 -2 (mg/kg/day) -1 , respectively. The combined incidence of
hepatocarcinomas and adenomas in females was used to derive the human cancer potency for dioxane
of 2.7 × 10 -2 (mg/kg/day) -1 . The airborne unit risk factor for dioxane of 7.7 E-6 (µg/m 3 ) -1 was
calculated by OEHHA/ATES assuming a human body weight of 70 kg and an inhalation rate of 20
m 3 /day.
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