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Table 4.
Derivation of cancer potencies from NCI (1979) and Innes et al.(1969).
study tumor/group q 1
*
(mg/kg-day) -1
q animal (mg/kgday)
-1
q human (mg/kgday)
-1
maximum
likelihood
estimate
LCB
(95%)
NCI, hepatoma/ 0.0017 12.1 0.021 0.016 0.004
1979 male
hepatoma/ 0.0006 12.6 0.008 0.003 0
female
Innes, reticulum cell 0.035 0.035 0.47 0.2 0.05
1969 sarcoma/ male
hepatoma/ 0.021 0.021 0.28 0.11 0.03
female
LCB = Lower confidence bound.
The highest upper bound cancer potency for humans (q human ) was derived from the results of the Innes
et al. (1969) study showing reticulum cell tumor induction in male B6C3F 1 mice. However, confidence
in this value is reduced because the number of animals used in the study is small (18/group) and data
were reported incompletely. Innes et al. (1969) and NCI (1979) both present data showing induction
of hepatomas in female B6C3F 1 mice. However, lack of overlap between the 95% confidence bounds
of the two potencies suggests there may be a greater sensitivity to this effect in the strain used by Innes
et al (1969). Selection of a cancer potency value is made based on the most sensitive species, site, and
study in the absence of evidence indicating the value is not representative (CDHS, 1985). On balance,
the evidence favors neither the higher sensitivity of the Innes et al. (1969) study nor the high quality of
the NCI (1979) study. For this reason, a method of Anderson (1983) was chosen for combining the
results of these studies. The resulting cancer potency derived from the geometric mean of the four
potencies shown in Table 4 is 0.07 (mg/kg-day) -1 .
A unit risk value based upon air concentrations was derived by OEHHA/ATES using an assumed
human breathing rate of 20 m 3 /day, 70 kg human body weight, and 100% fractional absorption after
inhalation exposure. The calculated unit risk value is 2.0 E-5 (µg/m 3 ) -1 .
V. REFERENCES
Anderson EL and the U.S. Environmental Protection Agency Carcinogen Assessment Group. 1983.
Quantitative approaches in use to assess cancer risk. Risk Anal 3:277-295.
Armitage P and Doll R. 1954. The age distribution of cancer and a multistage theory of carcinogenesis.
Br J Cancer 3:1-12.
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