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IV.
DERIVATION OF CANCER POTENCY
Basis for Cancer Potency
The IARC and US EPA carcinogen classifications for asbestos are 1 and A, respectively – that is, a
known human carcinogen. Ample data exists indicating that asbestos induces lung tumors and
mesotheliomas in both humans and animals. In cases where both human and animal cancer data exist
for a substance and both are suitable for quantitative risk assessment, use of the human data is preferred
by OEHHA. The cancer quantitative risk assessment of asbestos was therefore based on human
occupational lung cancer and mesothelioma incidence data, since both posed potential population risks
at ambient concentrations of asbestos.
Methodology
The CDHS (1986) risk assessment relies extensively on work done by the Consumer Product Safety
Commission (1983), the National Academy of Sciences (NRC, 1984), Nicholson (1985), and the
Ontario Royal Commission (1984). As with these risk assessments, the DHS risk assessment on
asbestos was based exclusively on the results of occupational epidemiologic studies.
Animal bioassay data were excluded from this analysis as there are numerous epidemiologic studies of
populations occupationally exposed to asbestos, which contain or have been supplemented with
exposure data adequate for purposes of quantitative risk assessment.
DHS adapted linear models developed and/or used in the work cited above to estimate risks of
mesothelioma and lung cancer to the general population. The models extrapolate risks observed in
numerous occupationally exposed cohorts to lower levels of asbestos found in the general environment.
In this case the range of extrapolation was four to five orders of magnitude. Results are presented
below.
Table 3: Estimated lifetime risks of lung cancer and mesothelioma due to continuous exposure to
0.0001 fibers/cm 3 of asbestos (expressed as cases per million population)
Exposure Group Lung Cancer Mesothelioma
Male Smokers 11 (110) 24 (120)
Female Smokers 5 (50) 32 (160)
Male Nonsmokers 2 (15) 32 (160)
Female Nonsmokers 1 (6) 38 (190)
Numbers in parentheses represent approximate upper 95% confidence limits.
The use of excess lung cancer lifetime risk values between 11 and 110 per million for 0.0001
fibers/cm 3 * of asbestos exposure were recommended. (*Fiber/cm 3 = asbestos fibers ≥ 5µm in length,
≥ 0.3 µm in width, with a length/width ratios of ≥ 3:1. Such fiber counts can be converted to total fibers
measurable by transmission electron microscopy (TEM) by multiplying by 100 to 1,000. Therefore,
0.0001 (PCM) fibers/cm 3 = 0.01 to 0.1 TEM fibers/cm 3 = 10,000 to 100,000 TEM fibers/m 3 .) For
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