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Methodology
The complex and potentially variable mix of chemical species in the condensed phase and the vapor
phase of diesel exhaust, required the measure of exposure related to carcinogenic risk to be specified.
The most commonly used measure of exposure is atmospheric concentration of particles in µg/m 3 . That
measure is obtained from the mass of particles collected on a filter per volume of the air that flowed
through the filter. On the basis of its relation to health studies and its general practicality, that measure
was used in the diesel exhaust TAC document cancer risk assessment (OEHHA, 1998).
OEHHA used two approaches to employing epidemiological studies for diesel exhaust quantitative risk
assessment. The first approach used the overall relative risks derived from the meta-analysis along with
an overall range of exposure for all the studies. The second approach focused upon the railroad worker
studies in developing the range of unit cancer risks.
Meta-analysis-Derived Cancer Unit Risks
The results of the meta-analysis provide information useful in bracketing the broadest likely range of
plausible carcinogenic potencies for diesel exhaust. The pooled relative risk values derived from the 12
epidemiological studies in the meta-analysis which adjusted for smoking were 1.44 (95% C.I. 1.32 -
1.56) for the fixed effects model and 1.43 (95% C.I. 1.31 -1.57) for the random effects model. The
magnitude of these relative risks provide information on the potential magnitude of the cancer risk
associated with diesel exhaust exposure. For the random effects model the upper 95% confidence limit
on excess relative risk is 0.57.
None of the studies in the meta-analysis provide direct measurements of exposure concentration over
the time of their follow up. Therefore, to the extent that the meta-analysis can be used to bracket the
carcinogenic potency of diesel exhaust, the exposures of the various study populations need to be
reconstructed. Hammond (1998) has reviewed the available industrial hygiene survey literature on the
occupations considered in the meta-analysis (bus garage workers, mechanics, truck drivers, heavy
equipment operators, railroad workers) and provided estimates of the plausible possible ranges of
workplace exposures of diesel exhaust respirable particulate matter for those occupations. Because of
the overall limitations in the data, the estimated ranges for each occupational subgroup of interest are
particularly broad. The lowest plausible estimate of occupational exposure for any such subgroup is 5
µg/m 3 (heavy equipment operators). The highest plausible estimate of any occupational subgroup is 500
µg/m 3 (bus garage workers, railroad workers, mechanics). The total range of plausible exposures for
the different populations therefore varies 100-fold. Using these air concentrations and the assumption
that workers inhaling 10m 3 of air per work shift were exposed to them for over 45 year period for a 70
year lifetime, it is possible to characterize a bracket of risks compatible with the results of the metaanalysis:
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