Health Assessment Document for Diesel Emissions - NSCEP | US ...

1 that their developing pulmonary and immunologic systems may be more susceptible than an
2 average adult's.
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4 12.3.5. Other Comprehensive Diesel Exhaust Health Assessments in the United States
5 In 1997 Cal-EPA (California Environmental Protection Agency) released a draft hazard
6 and .risk assessment for DE emissions. Cal-EPA concluded that a reasonable, very likely
7 explanation for the increased risks of lung cancer seen in experimental animal and epidemiologic
8 studies is a bona fide causal association between DE exposure and lung cancer. Because of
9 evidence for both carcinogenic and noncarcinogenic toxic effects, they also proposed to list DE as
1 0 a toxic air contaminant in California. Their conclusion for the carcinogenic hazard is similar but
11 not identical to EPA's, (i.e., DE is highly likely to be carcinogenic in humans). Cal-EPA also
12 provided a range for cancer risk per unit of lifetime exposure that is virtually identical to EPA's
13 risk range. The Cal-EPA risk range included a subrange of estimates from the animal studies and
14 a subrange of estimates from the human studies, as has EPA in this assessment. EPA indicates
15 that DE is "highly likely" to be ahuman carcinogen, whereas Cal-EPA is slightly more certain
16 about the likelihood. Given the health data uncertainties, data gaps, and the discretionary choices
17 that are made in risk assessment, the differences in the EPA and Cal-EPA assessment findings are
18 not significant. Cal-EPA's recommendations for noncancer respiratory.hazards are identical to
19 EPA's because they adopted the EPA RfC of5 !J.g/m 3 • As ofFebruary 1998, the Cal-EPA
20 assessment was still in draft.
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12.4. EXPOSURE PERSPECTIVE
Diesel emissions are complex mixtures containing thousands of organic and inorganic
constituents in both gas and particulate phases with differing chemical reactivities. After entering
the atmosphere, they are transported and transformed according to their distinctive characteristics,
undergoing physical and chemical changes that may form secondary pollutants more harmful than
their predecessors. Thus, a knowledge of diesel emissions at or near their sources is not sufficient
to fully assess their impact on human health and welfare. However, data on how DE contributes to
exposure levels for these secondary pollutants are currently lacking.
Determining the amount of DE present in the ambient air is also complicated by the
difficulty of distinguishing organic compounds and particles that originate in diesel engines from
those that originate in gasoline engines or come from other sources. This source speciation is not
well sorted out in the ambient characterization of DE.
Nonoccupational exposure to DE is worldwide in urban areas, with lesser exposure in rural
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areas. Certain working populations are also exposed to higher levels of DE than the rest of the
2/1/98 12-29 DRAFT--DO NOT CITE OR QUOTE