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In the United States, diesel engineexhaust has been identified as a potentially significant con-tributor to cancer risk from air pollution. California, for ex-ample, has quantified cancer impacts from diesel particulatematter (DPM) and identified DPM as the leading cancerrisk among air toxics in the state.1 By contrast, the U.S. Envi-ronmental Protection Agency (EPA) has noted the poten-tial for DPM to be carcinogenic, but has concluded that zerocancer risk cannot be ruled out and, therefore, has chosennot to quantify DPM’s cancer toxicity for ambient exposure.2

A recently released update of a key study at the core of thisdebate further compounds the controversy.3 As we considerpolicies to reduce cancer risk from environmental pollution,it is worthwhile to review some basic issues surrounding theestimation of cancer risk from DPM, consider their impactson policy-making, and place these estimates in context withthose associated with other endpoints and pollutants, suchas all-cause mortality from ambient PM.

ESTIMATING CANCER RISKSince ambient PM comprises thousands of chemical com-ponents, originates from many different sources, and isassociated with a variety of adverse health effects, there areseveral ways to sort the issue of airborne PM and publichealth. For starters, it may be practical to describe PM bycomponent since many of these components can be identi-fied in ambient monitor samples and may then be linked tofundamental studies of toxic mechanisms. In turn, thesemechanisms help explain the basis for health effects com-monly associated with PM in epidemiologic studies. How-ever, while sorting PM in this manner can be useful forestablishing a causal mechanism, it often does so at the ex-pense of being able to determine the source(s) from whichthe PM came. The latter is important because sources, ratherthan components, are often the target of pollution controlpolicy. Therefore, it may also be convenient, if more diffi-cult, to sort PM by source contribution, since it helps appor-tion ambient exposures to specific sources or source typesand aids regulatory impact analysis and policy-setting.

DPM presents that rare example of an aerosol that isdefined by its source, but also has a substantial number oftoxicologic and epidemiologic studies focused on its com-ponents, both separately and as groups. This has obviousappeal for policy-setting and analysis: adverse health effects(and the benefits of avoiding them) can be quantitativelylinked not only to the specific pollutant, but also its sourcetype. When a specific source type contributes a substantialportion of emissions and is readily identifiable, as are dieselvehicles, specifying the pathway between source and healtheffect holds the promise of designing policies that are bothhighly effective and efficient.

Yet for all the policy potential that DPM has, the history ofstudies connecting diesel and cancer risk is long, complicated,and not nearly complete. Several studies on lung cancer and

Andrew M. Wilson is a research associate at theHarvard Center for Risk Analysis in Boston, MA.

E-mail: awilson@hsph.harvard.edu.

DPM presents that rare

example of an aerosol that is

defined by its source, but also has

a substantial number of toxico-

logic and epidemiologic studies

focused on its components.

Copyright 2005 Air & Waste Management Association

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air pollution have providedgeneral, but not unequivo-cal, support for a causal linkbetween the two. Cohortstudies examining ambientPM exposure and lung can-cer, particularly a recentupdate including a longerfollow-up period more ap-propriate to the latencyassociated with lung cancer,have reported positive asso-ciations.4,5 Studies of com-mon components of dieselexhaust, such as benzeneand polycyclic aromatic hy-drocarbons, offer supportfor the notion that DPM maybe carcinogenic. Finally, andperhaps most directly, a largenumber of occupationalstudies of railroad, trucking,and other workers exposed todiesel exhaust report associa-tions with lung cancer.6-8

However, the same stud-ies that have helped per-suade many regulatoryagencies and research groupsthat DPM is likely carcino-genic have not led to similarconsensus on how this effectmight be quantified, or evenwhether it should be quanti-fied. California stands out asthe most prominent regula-tory supporter of a quantita-tive estimate, having proposedDPM as a toxic air contami-nant based on its potential to

cause cancer.1 In policy analyses, the California Air ResourcesBoard has developed and uses a cancer slope factor (CSF) basedon an occupational study of railroad workers. It interprets thisstudy as showing a quantifiable relationship between magni-tude of exposure and lung cancer rates. Using its central esti-mate CSF of 3 x 10-4 per µg/m3, a marginal DPM concentrationof approximately 1.8 µg/m3, and a population of 34 million,California analyses suggest an increase on the order of 240 can-cers annually. This represents approximately 70% of estimatedcancer risk to the state’s population from air toxics.

By comparison, EPA has concluded from these studiesthat DPM is “likely to be carcinogenic to humans by inhala-tion” for environmental, not just occupational, exposure set-tings.2 The agency stresses, however, that the evidence is notstrong enough to make this conclusion definitively. Thoughit presents sensitivity analyses of what these risks might be,based on estimates from occupational studies such as usedby California, EPA has elected not to develop and/or use a

REFERENCES1. Proposed Identification of Diesel Exhaust

as a Toxic Air Contaminant—Part B:Health Risk Assessment for Diesel Ex-haust; California Office of Environ-mental Health Hazard Assessment,Air Toxicology and EpidemiologySection, Oakland, CA, May 1998;available at ftp://ftp.arb.ca.gov/carbis/regact/diesltac/partb.pdf(accessed March 2005).

2. Health Assessment Document for DieselEngine Exhaust; EPA/600/8-90/057F. Prepared by the NationalCenter for Environmental Assess-ment, Washington, DC, for the U.S.Environmental Protection Agency,Office of Transportation and AirQuality, 2002; available atwww.epa.gov/ncea (accessedMarch 2005).

3. Garshick, E.; Laden, F.; et al. LungCancer in Railroad Workers Ex-posed to Diesel Exhaust; Environ.Health Perspect. 2004, 112 (15), 1539-1543.

4. Dockery, D.W.; Pope, C.A.; et al. AnAssociation between Air Pollutionand Mortality in 6 United StatesCities; N. Engl. J. Med. 1993, 329(24), 1753-1759.

5. Pope, C.A.; Burnett, R.T.; et al.Lung Cancer, CardiopulmonaryMortality, and Long-Term Expo-sure to Fine Particulate Air Pollu-tion; JAMA 2002, 287 (9), 1132-1141.

6. Steenland, et al. Case-Control Studyof Lung Cancer and Truck Drivingin the Teamsters Union; Am. J. Pub-lic Health 1990, 80 (6), 670-674.

7. Garshick E.; Schenker, M.B.;Munoz, A.; Segal, M.; Smith, T.J.;Woskie, S.R. A Case-Control Studyof Lung Cancer and Diesel ExhaustExposure in Railroad Workers; Am.Rev. Respir. Dis. 1987, 135 (6), 1242-1248.

8. Garshick, E.; Schenker, M.B.;Munoz, A.; Segal, M.; Smith, T.J.;Woskie, S.R. A Retrospective Co-hort Study of Lung Cancer and Die-sel Exhaust Exposure in RailroadWorkers; Am. Rev. Respir. Dis. 1988,137 (4):820-825.

CSF for DPM in its policy analyses, stating that the humanexposure–response data are too uncertain and the animalexposure–response data do not reflect human environmentalexposure levels. In its diesel health impacts estimates, EPA esti-mates the impact of DPM on all-cause mortality in adults andinfant mortality, but not direct effects on cancer incidence. Here,DPM is assumed to have the same toxicity as ambient PM. Indoing so, the same marginal increment of 1.8 µg/m3 DPM inthe California example above, treated as ambient PM aboveany potential threshold and combined with a commonly-usedall-cause mortality coefficient, would be associated with approxi-mately 1600 deaths in the same adult population.

This comparison has important implications for evaluat-ing policies to reduce cancer risk. First, while substantial un-certainty surrounds the quantified estimates of cancer casesattributable to DPM, the magnitude of this effect is poten-tially significant. Second, while DPM may pose the largestinfluence on excess cancer incidence within the set of pol-lutants categorized in regulation as air toxics, it is alsoassociated with another pollutant not included in thisclass—ambient PM—that is likely to have significant impactson another health endpoint (i.e., all-cause mortality). There-fore, evaluating pollution control policies for a limited setof pollutants over a limited set of health impacts may ignoreimportant collateral effects. Ignoring the broader picturemay lead to less efficient policy decisions.

These differences in characterizing health impacts fromDPM are not likely to remain at a perpetual impasse, for atleast two reasons. First, as diesel engine and control tech-nologies mature, both the exposure levels and compositionof DPM may change significantly, limiting the usefulness ofprevious exposure studies and conclusions about DPM tox-icity. In the front pages of its health assessment document,2EPA highlights this issue, declaring that future analyses willhave to revisit the toxicity issue in light of these changes.Second, new and/or revised toxicologic and epidemiologicstudies will emerge, offering new evidence to be considered.Already, an update to the railroad workers study underlyingthe California CSF has been released, allowing for a greaterfollow-up period and more insight into the critical issue ofthe healthy worker survivor effect.3

CONCLUSIONSThe controversies surrounding DPM and health impact as-sessments will evolve substantially in the future. Determin-ing whether, how, and with what potency DPM causes cancerwill, of course, remain an important issue. But the relevantevidence will change. Emerging studies—whether epidemio-logic, toxicologic, in vitro, or human experimental—thatcan directly inform the relationship between mortality andexposure intensity have the potential to shift the evidencebase away from occupational and toward environmental set-tings. In turn, these will affect the bases used to comparepolicy alternatives. As the United States and other developedcountries transition from generally high, visible levels ofpollution to lower pollution levels, a new generation of stud-ies to describe more subtle health effects is likely to rede-fine current debate and offer new insight for policy. em

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