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NIOSH Respiratory Diseases Research Program

Evidence Package for the National Academies' Review 2006-2007

NIOSH Programs > Respiratory Diseases > Evidence Package > 5. Respiratory Malignancies > 5.4 Lung Cancer Induced by Diesel Engine Exhaust

5.4a) Risk Assessment for Diesel Exhaust

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NIOSH has classified diesel exhaust as a potential occupational carcinogen since 1988 (A5-29); this classification was based primarily on data from experimental studies in animals; however, the excess risk of cancer in diesel-exhaust-exposed workers was not estimated in a quantitative fashion. In 1989, MSHA requested NIOSH, and in turn RDRP, to develop a quantitative estimate of the carcinogenic risk of diesel exhaust exposure. In 1990, the first risk assessment analysis by RDRP scientists was based on the results of animal inhalation studies, with no consideration of studies involving human exposure.151 Quantitative risk estimates for diesel exhaust based on animal inhalation data have been controversial, as some scientists consider the rodent response to be an example of a phenomenon known as “lung overload,” primarily documented in the rat. On this basis, the appropriateness of extrapolating rodent data to predict effects in humans has been questioned.


RDRP has attempted to address this controversy by developing human-based risk estimates for diesel exhaust and then systematically comparing the animal-based risk estimates to the human-based risk estimates.

To support of MSHA rulemaking, RDRP scientists prepared a report summarizing risk estimates from a variety of approaches using both toxicological and epidemiological data. In addition, RDRP scientists developed a quantitative risk estimate for occupational exposure to diesel exhaust, based on data from the trucking industry. RDRP scientists also collaborated in an analysis of potential confounding in risk estimates based on exposures in the railroad industry. These analyses have substantially extended the epidemiological database available for the development of quantitative risk estimates for diesel exhaust exposures applicable to the mining industry.

NIOSH and the National Cancer Institute (NCI) have undertaken a major epidemiological study of diesel exhaust and mortality, with a focus on lung cancer as an outcome. Study components consist of a large cohort mortality study, a nested case-control study of lung cancer, industrial hygiene surveys, and the development of retrospective exposure estimates. The study was initiated because of the need for epidemiologic information on the quantitative relationships between diesel exhaust and cancer, particularly lung cancer. A particular strength of the investigation is its focus on non-metal miners, because they receive much higher exposures to diesel exhaust than do other diesel-exposed workers. Non-metal miners typically have little exposure to other airborne contaminants (e.g. radon) associated with increased risk of lung cancer. The size of the cohort and case-control studies, together with the range of exposures, quantitative exposure data, and lack of confounders make this a particularly strong investigation.

Outputs and Transfer

RDRP scientist published six reports (A5-30), including two key reports (13, 14, A5-31, A5-32) used by MSHA as the primary basis for their risk assessment in their final rulemaking. The EPA made more than 25 references to NIOSH RDRP studies in its recent Diesel Emissions Health Assessment Document released March 2, 2007 ( [external link]).

Intermediate Outcome

On January 19, 2001, MSHA promulgated final rules to control diesel exhaust exposures in underground coal mines (66 FR 5526; 30 CFR 72) (A5-26), and in underground metal and nonmetal mines (66 FR 5706; 30 CFR 57) (A5-27).

 What’s Ahead

NIOSH and NCI are now in the process of completing their major epidemiological study of diesel exhaust and mortality. Progress on the study has been affected by intensive scrutiny and legal action by industry. The first scientific peer-review of the protocol by a tripartite group of industry, labor, and academic experts was contested on the basis it was an illegal federal advisory group. Judicial review led to nullification of the first peer-review and its findings, and the study was forced to repeat the protocol peer-review process from the beginning. This was undertaken using the NIOSH BSC as the peer-review panel. An industry-affiliated group then challenged the NIOSH BSC peer-review process. The U.S. Court of Appeals for the Fifth Circuit upheld the use of the NIOSH BSC but determined that the required notice of the committee's charter had been filed with the wrong Congressional Committee in the U.S. House of Representatives, thereby depriving that committee of effective oversight. Accordingly, the case was referred back to the district court to order a remedy. That court ordered NIOSH to provide the House Committee on Education and the Workforce with "all documentation of the BSC’ peer-review of the protocol." More importantly, the court also ordered NIOSH to "submit to the U.S. House of Representatives Committee on Education and the Workforce all diesel study data requested by the committee, as well as all draft reports, publications, and draft results or risk notification materials prepared in connection with the diesel study for review and approval prior to finalization and release and/or publication and distribution of such materials." The government appealed this order and the Court of Appeals reversed the lower court's finding that the Congressional Committee had the right to approve the study materials. Instead, the Court of Appeals required the government to refrain from publicly releasing all information submitted to the House Committee for a 90-day period. Work is proceeding and will be completed under the terms of this court order.


151. Smith R, Stayner S [1990]. An exploratory assessment of the risk of lung cancer associated with exposure to diesel exhaust based on a study in rats. Unpublished report, July 2, 1990, Division of Standards Development and Technology Transfer, NIOSH 1-54.