Scientific uncertainty of diesel exhaust epidemiology

Debra T. Silverman
Epidemiology 1998;9:4-5

In a recent study published in Epidemiology on diesel exhaust and lung cancer, researchers from the University of California (San Francisco) and the University of California (Berkeley) did a meta-analysis of 23 epidemiologic studies of diesel exhaust and lung cancer. (Note: 7 other diesel exhaust/lung cancer studies were excluded from the meta-analysis, 6 of which did not support the researcher's ultimate conclusion).

The researchers reported a relative risk of 1.33 (95 percent confidence interval 1.24. - 1.44).

But in an accompanying editorial, the National Cancer Institute's Debra T. Silverman wrote:

Skepticism regarding the carcinogenicity to the lung of diesel exhaust in humans arises from three main concerns about the epidemiologic evidence. First, and probably most important, the magnitude of the effect observed in most studies is low, with relative risks (RRs) typically under 1.5. Second, of the 30 studies conducted on the relation between diesel exhaust and lung cancer, only four have obtained either quantitative data on current exposure or semiquantitative data on historical exposure. None has obtained quantitative data on historical exposure, the measure most relevant to the development of lung cancer...Third, the effect of cigarette smoking has been controlled in only about one-half the studies...

[The authors] conclude that the data support a causal association between diesel exhaust and lung cancer in humans. Has science proven causality beyond any reasonable doubt? Probably not. The repeated finding of small effects, coupled with the absence of quantitative data on historical exposure, precludes a causal interpretation.

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