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estimate for those studies for which the exposure duration was not clear in the published reports was
1.24 (95% C.I. = 1.00-1.54) (six estimates from four studies) (see Table C-4 in Appendix C of
OEHHA (1998)).
These results were robust to a variety of sensitivity analyses. In an analysis of potential publication bias,
however, there appeared to be a modest increase in the RR estimates with increasing sample size
(reflected in a decreased standard error of the estimates). Publication bias, or the increased likelihood
or preference for the publication of statistically significant results compared to nonsignificant or null
results, may potentially distort pooled risk estimates. Publication bias is generally attributed to journal
editorial policies that prefer “positive” results, so that small, statistically nonsignificant studies are less
likely to be published than large, statistically nonsignificant studies (Greenland, 1994). However, it
should be noted that the studies with the smallest standard errors were almost exclusively cohort studies
that did not adjust for smoking and which also had a clear HWE, suggesting that other significant biases
are likely to have played a role in creating an appearance of publication bias. Therefore, although
publication bias cannot be ruled out, the inclusion of numerous studies of varying sample sizes and
statistically insignificant findings, as well as the uncontrolled confounding and likely selection bias
affecting many of the larger cohort studies, make it unlikely that the result of this meta-analysis can be
completely explained by publication bias.
In summary, the meta-analysis indicated a consistent positive association between occupations involving
diesel exhaust exposure and the development of lung cancer. Although substantial heterogeneity existed
in the initial pooled analysis, stratification on several factors identified a persistent positive relationship.
The major sources of heterogeneity included: (1) whether or not a study adjusted for smoking, (2) study
design (3) the exposure assessment, as developed from occupational categories, (4) and the presence
of selection bias, as manifested by an observed healthy worker effect. Taking these factors into account
tended to increase the estimates of relative risks of lung cancer from occupational exposure to diesel
exhaust.
Another independently conducted meta-analysis of diesel exhaust exposure and lung cancer produced
remarkably similar results, with an overall pooled relative risk estimate of 1.33 (95% C.I. = 1.24-1.44)
(Bhatia et al., 1998). In that analysis, the study inclusion and exclusion criteria were somewhat different
than those used by OEHHA staff, so that 23 studies were included. Consequently, the results of some
of their subset analyses differed from those described in OEHHA (1998). In addition, those authors
used only a fixed-effects model to derive pooled risk estimates, and did not focus on explorations of
sources of heterogeneity. Nevertheless, Bhatia and co-workers also found a persistent positive
relationship between diesel exhaust exposure and lung cancer that could not be attributed to potential
confounding by cigarette smoking. Moreover, in the narrower group of studies in their report, they
identified a positive exposure-response relationship in studies stratified by exposure duration.
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