home edit2 whole TSD November 2002 PDF format - OEHHA

and can distort the measure of effect (e.g., relative risk or odds ratio) (Rothman, 1986). With respect
to exposure assessment, statistically significant pooled estimates of elevated risk lacking evidence of
heterogeneity were identified in several occupational subgroup analyses, both with and without
additional stratification for smoking. Prior to stratifying by adjustment for smoking, the occupational
subgroups involving trucking (pooled RR = 1.47, 95% C.I. = 1.33-1.63), the railroad industry
(random-effects pooled RR = 1.45, 95% C.I. = 1.08-1.93), mechanics and garage workers (randomeffects
pooled RR = 1.35 (95% C.I. = 1.03-1.78), general transportation and professional drivers
(random-effects pooled RR = 1.45, 95% C.I. = 1.31-1.60) gave risk estimates greater than the overall
pooled risk estimate. The pooled RR estimates for trucking and general transportation and professional
drivers showed little to no evidence of heterogeneity; however, estimates for the railroad industry
demonstrated considerable heterogeneity (Q statistic = 30.90, p < 0.001).
Further stratification of the occupational subgroup analysis by adjustment for smoking produced a large
impact on the pooled risk estimates, with all smoking-adjusted subgroup estimates displaying little
evidence of heterogeneity and leading to increased risk estimates in all but one of the occupational
categories. Pooled risk estimates by occupation in smoking-adjusted studies showed little evidence of
heterogeneity for several occupations under both models, including truck drivers (random-effects
pooled RR = 1.53, 95% C.I. = 1.20-1.94), railroad workers (random-effects pooled RR = 1.68, 95%
C.I. = 1.28-2.19), and diesel mechanics and garage workers (random-effects pooled RR = 1.25, 95%
C.I. = 0.87-1.80). The pooled estimates for the heavy equipment operators and dock workers and for
the railroad industry studies adjusting for smoking displayed the most dramatic changes relative to the
occupational analysis without smoking stratification. Among the former subgroup, the pooled risk
estimate changed from 1.28 (random-effects model, 95% C.I. = 0.99-1.66) to 2.43 (95% C.I. = 1.21-
4.88). Among the railroad industry studies, the pooled risk estimate also increased substantially (from
1.45 to 1.68, 95% C.I. = 1.28-2.19). In both subgroups, the pooled smoking-adjusted estimates
showed little evidence of heterogeneity, though these estimates were based on two studies in the former
instance and three in the latter. However, the other two heavy equipment operator and dock worker
studies and the other three railroad industry studies that were not adjusted for smoking still displayed
evidence of heterogeneity (Q-statistics = 2.933, 1 d.f., p = 0.09, and 21.517, 2 d.f., p < 0.001,
respectively).
The meta-analysis also identified evidence of exposure-response relationships in the subgroup analyses
based on duration of employment. However, as noted in OEHHA (1998), this analysis was hampered
by the absence of duration-specific risk estimates in approximately one-half the studies. While the initial
analysis conducted on all the included studies resulted in elevated pooled risk estimates for strata in
which exposure durations were greater than 10 years relative to those with less than 10 years of
exposure or for which the exposure durations were not clear from the published reports, there was still
significant evidence of heterogeneity for several of the duration strata. In contrast, the analysis utilizing
only estimates from the smoking-adjusted studies showed some evidence of an exposure-response
gradient without evidence of statistical heterogeneity. The summary risks for all three exposure-duration
strata were: RR = 1.39 (95% C.I. 1.19-1.63) for < 10 years (based on three estimates from two
studies), RR = 1.64 (95% C.I. = 1.40-1.93) for 10 < to < 20 years (11 estimates from 6 studies), and
RR = 1.64 (95% C.I. = 1.26-2.14) for > 20 years (four estimates from four studies). The pooled risk
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