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employment (χ 2 test for trend: p < 0.05). Elevated, but nonsignificant, smoking-adjusted ORs were
also associated with the 31 lung cancer cases occurring in African-American railroad workers, OR =
2.6 (95% C.I. = 0.8-7.9) for 1-9 years and OR = 2.7 for ≥ 10 years of employment (95% C.I. = 0.6-
12.1).
Nokso-Koivisto and Pukkala (1994) compared the incidence of lung cancer among locomotive drivers
to the total Finnish population. The retrospective cohort consisted of the 8,391 members of the Finnish
Locomotive Drivers’ Association from 1953 until 1991 (retired drivers remain members until death).
After excluding 302 members for lack of personal identification information, an overall standardized
incidence ratio (SIR) of 0.86 (95% C.I. = 0.75-0.97) was found (236 cases). The overall incidence for
all cancer sites was also lower than expected, SIR 0.95 (95% C.I. = 0.89-1.01) but the incidence of
mesothelioma (SIR 4.05, 95% C.I. = 1.75-7.97) and oral cavity/pharyngeal cancers (SIR 1.75, 95%
C.I. = 1.02-2.80) were significantly increased. Prior to the 1970s Finnish drivers trained for 2 years in
railroad workshops, where significant exposure to asbestos occurred routinely during steam engine
maintenance, with little, if any, diesel exposure. Only drivers working after this period had the potential
for substantial exposure to diesel exhaust, and the electrification of the railroad in the 1970s and 1980s
may also have reduced the proportion of the cohort’s person-years that truly reflect exposure to diesel
exhaust. No data on smoking within the cohort were available, though a cross-sectional study of
locomotive drivers in 1976 showed that the prevalences of current smokers (40%), ex-smokers (34%),
and never-smokers (26%) were similar to those in the Finnish population as a whole.
All three population-based case-control studies found elevated risks for lung cancer in railroad workers
(Williams et al., 1977; Boffetta et al., 1988; Swanson et al., 1993); however, only the study by
Swanson et al. (1993) found a statistically significant increase, with a smoking-adjusted OR of 2.4
(95% C.I. = 1.1-5.1) for workers with ten or more years of employment. This study also found
evidence of a significant exposure-response relationship for the 67 cases observed in white railroad
workers. Williams et al. (1977) and Boffetta et al. (1988) had relatively fewer railroad workers (12
and 14 cases respectively) and no information on duration of exposure.
In the railroad industry-based studies, three of the larger studies identified statistically significant
increases in relative risk (Howe et al., 1983; Garshick et al., 1987a; Garshick et al., 1988). The large
cohort reported on by Howe et al. (1983) found elevated risks for individuals categorized as
“probably” and “possibly” exposed to diesel exhaust, but without adjustment for smoking or duration of
employment, the underlying risk is uncertain. In both the case-control and cohort studies by Garshick et
al., 1987a, 1988), significantly increased risks were associated with long-term employment in dieselrelated
railroad jobs. Both studies had substantial exposure assessment, sufficient latency, and duration
of employment data, and the case-control investigation also controlled for potential confounding by
smoking and by asbestos exposure. In contrast, the study by Nokso-Koivisto et al. (1994), found no
increase in lung cancer risk among Finnish locomotive engineers, though the description of the cohort
indicates the earlier cases were unlikely to have experienced any diesel exposure.
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