Health Assessment Document for Diesel Emissions - NSCEP | US ...

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classified as exposed or not exposed to diesel exhaust. The probable high-exposure category
c
consisted of bus and truck drivers, heavy equipment operators and repairmen, railroad workers,
and warehousemen. In the second method, guidelines set by NIOSH were used to classify
occupations based on exposure to diesel exhaust. In this method, the estimated proportion of
exposed workers was computed for each occupational category by using the NIOSH estimates of
the exposed population as the numerator and the estimates of individuals employed in each
occupational category from the 1970 census as the denominator. Occupations estimated to have
at least 20% of their employees exposed to diesel exhaust were defineq as "high exposure," those
with 10% to 19% oftheir employees exposed as "moderate exposure," and those with less than
10% oftheir employees exposed as "low exposure." The odds ratio was used as a measure of
association to assess the relationship between bladder cancer and diesel exhaust exposure. The
overall participation among those eligible and available for interview was 75% and 72% in cases
and controls, respectively.
A total of 194 bladder cancer cases and 582 controls were examined, and the two groups
were found to be comparable by age and education. Except for railroad workers, who had
relative odds of2.0 based on two cases and three controls (95% CI = 0.34, 11.61), the relative
odds were less than 1 for other diesel exhaust exposure occupations such as bus and truck
drivers, warehousemen, material handlers, and heavy equipment workers. When the risk was
examined using the NIOSH criteria for high, moderate, and low exposure, relative odds were
1.68 and 0.16 for high and moderate, respectively, with low as the referent group; neither was
statistically significant. Cases and controls were compared by smoking status. Cases were more
likely to be current cigarette smokers than were controls. Current smokers of l to 20
cigarettes/day had relative odds of 3.64 (95% CI = 2.04, 6.49), current smokers of 21 +
cigarettes/day had relative odds of 3.51 (95% CI = 2.00, 6.19), while ex-smokers had relative
odds of 1.72 (95% CI = 1.01, 2.92). After controlling for smoking, there was no significant
increase in the risk of bladder cancer for occupations with diesel exhaust exposure compared to
occupations with9ut diesel exhaust exposure." A synergistic effectbetween the two also was not
detected.
This study has two major methodologic limitations, both pertaining to exposure
classification. First, the use of "usual" occupation may have led to misclassification of those
individuals who had held a previous job with diesel exhaust exposure that was not their usual
occupation; this may have resulted in reduced power to detect weak associations. Second, since
· there was no information on amount or duration of diesel exhaust exposure, no analysis of doseresponse
relationships could be done. Also, no information was available on other confounding
·risk factors of bladder cancer such as urinary retention, amphetamine abuse, and smoking within
2/1/98 8-45 DRAFT --DO NOT CITE OR QUOTE