home edit2 whole TSD November 2002 PDF format - OEHHA

to a unit concentration (1 µg/m 3 ) of diesel exhaust.
The unit risks ultimately derived for the general population assume that the mass concentration of
particles governs the risk of diesel exhaust, regardless of the particular type of diesel engine or fuel. The
resulting estimate of risk entails uncertainties due primarily to the limited exposure information available
and to the choice of models and data used in the analysis.
These two studies are among a number of studies establishing excess relative risk of lung cancer among
workers exposed to diesel exhaust. These two studies were specifically selected for the quantitative
risk assessment because of their general excellence, their apparent finding of a relationship of cancer
rate to duration of exposure and because of the availability of measurements of diesel exhaust among
such railroad workers from the early 1980’s in other studies. The case-control study appears to have
an advantage in obtaining direct information on smoking rates, while the cohort study has an advantage
of smaller confidence intervals of the risk estimates.
Estimating Cumulative Exposure
The risk relationships developed for the case-control study and the initial analyses for the cohort study
used cumulative atmospheric exposure to diesel exhaust particles as the effective dose. The use of
cumulative exposure, defined as the area under the curve (AUC) of concentration versus time, required
a specification of the temporal pattern of exposure concentration. However, direct measurements of
exposure concentration over the time of the follow up were not available.
Therefore, the calculations required reconstruction of the exposure history in order to determine
cumulative exposure. The reconstruction was undertaken using (1) personal exposure measurements on
railroad workers just after the end of the follow-up period in that study, (2) historical data on the
dieselization of locomotives in the United States, and (3) descriptive information. The analysis included
workers on trains and excluded shop workers from the original cohort because of mixed exposures,
including no exposure to an unknown number in this group.
Exposure Measurements In The Early 1980s
Woskie et al. (1988b) estimated national average concentrations of respirable particulate matter (RSP)
for 13 job-groups. These concentrations were obtained by temperature correction of measurements of
respirable particulate matter (RSP) made in 1982-1983 in the northern region of the United States, as
reported in Woskie et al. (1988a). The investigators adjusted these concentrations to remove the
portion of RSP attributable to environmental tobacco smoke (ETS). The average values of the ETSadjusted
RSP for the principal categories of workers are listed in Table 3 for exposed and unexposed
workers.
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