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myelogenous leukemias (4 and 8 years exposure) and one primary macroglobulinemia (6 years
exposure).
In order to replicate these findings, Hogstedt et al. (1979b) conducted a cancer mortality study of the
cohort of ethylene oxide production workers originally studied by Ehrenberg and Hallstrom (1967).
The findings were similar, demonstrating elevated rates of leukemia and other cancers (Table 1). The
cohort was, however, exposed to other carcinogens, such as ethylene dichloride and
bis(2-chloroethyl)ether. Exposure to ethylene oxide was between 5.5 and 27.5 ppm (10 to 50 mg/m 3 )
in the 1960s and 0.55 to 5.5 ppm (2 to 10 mg/m 3 ) in the 1970s. Exposed workers developed 9 tumors
(including leukemias) where 3.4 would have been expected (SMR = 265)(p < 0.01).
Table 1:
Leukemia cases observed and expected in the studies of Hogstedt et al. (adapted from
Table 5 of Hogstedt et al., 1986)
Time interval
1960's * -1977 1978-1981 1960's * -1981
Plant Workers Obs. Exp. Obs. Exp. Obs. Exp.
Studied
1 203 2 ** 0.09 1 0.05 3 0.14
2 175 3 0.38 1 0.14 4 0.52
3 355 1 0.09 0 0.07 1 0.16
Total 733 6 0.56 2 0.27 8 0.83
* The initial year was 1968 for plant 1, 1961 for plant 2, and 1964 for plant 3.
** One of the original cases (macroglobulinemia) was later reclassified as a non-Hodgkin's
lymphoma.
Hogstedt et al. (1986) published findings from follow-up studies of the two cohorts described above
and a group of production workers from a third plant that had not been studied previously. This last
group included workers exposed only to ethylene oxide, exposed to ethylene oxide and propylene
oxide, or exposed to a mixture of chemicals. All workers from this third plant had been exposed for at
least one year. One leukemia was detected where 0.16 was expected. Leukemia cases from the three
plants are shown in Table 4.7. An excess of leukemia cases was reported in Plants 1 and 2 during the
1978-1981 follow-up study, but not in Plant 3. Eight cases of leukemia were observed among 733
workers in the three plants where 0.83 were expected (SMR = 964)(Table 1).
In addition to excess leukemia among the three groups of workers, six stomach cancers were reported
among Plant 2 workers where 0.65 were expected. Neither the leukemia nor stomach cancer cases
follow a dose-response pattern when analyzed by years of employment. However, years of
employment can be a poor surrogate for exposure if, for example, highly exposed workers tended to
shift to jobs with lower exposure or terminate their employment early.
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