home edit2 whole TSD November 2002 PDF format - OEHHA

statistically significant.
CDHS noted that the evidence supporting an association between working with ethylene oxide and
leukemia came from 4 out of 5 occupationally exposed cohorts. The overall evidence could not be
considered conclusive due to the small numbers of workers involved and to the possibility that other
workplace carcinogens may have been confounders. Nevertheless, the high estimates of risk were
striking; standardized incidence and mortality ratios for leukemia ranged from 6 to 21. Furthermore, the
replication of the early findings in other plants and in the follow-up of those same cohorts reduces the
probability that the observed excesses of leukemia were chance findings. Since worker recall was not
used to determine exposure, bias about exposure from that source is not present. The magnitude of the
effect argues against the findings being due to confounding, particularly since the other carcinogens were
different for the different plants. Though not conclusive, these studies provide substantial evidence of
ethylene oxide's carcinogenicity in humans.
Animal Studies
Rats
A dose-related increase in local tumors, mainly squamous cell carcinomas of the forestomach, was
observed in female Sprague-Dawley rats given doses of 7.5 or 30 mg/kg body weight of 99.7% pure
ethylene oxide twice a week by gastric intubation for 107 weeks (average total dose of 1186 and 5112
mg/kg body weight, respectively)(Dunkelberg 1982, US EPA 1985). Rats treated with the high dose
of ethylene oxide showed increased tumor-related mortality as well as decreased tumor latency
compared to the low dose or the control groups. The incidence of local tumors was 0/50 in both
control groups, 8/50 in the low-dose group, and 31/50 in the high-dose group. The frequency of
tumors at other sites was not increased by ethylene oxide treatment (Dunkelberg, 1982; US EPA
1985).
In a two-year inhalation study in rats, Snellings et al. (1984) found that ethylene oxide increased the
incidence of mononuclear cell leukemia in animals of both sexes, and peritoneal mesotheliomas in males.
Tumor frequency among female rats was greater in all exposed groups than in controls. In addition,
brain gliomas were observed in male and female exposed rats. Since this tumor has an historically rare
background occurrence in Fischer 344 (F344) rats, it was considered to be a tumor type appropriate
for use in risk evaluation.
Eight-week-old F344 rats were exposed in inhalation chambers to 10, 33, or 100 ppm of 99.9% pure
ethylene oxide 6 hours/day, 5 days/week, for two years. Initially, 120 males and 120 females were
exposed per dose. Two control groups (C1 and C2) of 120 rats per sex were exposed in inhalation
chambers to room air.
Planned terminations of 10 rats per sex per dose were performed at 6 and 12 months of exposure and
of 20 rats per sex per dose at 18 months. The remainder of the females were sacrificed at 24 months
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