Priority Substances List - Statement of the Science Report for Ethylene Oxide

Synopsis

Ethylene oxide (CAS No. 75-21-8) is a colourless, highly reactive gas at room temperature and pressure. It has a high vapour pressure and high water solubility.

Domestic production of ethylene oxide in 1996 was 625 kilotonnes, 95% of which was used in the manufacture of ethylene glycol. An estimated 4% was used in the manufacture of surfactants. Ethylene oxide is also used as a sterilant for health care materials and other heat-sensitive products. Releases of ethylene oxide from natural sources, such as waterlogged soil, are expected to be negligible. Anthropogenic sources, not including sterilization, released an estimated 22.8 tonnes, all to the atmosphere, in 1996, down from 104 tonnes in 1993. An estimated 3 tonnes per year were lost to the atmosphere in 1996 from servicing medical facilities using ethylene oxide in sterilization processes and commercial sterilization operations.

Based on empirical fate data, release of ethylene oxide to the atmosphere is unlikely to result in transfer to other environmental compartments in significant quantities.

Atmospheric half-lives are based on reaction with photogenerated hydroxyl radicals and range from 38 to 382 days. In the event of release or spill to water, ethylene oxide is expected to be susceptible to evaporation, hydrolysis, and aerobic and, to a lesser extent, anaerobic biodegradation. In water, experimental results show the volatilization half-life to be 1 hour, the hydrolysis half-life to be 12-14 days, the aerobic biodegradation half-life to be from 20 days to 6 months and the anaerobic biodegradation half-life to be from 4 months to 2 years. In soil, ethylene oxide is expected to volatilize rapidly. The hydrolysis half-lives for soil and groundwater are estimated to be between 10.5 and 11.9 days.

Data on toxicity for organisms in the aquatic and terrestrial environments are limited. Most ethylene oxide is released to the atmosphere, and little transfer to water or soil is expected. Therefore, the potential for adverse effects is greatest for terrestrial organisms exposed to contaminated air. There were no available studies examining the effects on wild mammal and bird species; consequently, chronic effects observed in laboratory animals were assumed to reflect those in wild species. From these studies, the most significant endpoint with the greatest potential to result in population-level effects in wildlife was the induction of adverse reproductive effects in rats following inhalation at 183 mg/m³. This study was chosen as the most critical to the assessment and was used as a basis for the generation of an Estimated No-Effects Value (ENEV). Comparison of the worst-case average concentration in air (956 µg/m3) with the ENEV (1830 µg/m3) indicates that it is unlikely that terrestrial organisms are exposed to harmful levels of ethylene oxide in the Canadian environment.

Ethylene oxide is not expected to contribute to the formation of ground-level ozone or to the depletion of the stratospheric ozone layer. Its contribution as a greenhouse gas is also considered to be negligible.

The focus of the human health assessment is airborne exposure. Based on studies in animals, cancer is considered the critical endpoint for effects of ethylene oxide on human health. In inhalation studies, ethylene oxide has induced a wide range of tumours, with a strong likelihood that the mode of action involves direct interaction with genetic material. As a result, there is considered to be a probability of harm at any level of exposure. While there is some evidence of an association between exposure to ethylene oxide and the development of hematological cancers in epidemiological studies of occupationally exposed populations, limitations of the data preclude definitive conclusions.

Based on the information available, it is concluded that ethylene oxide is not entering the environment in a quantity or concentration or under conditions that have or may have an immediate or long-term harmful effect on the environment or its biological diversity, or that constitute or may constitute a danger to the environment on which life depends. Ethylene oxide is considered to be entering the environment in a quantity or concentration or under conditions that constitute or may constitute a danger in Canada to human life or health. Therefore, ethylene oxide is considered to be "toxic" as defined in Section 64 of the Canadian Environmental Protection Act, 1999 (CEPA 1999).

Based on comparison of extremely limited monitoring data and primarily predicted concentrations of ethylene oxide in air with tumorigenic potency, it is recommended that options to reduce exposure, particularly in the vicinity of point sources, be investigated. It is also recommended that there be additional investigation of the magnitude of exposure of populations in the vicinity of point sources to assist risk management actions.