This report contains the collective views of an international group of experts and does not necessarily represent the decisions or the stated policy of the United Nations Environment Programme, the International Labour Organization, or the World Health Organization.

Concise International Chemical Assessment Document 54

First draft prepared by R.G. Liteplo and M.E. Meek, Health Canada, Ottawa, Canada; and

M. Lewis, Environment Canada, Ottawa, Canada

Published under the joint sponsorship of the United Nations Environment Programme, the International Labour Organization, and the World Health Organization, and produced within the framework of the Inter-Organization Programme for the Sound Management of Chemicals.

World Health Organization

Geneva, 2003

The International Programme on Chemical Safety (IPCS), established in 1980, is a joint venture of the United Nations Environment Programme (UNEP), the International Labour Organization (ILO), and the World Health Organization (WHO). The overall objectives of the IPCS are to establish the scientific basis for assessment of the risk to human health and the environment from exposure to chemicals, through international peer review processes, as a prerequisite for the promotion of chemical safety, and to provide technical assistance in strengthening national capacities for the sound management of chemicals.

The Inter-Organization Programme for the Sound Management of Chemicals (IOMC) was established in 1995 by UNEP, ILO, the Food and Agriculture Organization of the United Nations, WHO, the United Nations Industrial Development Organization, the United Nations Institute for Training and Research, and the Organisation for Economic Co-operation and Development (Participating Organizations), following recommendations made by the 1992 UN Conference on Environment and Development to strengthen cooperation and increase coordination in the field of chemical safety. The purpose of the IOMC is to promote coordination of the policies and activities pursued by the Participating Organizations, jointly or separately, to achieve the sound management of chemicals in relation to human health and the environment.

WHO Library Cataloguing-in-Publication Data

Ethylene oxide.

(Concise international chemical assessment document ; 54)

1.Ethylene oxide - toxicity 2.Risk assessment 3.Environmental exposure I.International Programme on Chemical Safety II.Series

ISBN 92 4 153054 5         (LC/NLM Classification: QD 305.E7)

ISSN 1020-6167

©World Health Organization 2003

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FOREWORD

Concise International Chemical Assessment Documents (CICADs) are the latest in a family of publications from the International Programme on Chemical Safety (IPCS) — a cooperative programme of the World Health Organization (WHO), the International Labour Organization (ILO), and the United Nations Environment Programme (UNEP). CICADs join the Environmental Health Criteria documents (EHCs) as authoritative documents on the risk assessment of chemicals.

International Chemical Safety Cards on the relevant chemical(s) are attached at the end of the CICAD, to provide the reader with concise information on the protection of human health and on emergency action. They are produced in a separate peer-reviewed procedure at IPCS. They may be complemented by information from IPCS Poison Information Monographs (PIM), similarly produced separately from the CICAD process.

CICADs are concise documents that provide summaries of the relevant scientific information concerning the potential effects of chemicals upon human health and/or the environment. They are based on selected national or regional evaluation documents or on existing EHCs. Before acceptance for publication as CICADs by IPCS, these documents undergo extensive peer review by internationally selected experts to ensure their completeness, accuracy in the way in which the original data are represented, and the validity of the conclusions drawn.

The primary objective of CICADs is characterization of hazard and dose–response from exposure to a chemical. CICADs are not a summary of all available data on a particular chemical; rather, they include only that information considered critical for characterization of the risk posed by the chemical. The critical studies are, however, presented in sufficient detail to support the conclusions drawn. For additional information, the reader should consult the identified source documents upon which the CICAD has been based.

Risks to human health and the environment will vary considerably depending upon the type and extent of exposure. Responsible authorities are strongly encouraged to characterize risk on the basis of locally measured or predicted exposure scenarios. To assist the reader, examples of exposure estimation and risk characterization are provided in CICADs, whenever possible. These examples cannot be considered as representing all possible exposure situations, but are provided as guidance only. The reader is referred to EHC 170.¹

While every effort is made to ensure that CICADs represent the current status of knowledge, new information is being developed constantly. Unless otherwise stated, CICADs are based on a search of the scientific literature to the date shown in the executive summary. In the event that a reader becomes aware of new information that would change the conclusions drawn in a CICAD, the reader is requested to contact IPCS to inform it of the new information.

Procedures

The flow chart on page 2 shows the procedures followed to produce a CICAD. These procedures are designed to take advantage of the expertise that exists around the world — expertise that is required to produce the high-quality evaluations of toxicological, exposure, and other data that are necessary for assessing risks to human health and/or the environment. The IPCS Risk Assessment Steering Group advises the Coordinator, IPCS, on the selection of chemicals for an IPCS risk assessment based on the following criteria:

• there is the probability of exposure; and/or
• there is significant toxicity/ecotoxicity.

Thus, it is typical of a priority chemical that

• it is of transboundary concern;
• it is of concern to a range of countries (developed, developing, and those with economies in transition) for possible risk management;
• there is significant international trade;
• it has high production volume;
• it has dispersive use.

The Steering Group will also advise IPCS on the appropriate form of the document (i.e., EHC or CICAD) and which institution bears the responsibility of the document production, as well as on the type and extent of the international peer review.

The first draft is based on an existing national, regional, or international review. Authors of the first draft are usually, but not necessarily, from the institution that developed the original review. A standard outline has been developed to encourage consistency in form. The first draft undergoes primary review by IPCS to ensure that it meets the specified criteria for CICADs.

The second stage involves international peer review by scientists known for their particular expertise and by scientists selected from an international roster compiled by IPCS through recommendations from IPCS national Contact Points and from IPCS Participating Institutions. Adequate time is allowed for the selected experts to undertake a thorough review. Authors are required to take reviewers’ comments into account and revise their draft, if necessary. The resulting second draft is submitted to a Final Review Board together with the reviewers’ comments. At any stage in the international review process, a consultative group may be necessary to address specific areas of the science.

The CICAD Final Review Board has several important functions:

• to ensure that each CICAD has been subjected to an appropriate and thorough peer review;
• to verify that the peer reviewers’ comments have been addressed appropriately;
• to provide guidance to those responsible for the preparation of CICADs on how to resolve any remaining issues if, in the opinion of the Board, the author has not adequately addressed all comments of the reviewers; and
• to approve CICADs as international assessments.

Board members serve in their personal capacity, not as representatives of any organization, government, or industry. They are selected because of their expertise in human and environmental toxicology or because of their experience in the regulation of chemicals. Boards are chosen according to the range of expertise required for a meeting and the need for balanced geographic representation.

Board members, authors, reviewers, consultants, and advisers who participate in the preparation of a CICAD are required to declare any real or potential conflict of interest in relation to the subjects under discussion at any stage of the process. Representatives of nongovernmental organizations may be invited to observe the proceedings of the Final Review Board. Observers may participate in Board discussions only at the invitation of the Chairperson, and they may not participate in the final decision-making process.

This CICAD on ethylene oxide was prepared jointly by the Environmental Health Directorate of Health Canada and the Commercial Chemicals Evaluation Branch of Environment Canada, based on documentation prepared as part of the Priority Substances Program under the Canadian Environmental Protection Act (CEPA). The objective of assessments on priority substances under CEPA is to assess potential effects of indirect exposure in the general environment on human health as well as environmental effects. Data identified as of the end of May 1998 (environmental effects) and August 1999 (human health effects) were considered in this review.² Information on the nature of the peer review and availability of the source document (Environment Canada & Health Canada, 2001) is presented in Appendix 2. Other reviews that were also consulted include ATSDR (1990), BUA (1995), IARC (1976, 1994), US EPA (1985), and an earlier EHC monograph on this chemical (IPCS, 1985). Information on the peer review of this CICAD is presented in Appendix 3. This CICAD was approved as an international assessment at a meeting of the Final Review Board, held in Monks Wood, United Kingdom, on 16–19 September 2002. Participants at the Final Review Board meeting are presented in Appendix 4. The International Chemical Safety Card for ethylene oxide (ICSC 0155), produced by the International Programme on Chemical Safety (IPCS, 1999), has also been reproduced in this document.

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

Production of ethylene oxide in the source country for this CICAD (i.e., Canada) 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.

Most ethylene oxide is released to the atmosphere. Releases of ethylene oxide from natural sources, such as waterlogged soil, are expected to be negligible. In the source country (Canada), anthropogenic sources, not including sterilization, released an estimated 23.0 tonnes, all to the atmosphere, in 1996. An estimated additional 3.0 tonnes/year were lost to the atmosphere in 1996 from the servicing of medical facilities using ethylene oxide in sterilization processes and commercial sterilization operations.

Release of ethylene oxide to the atmosphere is unlikely to result in transfer to other environmental compartments in significant quantities. Atmospheric half-lives based on reaction with photogenerated hydroxyl radicals 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, the volatilization half-life is about 1 h, the hydrolysis half-life about 12–14 days, the aerobic biodegradation half-life 20 days to 6 months, and the anaerobic biodegradation half-life 4 months to 2 years. In soil, ethylene oxide is expected to volatilize rapidly. Hydrolysis half-lives for soil and groundwater are estimated to be between 10.5 and 11.9 days. Ethylene oxide is not expected to bioaccumulate on the basis of its very low log octanol/water partition coefficient (K_ow).

Ethylene oxide is rapidly taken up via the lungs, distributed, and metabolized to ethylene glycol and to glutathione conjugates. Ethylene oxide can be absorbed through the skin from the gas phase or from aqueous solutions and is uniformly distributed throughout the body. Ethylene oxide is an alkylating agent and forms protein and DNA adducts. Haemoglobin adducts have been used for biomonitoring.

The acute inhalation toxicity of ethylene oxide in rodents and dogs is low, with 4-h LC₅₀s generally being greater than 1500 mg/m³. Available data on the non-neoplastic effects of repeated exposure to ethylene oxide in studies are limited, with past focus being primarily on the carcinogenicity of the compound. Reported effects in studies in animals were restricted primarily to those on the haematological and nervous systems.

Based on studies primarily in occupationally exposed populations, ethylene oxide is an ocular, respiratory, and dermal irritant and a sensitizing agent. Neurological effects (primarily sensorimotor polyneuropathy) have been observed in workers exposed to relatively high concentrations and in animals exposed to levels greater than those at which increases in tumours have been reported.

The route of likely greatest exposure and focus of the human health assessment is inhalation from air. Based on studies in animals, cancer is considered the critical end-point for effects of ethylene oxide on human health for long-term exposure of the general population. In inhalation studies, ethylene oxide has induced a wide range of tumours (e.g., leukaemia, lymphoma, brain, lung), with a strong likelihood that the mode of action involves direct interaction with genetic material, for which there is consistent and convincing evidence. While there is some evidence of an association between exposure to ethylene oxide and the development of haematological cancers in epidemiological studies of occupationally exposed populations, limitations of the data preclude definitive conclusions.

Ethylene oxide induces gene mutations at all phylogenetic levels tested in vitro and in vivo. It also induces germ cell mutations and clastogenic effects in experimental animals. There is consistent evidence that ethylene oxide has induced clastogenic changes in exposed workers.

In experimental animals, ethylene oxide is fetotoxic in the presence and absence of maternal toxicity at concentrations higher than those associated with cancer and other non-cancer (i.e., neurological) effects; it is teratogenic only at very high concentrations (above about 1600 mg/m³). Evidence from epidemiological studies of reproductive effects (primarily spontaneous abortions) of ethylene oxide in humans is limited. In experimental animals, among non-neoplastic effects, reproductive effects occur at lowest concentration (>90 mg/m³). These include reductions in litter size, increased post-implantation losses, alterations in sperm morphology, and changes in sperm count and motility.

Cancer is considered the critical end-point for quantification of exposure–response for risk characterization for ethylene oxide. The lowest concentration causing a 5% increase in tumour incidence above background (TC₀₅) from a study in rats or mice that had optimal characterization of exposure–response was 2.2 mg/m³ (unit risk = 0.05/2.2 mg/m³ = 0.023 per mg/m³) for the development of mononuclear leukaemias in female F344 rats exposed via inhalation to ethylene oxide; the lower 95% confidence limit was 1.5 mg/m³. Primarily as a basis for comparison with the tumorigenic potencies for cancer, the concentration associated with a 5% increase in the incidence of germ cell mutations (BMC₀₅) is also presented (46 mg/m³), although it is based on dominant visible mutations only and does not take into account other genetic end-points in live offspring. Similarly, tolerable concentrations based on observed neurological or reproductive effects would be in the range of tens of micrograms per cubic metre.

On this basis, predicted cancer risks in the vicinity of industrial point sources, based on limited modelling and monitoring data, are greater than 10^–5.

Since ethylene oxide is expected to be present primarily in air, the potential for adverse effects is greatest for terrestrial organisms, for which available data are limited. The most significant end-point with the greatest potential to result in population-level effects in wildlife was the induction of adverse reproductive effects. Comparison of the worst-case average concentration in air with the estimated no-effects value indicates that it is unlikely that terrestrial organisms are exposed to harmful levels of ethylene oxide in air.

Ethylene oxide (CAS registry number 75-21-8) is also known as diethylene oxide, E.O., epoxyethane, 1,2-epoxyethane, oxane, oxidoethane, and oxirane. The chemical structure of ethylene oxide is shown in Figure 1.

Fig. 1: Chemical structure of ethylene oxide

The molecular formula for ethylene oxide is H₂COCH₂, and its relative molecular mass is 44.05. At room temperature (25 °C) and normal atmospheric pressure, ethylene oxide is a colourless, highly reactive, and flammable gas with a characteristic ethereal odour. It has a high vapour pressure (~146 kPa) and high water solubility (completely miscible). It is reactive in both the liquid and vapour phases (IPCS, 1985). Table 1 summarizes the physical and chemical properties of ethylene oxide. Additional properties are presented in ICSC 0155, which is reproduced in this document.

Table 1: Physical and chemical properties of ethylene oxide.

The conversion factors³ for ethylene oxide in air (at 20 °C and 101.3 kPa) are as follows:

1 ppm = 1.83 mg/m³

1 mg/m³ = 0.55 ppm

The most common method for the identification of ethylene oxide in various media is gas chromatography (GC). GC with an electron capture detector (ECD) is often used to measure concentrations in workplace air. The sample is first adsorbed on hydrobromic acid-coated charcoal, desorbed with dimethylformamide, and then derivatized to 2-bromoethylheptafluorobutyrate for analysis. This method (NIOSH Method 1614) has an estimated detection limit of 1 µg ethylene oxide per sample (Eller, 1987a). The US Occupational Health and Safety Administration delineates a method with slight modification to sample collection: adsorption onto charcoal, desorption with a benzene:carbon disulfide solution, and conversion to 2-bromoethanol prior to analysis (Tucker & Arnold, 1984). NIOSH Method 3702 describes analysis with a portable gas chromatograph and a photoionization detector for ethylene oxide in workplace air. The sample is drawn directly into a syringe or collected as a bag sample and injected directly into the gas chromatograph. This method has an estimated detection limit of 2.5 pg/ml injection (Eller, 1987b). Passive samplers are also available for ethylene oxide sample collection.

Haemoglobin adducts of ethylene oxide (hydroxyethyl valine and hydroxyethyl histidine) have been determined by radioimmunological techniques, a modified Edman degradation procedure with GC/mass spectrometry (MS), GC with selective ion MS, and GC/ECD (IARC, 1994).

Ethylene oxide has been determined in emissions from production plants and commercial sterilizers by GC/flame ionization detection. GC and headspace GC have also been used to analyse ethylene oxide residues in sterilants, drugs, plastics, ethoxylated surfactants and demulsifiers, packaging materials, and processed food products (IARC, 1994).

Data on production, sources, and emissions primarily from the source country of the national assessment on which the CICAD is based (i.e., Canada) are presented here as an example. Sources and patterns of emissions in other countries are expected to be similar, although quantitative values may vary.

Ethylene oxide is produced from a few natural sources. In certain plants, ethylene (a natural plant growth regulator) is degraded to ethylene oxide (Abeles & Dunn, 1985). It is also a product of ethylene catabolism in certain microorganisms (De Bont & Albers, 1976). Ethylene oxide can be generated from waterlogged soil (Smith & Jackson, 1974; Jackson et al., 1978), manure, and sewage sludge (Wong et al., 1983). Quantitative estimates of production from these natural sources are not available, but emissions are expected to be negligible.

4.2.1 Production and use

Canadian production of ethylene oxide was estimated at 625 kilotonnes in 1996, and the forecasted total estimated for 1999 was 682 kilotonnes (CIS, 1997).

Virtually all of the ethylene oxide produced is used as an intermediate in the production of various chemicals (ATSDR, 1990). In 1993, 89% of the total Canadian production of ethylene oxide was used in the production of ethylene glycol (SRI, 1993); in 1996, 95% was used for this purpose (CIS, 1997). An estimated 4% (26 kilotonnes) is used in the manufacture of surfactants (CIS, 1997). Ethylene oxide, alone or in combination with other gases, such as carbon dioxide and nitrogen, is used to sterilize instruments from the health care, publication, and wood products sectors. Ethylene oxide is also used in other industries where heat-sensitive goods are sterilized (How-Grant, 1991; BUA, 1995) and in the manufacture of choline chloride, glycol ethers, and polyglycols (CIS, 1997). Other minor uses worldwide include its application in the manufacture of rocket propellant and petroleum demulsifiers (Lewis, 1993).

Ethylene oxide is used for the control of insects in stored products and for the control of bacteria in spices and natural seasonings (J. Ballantine, personal communication, 1997). Ethylene oxide is also present as a formulant or component of a formulant in pest control products at concentrations up to 0.4%. The formulants include fungicides, insecticides, herbicides, and an adjuvant (J. Ballantine, personal communication, 1997).

4.2.2 Non-point sources

Non-point sources of release of ethylene oxide include its formation from fossil fuel combustion (US EPA, 1984) and presence in tobacco smoke (Howard, 1989). Neither source is expected to be significant (US EPA, 1984). Ethylene oxide is used as a component in the production of polyoxyethylene surfactants (Gaskin & Holloway, 1992). Ethylene oxide in this form is bound within the surfactant molecule, and any release is expected to be minimal. Similarly, ethylene oxide may be present in nonylphenol ethoxylate formulations at concentrations below 10 mg/litre (Talmage, 1994), and ethylene oxide may remain as a contaminant at 10 mg/kg in liquid detergents. A variety of other products, including paints and coatings, were reported to contain ethylene oxide at levels ranging from trace to <0.5%.

Ethylene oxide is used for the control of insect (i.e., fumigation) and microbial (i.e., sterilization) infestations (Agriculture and Agri-Food Canada, 1996; Health Canada, 1999a; S. Conviser, personal communication, 1999). Following fumigation, concentrations of ethylene oxide generally fall to negligible levels within a few hours (IARC, 1976).

4.2.3 Point sources

Gaseous and liquid forms of ethylene oxide can be released during production and use, as well as during the manufacture of ethylene glycol, ethoxylates, ethers, and ethanolamines (Howard, 1989). Ethylene oxide releases to the Canadian environment totalled 23.0 tonnes in 1996, with the reporting sectors being plastics and synthetics (0.24 tonnes), inorganic chemicals (6.1 tonnes), industrial organic chemicals (8.7 tonnes), and soap and cleaning compounds (8.0 tonnes) (NPRI, 1996). By 1997, emissions were reduced by 82% from the 1993 levels (ARET, 1999). An estimated additional 3.0 tonnes were lost to the atmosphere in 1996 from the servicing of medical facilities using ethylene oxide in sterilization processes and commercial sterilization operations.

Although sterilization is not a major use of ethylene oxide in terms of volumes consumed, it may be a very significant source of release to the environment (IPCS, 1985). Based on a survey of hospitals using ethylene oxide as a sterilizing agent, conducted in April 1994, the amount of ethylene oxide used as a sterilant was estimated to be 40 tonnes/year. Because many facilities now have improved control measures in place (Havlicek et al., 1992; Canadian Hospital Association & Environment Canada, 1994) and because of the use of alternative equipment that does not involve the use of ethylene oxide (S. Smyth-Plewes, personal communication, 1998), the current volumes of ethylene oxide used and released may be significantly less than the 1994 estimates.

In an examination of the primary US sources of ethylene oxide releases, sterilization/fumigation sites, production/captive use, medical facilities, and ethoxylation accounted for 57%, 31%, 8%, and 4% of total emissions, respectively (Markwordt, 1985). In an early US study, it was estimated that <0.1% of ethylene oxide produced is used as a sterilizing agent or fumigant, yet this accounts for the majority of ethylene oxide released into the atmosphere (Markwordt, 1985). Similarly, Berkopec & Vidic (1996) reported that in Slovenia, emissions to the atmosphere during sterilization were higher than emissions from other processes, such as synthesis of glycols and other derivatives in the chemical industry, although the sterilization process accounts for only 2% of total ethylene oxide use. In Belgium, an estimated 0.07% of the total consumption of ethylene oxide was used in sterilization operations in health care and medical products industries (Wolfs et al., 1983).

In facilities that have recirculating-water vacuum pumps, there is practically no loss of ethylene oxide through the water drain (Meiners & Nicholson, 1988; US EPA, 1992, 1994). In those facilities that use once-through water-sealed vacuum pumps, some ethylene oxide dissolved in the water will be directed to a floor drain and will likely volatilize to the atmosphere at an outdoor ground-level drain near the facility or wastewater treatment facility (US EPA, 1992; WCB, 1994).

Based on empirical data on fate, release of ethylene oxide to the atmosphere is unlikely to result in transfer to other environmental compartments in significant quantities. Reaction half-lives in the atmosphere may be significantly long (between 38 and 382 days). Although water solubility suggests that washout from the atmosphere by precipitation could be important, volatilization from the water is too rapid to suggest this to be a significant fate process. On the basis of a low log K_ow (–0.30), the potential for bioaccumulation of ethylene oxide is considered to be very low. As a result of its high water solubility and vapour pressure, ethylene oxide is not expected to bioaccumulate or accumulate in sediment or soil.

The atmospheric half-lives for ethylene oxide following vapour-phase reactions with photochemically produced hydroxyl radicals, assuming an atmospheric concentration of 1 × 10⁶ radicals/cm³, were estimated to be 120 days (Atkinson, 1986), 99 days (Lorenz & Zellner, 1984), 151 days (C. Zetzsch, personal communication, 1985, cited in Atkinson, 1986), and between 38 and 382 days (Howard et al., 1991).

The theoretical atmospheric lifetimes (approximately 1.43 × t_½) for ethylene oxide were estimated at ~200 days (Bunce, 1996) and 330 days (Winer et al., 1987) and were calculated based on the reaction with hydroxyl radicals at a concentration of 8.0 × 10⁵ and 1.0 × 10⁶ radicals/cm³, respectively. Such lifetimes are expected to be long enough to allow a very small percentage of the amount emitted to reach the stratosphere (Bunce, 1996).

Ethylene oxide has a very high water solubility (completely miscible), which would suggest that some washout via precipitation can be expected; however, its high vapour pressure (~146 kPa) and rapid volatilization rate may limit the effectiveness of this process. An examination of the effect of atmospheric precipitation was conducted in a laboratory setting (Winer et al., 1987), resulting in evidence that washout has little impact on reducing atmospheric concentrations.

Ethylene oxide is expected to undergo numerous fate processes in water, including evaporation, hydrolysis, and aerobic and anaerobic degradation. The reported experimental aquatic half-life for evaporation of ethylene oxide in water is 1 h with no wind and 0.8 h with a 5 m/s wind (Conway et al., 1983). Ethylene oxide degrades in water by hydrolysis and other nucleophilic reactions (US EPA, 1985). Ethylene oxide is hydrolysed in fresh water to ethylene glycol and in salt water to ethylene glycol and ethylene chlorohydrin. The half-life was estimated experimentally to be 12–14 days for hydrolysis at pH 5–7 in fresh water and 9–11 days for hydrolysis in salt water (Conway et al., 1983). The aqueous aerobic biodegradation half-life of ethylene oxide was approximately 20 days from a lightly seeded biological oxygen demand (BOD) test, and the rate in a biological waste treatment system is expected to be much faster (Conway et al., 1983). Based on the BOD test results of Bridié et al. (1979a) and Conway et al. (1983), Howard et al. (1991) estimated the unacclimated aqueous biodegradation half-life to be from 1 to 6 months. The aqueous anaerobic half-life, based on the estimated aerobic biodegradation half-life, is 4–24 months (Howard et al., 1991). The 5-day BOD was 3% of the theoretical oxygen demand of 1.82 g/g (Bridié et al., 1979a).

Ethylene oxide is miscible in water and poorly adsorbed to soil; however, owing to its high vapour pressure (146 kPa), a spill of ethylene oxide to soil will result in most volatilizing to the atmosphere, with only a small fraction infiltrating the soil. Evaporation will continue within the soil, but at a reduced rate (Environment Canada, 1985). Dilution with water will reduce the velocity at which the ethylene oxide moves downward and at the same time diminish the vapour pressure and reduce the rate of evaporation. Upon reaching the groundwater table, ethylene oxide will move in the direction of groundwater flow. The half-lives for hydrolysis in groundwater and soil are estimated to be between 10.5 and 11.9 days, based on measured rate constants at pH 5, 7, and 9 (Mabey & Mill, 1978; Howard et al., 1991). In general, volatilization is the primary removal mechanism, but ethylene oxide is expected to hydrolyse and be biodegraded relatively rapidly in most soils.

Information on the environmental fate of ethylene oxide in sediment has not been identified. Because of its physical and chemical properties, ethylene oxide is not expected to be sorbed by sediment or soil.

Reported levels of ethylene oxide in environmental biota have not been identified. On the basis of a low log K_ow (–0.30), the potential for bioaccumulation of ethylene oxide is expected to be very low (Verschueren, 1983; Howard, 1989).

Fugacity modelling was conducted to characterize key reaction, intercompartment, and advection (movement out of a system) pathways for ethylene oxide and its overall distribution in the environment in the source country (Canada). A steady-state, non-equilibrium model (Level III fugacity model) was run using the methods developed by Mackay (1991) and Mackay & Paterson (1991). All physical and chemical property input values were selected from a compilation of literature values based on criteria for integrity (for details, see DMER & AEL, 1996).

Based on the ChemCAN Level III fugacity model, which depicts a mixedwood plain region of a densely populated area of southern Ontario, it is estimated that ethylene oxide will have an overall persistence of 3 days in that region from a reaction persistence estimated at 70 days. Owing to its short overall persistence, higher ethylene oxide concentrations will likely be centralized in areas close to discharges. Based on the 1993 release volume of 53 200 kg to the atmosphere in southern Ontario (NPRI, 1993), the average steady-state levels in the southern Ontario region are estimated to be 1.02 ng/m³ in air (344 kg), 0.067 ng/litre in water (99.0 kg), 6.03 × 10^–5 ng/g in soil (0.858 kg), and 3.27 × 10^–5 ng/g in sediment (0.034 kg). Bioaccumulation is not expected (DMER & AEL, 1996).

The concentrations of ethylene oxide predicted above are based on the assumption that air entering southern Ontario from neighbouring regions contains no ethylene oxide. Estimates of concentrations of ethylene oxide in air in the 48 contiguous states of the USA, derived from atmospheric dispersion modelling and US emission inventories, are available (Woodruff et al., 1998). Mean concentrations predicted for 1990 in Michigan and New York, which border southern Ontario, were 4.9 ng/m³ and 5.9 ng/m³, respectively. When the average of these concentrations was assumed for the concentration of ethylene oxide in air advected into southern Ontario, the concentrations predicted by the ChemCAN model increased approximately 6-fold, to 6.2 ng/m³ in air, 0.4 ng/litre in water, 3.7 × 10^–4 ng/g in soil, and 2.0 × 10^–4 ng/g in sediment. Concentrations predicted in the ChemCAN auxiliary compartments of terrestrial animals and terrestrial plants were 4.3 × 10^–5 ng/g and 1.4 × 10^–3 ng/g, respectively, when this additional advective input was included in the fugacity modelling (Health Canada, 1999a).

Data on concentrations in the environment from the source country of the national assessment on which the CICAD is based (i.e., Canada) are presented here as a basis for the sample risk characterization. Patterns of exposure in other countries are expected to be similar, although quantitative values may vary.

6.1.1 Ambient air

Data on concentrations of ethylene oxide in emissions or ambient air are very limited.

Ethylene oxide was detected at concentrations of 3.7, 3.9, and 4.9 µg/m³ in 3 of 50 24-h samples of air collected outside of randomly selected residences during a multimedia exposure study conducted in Canada (Health Canada, 1999a). The censored mean value was 0.34 µg/m³ when a concentration equivalent to one-half the limit of detection (i.e., ½ × 0.19 µg/m³ = 0.095 µg/m³) was assumed for the 47 samples in which ethylene oxide was not detected. Ethylene oxide was detected at 3 (or 33%) of 9 locations in Alberta, but at none of the 35 locations in Ontario or the 6 locations in Nova Scotia during this study (Health Canada, 1999a).

Based on data on 1993 air quality modelling predictions from a Canadian production facility, (Environment Canada, 1997), it was estimated that 1-h average ground-level concentrations of ethylene oxide would exceed 12 µg/m³ for a total of 17 h a year in the immediate vicinity of the plant. Predicted maximum 1-h-average ground-level concentrations ranged from 3.7 to 20.1 µg/m³ at distances of 5 km and 2.7 km from the plant, respectively. No measurements were available to validate these predictions.

Estimated maximum average daily concentrations of ethylene oxide in the vicinity of Canadian hospitals were 0.26, 0.83, 1.3, and 2.12 µg/m³ between 100 and 70 m from the emission source and from stack heights of 30 m, 18 m, 15 m, and 12 m, respectively (Environment Canada, 1999). Concentrations closer to or farther from the source are predicted to be less. The estimate was based on the US EPA "SCREEN3" Gaussian plume model, which incorporates source-related and meteorological factors to estimate pollutant concentrations from continuous sources. The model assumes that the pollutant does not undergo any chemical reactions and that no other removal processes, such as wet or dry deposition, act on the plume during its transport from the source (for input parameters, see US EPA, 1995).

In an assessment of emissions and concentrations of ethylene oxide throughout California, USA, mean 24-h ambient air concentrations sampled in Los Angeles ranged from 0.038 to 955.7 µg/m³ (n = 128) (Havlicek et al., 1992). The authors reported that heavy usage of ethylene oxide within the Los Angeles basin coupled with restricted airflow out of the basin likely led to the large range of concentrations. There was a high degree of local variability that would be consistent with the release of ethylene oxide during a sterilization cycle. Concentrations in air sampled in northern California ranged from 0.032 to 0.40 µg/m³ (n = 36). At remote coastal locations in California, concentrations ranged from 0.029 to 0.36 µg/m³ (n = 22). The authors cautioned that it was not possible to draw definitive conclusions regarding the spatial and temporal distribution of ethylene oxide based on the samples collected. Levels were highly variable, especially in urban areas, with 100-fold shifts in airborne concentrations over a few minutes.

Peak short-term and long-term ambient concentrations of ethylene oxide as a result of emissions from four sterilization facilities in Duval County, Florida, USA, were estimated based on the US EPA SCREEN and Industrial Source Complex Short-Term dispersion models (Tutt & Tilley, 1993). These included a commercial spice fumigation facility (with estimated annual emissions of 1959.5 kg ethylene oxide) and three hospitals of decreasing emission profiles (i.e., from 210.9 to 2.1 kg/year). The predicted maximum average annual concentrations from the two highest emitters were 11 µg/m³ for a sterilization facility in Florida and 2 µg/m³ in the vicinity of a hospital in Florida, both at a distance of 32 m from their respective point sources.

6.1.2 Indoor air

Ethylene oxide was detected at a concentration of 4 µg/m³ in only 1 of 50 24-h samples of air collected inside randomly selected residences during a multimedia exposure study conducted in Canada (Health Canada, 1999a). The censored mean value was 0.17 µg/m³ when a concentration equivalent to one-half the limit of detection (i.e., ½ × 0.19 µg/m³ = 0.095 µg/m³) was assumed for the 49 samples in which ethylene oxide was not detected. Ethylene oxide was detected at concentrations of 5 µg/m³ in 3 of 24 personal air samples collected from an occupant of each of the 50 residences (Conor Pacific Environmental, 1998).

6.1.3 Water, sediment and soil, and biota

Data on concentrations of ethylene oxide in drinking-water, surface water, groundwater, sediment, soil, or biota were not identified.

6.1.4 Food

Ethylene oxide was detected in 96 (or 47%) of 204 samples of food products taken from retail stores in Denmark in 1985 (Jensen, 1988). The reported concentrations reflect the total amount of ethylene chlorohydrin and ethylene oxide present at the time of analysis. These concentrations ranged from <0.05 to 1800 µg/g in the individual samples, without correction for recoveries. Ethylene oxide was detected frequently among 24 samples of spices (Jensen, 1988), at a mean concentration of 84 µg/g and a maximum concentration of 580 µg/g.

Ethylene oxide was detected, but not quantified, in 1 of 2372 samples of eggs and in 1 of 3262 samples of fish collected in the USA in 1975 (Duggan et al., 1983).

6.1.5 Consumer products

Ethylene oxide may be present in tobacco as a result of its use as a fumigant and sterilizing agent (ATSDR, 1990). It has been detected in smoke from fumigated and unfumigated tobacco at levels of 0.3 µg/ml and 0.02 µg/ml, respectively (Binder, 1974).

Ethylene oxide may also be present as a contaminant of skin care products. Current commercial preparations of polyglycol ethers may contain residues of ethylene oxide monomer up to approximately 1 µg/g, according to a European study (Filser et al., 1994). Kreuzer (1992) reported concentrations of ethylene oxide monomer in skin care products ranging from 1.9 to 34 nmol/cm³ (0.08–1.5 mg/litre) and a range of maximum skin penetration of ethylene oxide of 1.0–14% in various product formulations.

6.1.6 Medical devices

Ethylene oxide is the most common agent currently used for sterilizing disposable dialysers, blood tubing, and heat-sensitive medical items (Henne et al., 1984; Babich, 1985). Ethylene oxide may be absorbed by medical equipment during sterilization and may remain there as the unchanged compound or as one of its reaction products (IPCS, 1985). Concentrations of residual ethylene oxide in medical devices immediately following their sterilization have ranged up to 1 or 2% (Gillespie et al., 1979; Gilding et al., 1980). These concentrations generally declined rapidly after a few days’ aeration, although levels exceeding 180 mg/m³ were sometimes measured following aeration.

The focus of this section and the basis for risk characterization is exposure in air, for which there are at least some data as a basis to estimate exposure. This is justified on the basis that most ethylene oxide is released to air and is unlikely to be transferred to other media. Moreover, ethylene oxide is not expected to accumulate in sediment or soil or to bioaccumulate, as a result of its high water solubility and vapour pressure.

The concentration of ethylene oxide predicted for ambient air (i.e., 6.2 × 10^–3 µg/m³) by ChemCAN fugacity modelling, assuming that air entering southern Ontario from the USA contains ethylene oxide at a concentration of 5.4 × 10^–3 µg/m³, was considered the basis for the minimum estimate of exposure via inhalation. Censored mean concentrations of ethylene oxide in outdoor and indoor air (i.e., 0.34 µg/m³ and 0.17 µg/m³, respectively), derived from the multimedia exposure study, were considered to represent the maximum concentrations to which the general population is exposed daily outdoors and indoors, respectively. Upper-bounding estimates of exposure via inhalation for the general population in Canada were based upon the maximum concentrations of ethylene oxide in outdoor and indoor air (i.e., 4.9 µg/m³ and 4.0 µg/m³, respectively) reported from the multimedia exposure study (Conor Pacific Environmental, 1998). Mean concentrations in ambient air sampled in Los Angeles, California, ranged from 0.038 to 955.7 µg/m³ (Havlicek et al., 1992).

Exposure to ethylene oxide in ambient air may be substantially higher for populations residing in the vicinity of point sources. An ethylene oxide concentration of 2 µg/m³ was predicted for outdoor air in close proximity to hospitals in Canada (Environment Canada, 1999) and Florida (Tutt & Tilley, 1993). A concentration of 11 µg ethylene oxide/m³ was predicted for outdoor air in close proximity to a sterilization facility in Florida (Tutt & Tilley, 1993). A maximum 1-h concentration of 20.1 µg ethylene oxide/m³ was predicted for outdoor air near a production facility for ethylene glycol in Alberta (Environment Canada, 1997).

Limitations of the data preclude development of meaningful probabilistic estimates of exposure of the general population to ethylene oxide in air.

Workers may be exposed to ethylene oxide during its production or use in the manufacture of other chemicals. Because ethylene oxide is highly explosive and reactive, the processing equipment generally consists of tightly closed and highly automated systems, which limit occupational exposure. Exposures occur primarily during the loading or unloading of transport tanks, product sampling procedures, and equipment maintenance and repair (CHIP, 1982). The Toxic Chemical Release Inventory listed 197 industrial facilities that produced, processed, or otherwise used ethylene oxide in 1988 (US EPA, 1990).

Industrial workers may also be exposed to ethylene oxide during sterilization of a variety of products, such as medical equipment and products (surgical products, single-use medical devices, etc.), disposable health care products, pharmaceutical and veterinary products, spices, and animal feed. Although much smaller amounts of ethylene oxide are used in sterilizing medical instruments and supplies in hospitals and for the fumigation of spices, it is during these uses that the highest occupational exposure levels have been measured (IARC, 1994). There was a wide range in reported concentrations (from 0 to about 1500 mg/m³), depending on operation, conditions, and duration of sampling for workers in US hospitals where ethylene oxide is used as a gaseous sterilant for heat-sensitive medical items, surgical instruments, and other objects and fluids coming in contact with biological tissues. Based on a limited field survey of hospitals, it was reported that concentrations of ethylene oxide near malfunctioning or improperly designed equipment may reach transitory levels of hundreds or even a few thousand milligrams per cubic metre, but time-weighted average (TWA) ambient and breathing zone concentrations were generally below about 90 mg/m³ (CHIP, 1982).

Information on the kinetics and metabolism of ethylene oxide has been derived primarily from studies conducted with laboratory animals exposed via inhalation, although some limited data from humans have been identified.

Ethylene oxide is very soluble in blood, and pulmonary uptake is expected to be rapid, dependent only upon the alveolar ventilation rate and concentration in the inspired air (IPCS, 1985). There is a lack of quantitative data on the absorption of ethylene oxide in various species; however, studies have revealed that ethylene oxide is absorbed rapidly through the respiratory tract in rats (Filser & Bolt, 1984; Koga et al., 1987; Tardif et al., 1987), mice (Ehrenberg et al., 1974; Tardif et al., 1987), and rabbits (Tardif et al., 1987). Ehrenberg et al. (1974) estimated that close to 100% of the inhaled dose of ethylene oxide was absorbed by mice exposed for 1–2 h to average concentrations between 2 and 55 mg/m³.

Quantitative information on the absorption of ethylene oxide in laboratory animals following ingestion or dermal exposure was not identified.

Ethylene oxide and its metabolites are rapidly distributed throughout the body. In the study performed by Ehrenberg et al. (1974) with mice exposed to [¹⁴C]ethylene oxide at concentrations of 2–55 mg/m³ for between 1 and 2 h, the greatest amount of radioactivity was detected in the liver, kidneys, and lungs, with smaller amounts in the spleen, testes, and brain. In rats exposed to [¹⁴C]ethylene oxide vapour at concentrations of 18.3, 183, or 1830 mg/m³ for 6 h (estimated mean absorbed doses of 2.7, 20.2, and 106.8 mg/kg body weight, respectively), the greatest amounts of radioactivity were found in the urinary bladder, liver, packed blood cells, and adrenal glands; the lowest levels were in fat (Tyler & McKelvey, 1982).

Ehrenberg et al. (1974) reported that in mice, about 78% of an inhaled dose was eliminated in the urine within 48 h, with the majority excreted within the first 24 h. Tyler & McKelvey (1982) observed that at all exposure levels tested, the primary route of [¹⁴C]ethylene oxide elimination in rats following inhalation exposure was in the urine (mean value of 59% recovered radioactivity), with lesser amounts expired as carbon dioxide (12%) and ethylene oxide (1%) or eliminated in the faeces (4.5%). After a single dose of 2 mg [¹⁴C]ethylene oxide/kg body weight (in propanediol) was administered intraperitoneally to rats, 43% of the radioactivity was excreted in the urine within 50 h, most (approximately 40%) appearing within the first 18 h following injection (Jones & Wells, 1981); 9% was identified as S-(2-hydroxyethyl)cysteine, and 33% was identified as N-acetyl-S-(2-hydroxyethyl)cysteine (both products of glutathione conjugation). In addition, 1.5% was exhaled via the lungs as carbon dioxide, and 1% was exhaled as unmetabolized ethylene oxide.

Brown et al. (1996) examined the distribution and elimination of ethylene oxide in rats and mice following inhalation exposure. Loss of ethylene oxide from the blood (and other tissues) was approximately 3- to 4-fold faster in mice than in rats. Following exposure, similar levels of ethylene oxide were measured in the brain, blood, and muscle within each species. However, concentrations in the testes of rats were 20% of those in other tissues; in mice, levels in the testes were 50% of those measured in other tissues.

In animals and humans, there are two routes of ethylene oxide catabolism, both of which are considered to be detoxification pathways. The first involves hydrolysis to ethylene glycol, with subsequent conversion to oxalic acid, formic acid, and carbon dioxide. The second involves conjugation with glutathione, with subsequent metabolic steps yielding S-(2-hydroxyethyl)cysteine [S-(2-carboxymethyl)cysteine] and N-acetylated derivatives (i.e., N-acetyl-S-(2-hydroxyethyl)cysteine [and N-acetyl-S-(2-carboxymethyl)cysteine]) (Wolfs et al., 1983; IPCS, 1985; ATSDR, 1990; Popp et al., 1994). Based upon available data, the route involving conjugation with glutathione appears to predominate in rats and mice; in larger species (rabbits, dogs), the conversion of ethylene oxide is primarily via hydrolysis through ethylene glycol (Jones & Wells, 1981; Martis et al., 1982; Gérin & Tardif, 1986; Tardif et al., 1987; Brown et al., 1996). Ethylene oxide may also be formed from the metabolism of ethylene (IARC, 1994).

A physiologically based pharmacokinetic (PBPK) model for the dosimetry of inhaled ethylene oxide was first developed for rats and included binding of ethylene oxide to haemoglobin and DNA in addition to tissue distribution, metabolic pathways (i.e., hydrolysis by epoxide hydrolase and conjugation by glutathione-S-transferase), and depletion of hepatic and extra-hepatic glutathione (Krishnan et al., 1992). The model was then refined and extended to mice and humans (Fennell & Brown, 2001). Simulations indicate that in mice, rats, and humans, about 80%, 60%, and 20%, respectively, would be metabolized via glutathione conjugation (Fennell & Brown, 2001).

This is consistent with observed levels of theta-class glutathione S-transferase (GSTT1) enzyme activity in the order mice > rats > humans.⁴ In rats and mice, GSTT1 activity was highest in the liver, followed by the kidney and testes. Rat brain and rat and mouse lung contained small amounts of activity compared with other tissues (enzyme activity in mouse brain was not examined).⁵ Ethylene oxide is a substrate for the human GSTT1 enzyme (Hallier et al., 1993; Pemble et al., 1994; Hayes & Pulford, 1995).

Ethylene oxide is an electrophilic agent that alkylates nucleophilic groups in biological macromolecules, including DNA and proteins. In haemoglobin, for example, adducts can be formed at cysteine residues, N-terminal valine, as well as N’- and N’-histidine (Segerbäck, 1990). Since ethylene oxide is formed during the metabolism of ethylene, a natural body constituent, endogenous as well as exogenous sources of ethylene and ethylene oxide contribute to background alkylation of proteins such as haemoglobin and albumin, as well as DNA (Bolt, 1996). N-(2-Hydroxyethyl)valine (HEVal) and hydroxyethylhistidine (HEHis) adducts have been frequently monitored in tissues of workers exposed to ethylene oxide in occupational settings (see IARC, 1994). Background levels of HEVal in non-smokers ranged from 9 to 188 pmol/g globin (Törnqvist et al., 1986, 1989; Bailey et al., 1988; Hagmar et al., 1991; Sarto et al., 1991; Tates et al., 1991, 1992; van Sittert et al., 1993; van Sittert & van Vliet, 1994; Farmer et al., 1996; Granath et al., 1996). Ethylene oxide binding to DNA results primarily in the formation of 7-(2-hydroxyethyl)guanine (7-HEGua) (Föst et al., 1989; Li et al., 1992); other adducts have also been identified at much lower levels. In DNA extracted from the lymphocytes of unexposed individuals, mean background levels of 7-HEGua ranged from 2 to 8.5 pmol/mg DNA (Föst et al., 1989; Bolt et al., 1997). Although these levels were similar to those measured in rodents not exposed to ethylene oxide (Föst et al., 1989; Walker et al., 1992), Wu et al. (1999a), using a more sensitive technique, reported that human tissue contains 10- to 15-fold higher levels of endogenous 7-HEGua than rodent tissue.

Studies of smokers exposed to ethylene oxide in cigarette smoke (Fennell et al., 2000) and occupationally exposed workers (Yong et al., 2001) have revealed higher levels of haemoglobin HEVal adducts among individuals with a GSTT1 "null genotype" (i.e., homozygous deletion of GSTT1 gene) than among those with a GSTT1 "positive genotype" (i.e., having at least one copy of the GSTT1 gene). In mice, half-lives for the removal of 7-HEGua in DNA from a variety of tissues (brain, lung, spleen, liver, and testes) were 1.5- to 3.9-fold lower than in rats (Walker et al., 1992). In both rats and mice, substantive depletion of glutathione pools has been observed following single exposure to high levels (i.e., >550 mg/m³) of ethylene oxide (McKelvey & Zemaitis, 1986; Brown et al., 1998), although it should be noted that increases in tumour incidence have been observed at lower concentrations. Reports on two PBPK models for ethylene oxide in rodents and humans have recently appeared (Csanády et al., 2000; Fennell & Brown, 2001). The models for rats, mice, and humans are qualitatively similar in their elements and provide for interspecies comparisons of internal ethylene oxide dose. The models are consistent with the conclusion that ethylene oxide is acting as a direct-acting alkylating agent in humans and rodents. Quantitative differences in response in biomarkers of exposure and effect are accounted for by differences in basic physiology between rodents and humans, rather than by factors suggesting a different mode of action.

Ethylene oxide is of low acute toxicity following inhalation, with 4-h LC₅₀s of about 2700, 1500, and 1800 mg/m³ for rats, mice, and dogs, respectively (Jacobson et al., 1956). LD₅₀s for ethylene oxide administered orally (in water) were 330 mg/kg body weight for male rats, 280 and 365 mg/kg body weight for female and male mice, respectively, and 270 mg/kg body weight for guinea-pigs (both sexes) (Smyth et al., 1941; Woodard & Woodard, 1971).

The lungs (oedema, congestion, haemorrhage) and nervous system (convulsions, prostration) are the principal organs affected following inhalation exposure to acutely toxic levels of ethylene oxide.

Available data on the repeated-dose toxicity of ethylene oxide are limited, being restricted primarily to inhalation studies in which animals were exposed to single concentrations.

Mortality was increased following the inhalation exposure of rats, mice, guinea-pigs, rabbits, and monkeys to concentrations of ethylene oxide ranging from about 730 to 1500 mg/m³ for 10 days to 8 weeks (Hollingsworth et al., 1956; Jacobson et al., 1956; Snellings, 1982; NTP, 1987). In rats exposed to ethylene oxide at concentrations between 180 and 915 mg/m³ for several weeks, there were haematological effects, changes in clinical chemistry, as well as histopathological alterations in various tissues (Jacobson et al., 1956; Snellings, 1982; Mori et al., 1990). Effects in mice exposed to ethylene oxide at 810 mg/m³ for 3 weeks included reduced body weight gain, poor coordination of the hindquarters, irregular breathing, convulsions, and red urine. In both rats and mice, body weight gain was reduced following repeated exposure to levels of ethylene oxide as low as 90 mg/m³ for approximately 7 weeks (Snellings, 1982).

Reductions in haemoglobin concentration, haematocrit, and red blood cell counts, accompanied by increases in reticulocytes, were observed in rats exposed to 915 mg ethylene oxide/m³ for 13 weeks (Fujishiro et al., 1990; Mori et al., 1990). This exposure regimen also produced declines in the activities of glutathione reductase and creatine kinase in blood and various tissues (Katoh et al., 1988, 1989; Matsuoka et al., 1990; Mori et al., 1990; Fujishiro et al., 1991), as well as increased hepatic lipid peroxidation (Katoh et al., 1988, 1989). Other effects observed in rats following medium-term exposure to ethylene oxide at concentrations ranging from 370 to 915 mg/m³ included those on the nervous system (Hollingsworth et al., 1956; Ohnishi et al., 1985, 1986; Matsuoka et al., 1990; Mori et al., 1990), disturbances in hepatic porphyrin-haem metabolism (Fujishiro et al., 1990), and histopathological changes in the testes, kidneys, and lungs (Hollingsworth et al., 1956). Effects in mice exposed to ethylene oxide are similar to those in rats (Snellings et al., 1984a; Popp et al., 1986); renal tubular degeneration was observed at concentrations as low as 183 mg/m³ (NTP, 1987). Exposure to concentrations as low as 86 mg/m³ reduced locomotor activity (Snellings et al., 1984a).

No differences in haematological parameters (red or white blood cell counts, haematocrit, haemoglobin, or white cell differential) were observed in rabbits exposed to 458 mg ethylene oxide/m³ for 12 weeks, compared with unexposed controls (Yager & Benz, 1982).

In the only short- or medium-term study identified on the oral toxicity of ethylene oxide, there was a loss of body weight, gastric irritation, and slight liver damage following exposure of rats to 100 mg ethylene oxide/kg body weight, 5 times/week, for a total of 15 doses in 21 days (Hollingsworth et al., 1956).

8.3.1 Chronic toxicity

Non-neoplastic effects associated with long-term exposure to ethylene oxide have not been investigated extensively, most studies having focused on the carcinogenicity of this substance. In several investigations conducted with rats exposed to ethylene oxide for 2 years, significant reductions in body weight gain at concentrations as low as 60.4 mg/m³ and decreased survival time at exposures of >92 mg/m³ have been observed (Lynch et al., 1984a,b; Snellings et al., 1984b; Garman et al., 1985; Garman & Snellings, 1986). Additional non-neoplastic effects observed at exposures of >92 mg ethylene oxide/m³ include increased levels of aspartate aminotransferase in serum, reduced absolute kidney and adrenal weights, an increased incidence of inflammatory lesions in the lungs, nasal cavity, trachea, and internal ear, proliferative and degenerative lesions in the adrenal cortex, increased splenic extramedullary haematopoiesis, as well as an increased incidence of multifocal mineralization of the posterior layers of the choroid/sclera portion of the eye (Lynch et al., 1984a,b). Skeletal muscular atrophy (in the absence of sciatic nerve neuropathy) was noted following exposure to 183 mg ethylene oxide/m³ (Lynch et al., 1984a,b).

No exposure-related effects upon survival, body weight gain, clinical signs, or other non-neoplastic end-points (examined in a wide range of tissues) were observed in B6C3F₁ mice exposed to 92 or 183 mg ethylene oxide/m³ for 2 years (NTP, 1987).

Monkeys exposed for 2 years to >92 mg ethylene oxide/m³ developed axonal dystrophy in the nucleus gracilis of the medulla oblongata of the brain, along with demyelination in the distal portion of the fasciculus gracilis (Sprinz et al., 1982; Lynch et al., 1984b). Decreased nerve conduction velocity was observed in only 2 of 12 monkeys exposed to 183 mg/m³. Weight gain was significantly reduced following exposure to 183 mg ethylene oxide/m³ (Lynch et al., 1984a,b; Setzer et al., 1996). Lynch et al. (1992) indicated subsequently that in animals exposed to 0, 92, or 183 mg ethylene oxide/m³, the incidence of lens opacities was 0/12, 2/11, and 3/11, respectively, when assessed during the last month of exposure, or 2/4, 2/3, and 4/4, respectively, when assessed 10 years after the cessation of exposure.

8.3.2 Carcinogenicity

Substance-related increases in a variety of tumour types have been observed in rodents exposed to ethylene oxide. Descriptions of study protocols and results (including incidence of tumours) are presented in Table 2 (rats) and Table 3 (mice). In two studies, inhalation exposure increased the incidence of mononuclear cell leukaemia⁶ and gliomas of the brain in F344 rats of both sexes and of peritoneal mesotheliomas in male rats. In mice, increased incidences of alveolar/bronchiolar adenomas or carcinomas and Harderian gland papillary cystadenomas were observed in both sexes, while the incidences of malignant lymphomas, uterine and mammary gland adenocarcinomas, and mammary gland adenocarcinomas or adenosquamous carcinomas (combined) were increased in females. An increase in squamous cell carcinomas of the forestomach was observed in female rats following the administration (by gavage) of ethylene oxide; the subcutaneous injection of ethylene oxide to female mice induced local fibrosarcomas.

Table 2: Incidence of tumours in Fischer 344 rats exposed to ethylene oxide by inhalation.

Table 3: Incidence of tumours in B6C3F₁ mice exposed to ethylene oxide.^a

In male Fischer 344 rats exposed to 0, 92, or 183 mg ethylene oxide/m³, the incidence of mononuclear cell leukaemia was increased, notably in the low exposure group (Lynch et al., 1984a,b). The incidence of peritoneal mesotheliomas and mixed cell gliomas in brain tissue was increased in an exposure-related fashion.

Results were similar when groups (n = 120 per sex) of male and female Fischer 344 rats were exposed to 0, 18.3, 60.4, or 183 mg ethylene oxide/m³ (Snellings et al., 1984b; Garman et al., 1985; Garman & Snellings, 1986). Trend analysis of the incidence of mononuclear cell leukaemia revealed a significant association for both sexes, although the increase was clearly concentration related only in females and was significantly different from the control group in females at the highest concentration only (Snellings et al., 1984b). Among males, trend analysis of the incidence of peritoneal mesotheliomas indicated a relationship between exposure to ethylene oxide and tumour induction after adjustment for mortality (Snellings et al., 1984b). Concentration-related increases in primary brain tumours (gliomas, malignant reticuloses, and granular cell tumours) were observed in both sexes (Garman et al., 1985; Garman & Snellings, 1986). The incidence of subcutaneous fibroma (15/58) was significantly increased in male rats in the highest exposure group (i.e., 183 mg/m³) (Snellings et al., 1984b). The increase in the incidence of mononuclear leukaemia, mesothelioma, and brain tumours in these animals occurred during the later stages of this study (i.e., after about 20–24 months of exposure to ethylene oxide) (Snellings et al., 1984b; Golberg, 1986).

In male and female B6C3F₁ mice exposed to 0, 92, or 183 mg ethylene oxide/m³, there was a significant concentration-related increase in the incidence of alveolar/bronchiolar carcinomas and papillary cystadenomas in the Harderian gland (NTP, 1987). In females, there were concentration-related increases in the incidence of malignant lymphomas of the haematopoietic system and uterine adenocarcinoma; the incidence of mammary adenocarcinoma and adenosquamous carcinoma was elevated in both treated groups (NTP, 1987).

In a strain A mouse short-term test for carcinogenicity, a concentration-related increase in the incidence of pulmonary adenomas was observed following exposure (6 h/day, 5 days/week) to 128 and 366 mg ethylene oxide/m³ for 6 months (Adkins et al., 1986).

In a carcinogenicity study involving oral exposure, the intragastric administration of 7.5 or 30 mg ethylene oxide/kg body weight to female Sprague-Dawley rats twice weekly for 150 weeks produced a dose-related increase in the incidence of forestomach tumours (mainly squamous cell carcinomas) (Dunkelberg, 1982).

In female NMRI mice, the subcutaneous injection of ethylene oxide for 95 weeks (mean total doses up to 64.4 mg/mouse) resulted in a significant dose-dependent increase in the number of tumours (i.e., sarcomas) at the site of injection (Dunkelberg, 1981). No skin tumours were observed in female ICR/Ha Swiss mice following the dermal application of approximately 100 mg ethylene oxide (10% in acetone) 3 times weekly for life (Van Duuren et al., 1965).

The genotoxicity of ethylene oxide has been reviewed extensively (IARC, 1994). Owing to the consistency of the results, only a brief summary of studies conducted with in vitro systems or with laboratory animals is provided here. Ethylene oxide is a potent alkylating agent that has been genotoxic in virtually all studies in which it was examined (reviewed in IARC, 1994). In in vitro testing, it induced DNA damage and gene mutations in bacteria, yeast, and fungi and gene conversion in yeast. In mammalian cells, observed effects include gene mutations, micronucleus formation, chromosomal aberrations, cell transformation, unscheduled DNA synthesis, sister chromatid exchange, and DNA strand breaks. Notably, Hallier et al. (1993) observed that the frequency of sister chromatid exchange in human peripheral blood lymphocytes exposed in vitro to ethylene oxide was higher in cells isolated from individuals expressing low levels of GSTT1 than in cells from subjects expressing higher levels of this enzyme.

The results of in vivo studies on the genotoxicity of ethylene oxide have also been consistently positive (see IARC, 1994) following ingestion, inhalation, or injection. In vivo exposure to ethylene oxide induced gene mutation at the hypoxanthine phosphoribosyl transferase (Hprt) locus in mouse and rat splenic T-lymphocytes; sister chromatid exchange was induced in lymphocytes from rabbit, rat, and monkey, in bone marrow cells from mouse and rat, and in rat spleen. Increases in the frequency of gene mutations in the lung (lacI locus) (Sisk et al., 1997) and in T-lymphocytes (Hprt locus) (Walker et al., 1997a) have been observed in transgenic mice exposed to ethylene oxide via inhalation, at concentrations similar to those in carcinogenesis bioassays with this species (NTP, 1987).

In male Big Blue^® (lacI transgenic) B6C3F₁ mice exposed to 0, 92, 183, or 366 mg ethylene oxide/m³ for 6 h/day, 5 days/week, for 4 weeks, the observed mean (±SE) frequency of mutation at the Hprt locus in splenic T-lymphocytes was 2.2 (±0.03) × 10^–6, 3.8 (±0.5) × 10^–6 (P = 0.009), 6.8 (±0.9) × 10^–6 (P = 0.001), and 14.1 (±1.1) × 10^–6 (P < 0.001), respectively (Walker et al., 1997a). The frequency of Hprt mutations in splenic T-lymphocytes was increased (compared with unexposed controls) 5.0- to 5.6-fold in male F344 rats and (non-transgenic) male B6C3F₁ mice exposed to 366 mg ethylene oxide/m³ for 6 h/day, 5 days/week, for 4 weeks (Walker et al., 1997b). Similarly, the frequency of lacI mutations in the lungs, bone marrow, and spleen, but not in germ cells, was increased in male Big Blue^® (lacI transgenic) B6C3F₁ mice exposed to 0 or 366 mg ethylene oxide/m³ (Sisk et al., 1997; Recio et al., 1999).

In vivo exposure to ethylene oxide also induced heritable mutations or effects in germ cells in rodents (IARC, 1994). Ethylene oxide induced dominant lethal effects in mice and rats and heritable translocations in mice. There were dominant visible and electrophoretically detectable mutations in the offspring of male mice exposed (by inhalation) to 366 mg ethylene oxide/m³ for 6 h/day, 5 days/week, for 7 weeks and then mated. This exposure regimen was adopted to ensure that all progeny originated from sperm exposed during the entire spermatogenic process (Lewis et al., 1986). In a study in which male (C3H × 101)F₁ mice were exposed by inhalation to 0, 302, 373, 458, or 549 mg ethylene oxide/m³, 6 h/day, 5 days/week, for 6 weeks, then daily for an additional 2.5 weeks, and subsequently mated to T-stock (or [SEC × 101]F₁) females, the percent dominant lethals (P < 0.01 at concentrations >373 mg/m³, compared with controls) was 0 (0), 6 (8), 14 (13), 23 (24), and 60 (45), respectively (Generoso et al., 1990). The frequency of translocation carriers (P < 0.01 at all concentrations, compared with controls) among the progeny of these groups of ethylene oxide-exposed male mice mated to T-stock (or [SEC × C57BL]F₁) females (data combined) was 1/2068 (0.05%), 32/1143 (2.8%), 52/1021 (5.1%), 88/812 (10.8%), and 109/427 (25.5%), respectively (Generoso et al., 1990).

8.5.1 Effects on fertility

Degeneration of the seminiferous tubules and germ cells, decreased epididymal weight, decreased sperm count, and an increase in the percentage of abnormal sperm were observed in Wistar rats exposed to >458 mg ethylene oxide/m³ for 13 weeks (Mori et al., 1989, 1991). When abnormal sperm heads were classified into immature and teratic types, the frequency of teratic types was increased at exposures of >92 mg/m³, although it was not concentration dependent (Mori et al., 1991). Decreased relative testicular weight was observed in rats after exposure to 915 mg ethylene oxide/m³ (Mori et al., 1989). In a limited study in rats, slight degeneration of the tubules in the testes was observed after exposure to 370 mg ethylene oxide/m³ for 25–32 weeks (Hollingsworth et al., 1956). Embryotoxic and fetotoxic effects were observed in reproductive studies with rats after exposure of the dams via inhalation to concentrations of ethylene oxide between 183 and 275 mg/m³, prior to mating and throughout gestation. These effects included a decrease in the number of implantation sites per pregnant female, an increase in the incidence of resorptions, a decrease in the median number of pups born on day 0 postpartum per litter, as well as a lower ratio of the number of fetuses born to the number of implantation sites per female (Hackett et al., 1982; Snellings et al., 1982a,b; Hardin et al., 1983). Under these exposure conditions, adverse effects on the dams were not observed (based simply upon clinical appearance and demeanour).

Reproductive effects in mice are similar to those observed in rats. These include increases in the number of resorption bodies and reductions in the number of implants per female and in the number of living embryos per female in female animals exposed to 549 or 2196 mg ethylene oxide/m³ prior to mating (Generoso et al., 1987), concentration-related increases in the percentage of abnormal sperm in animals exposed to 366 mg ethylene oxide/m³ for 5 days (Ribeiro et al., 1987), and a decline in absolute but not relative testicular weight without histological changes in mice exposed to 86 mg ethylene oxide/m³ for 10 weeks (Snellings et al., 1984a).

A decline in sperm count and motility was observed in monkeys exposed to concentrations of ethylene oxide as low as 92 mg/m³ for 24 months (Lynch et al., 1984b,c).

8.5.2 Developmental toxicity

Exposure of Sprague-Dawley rats to a maternally toxic concentration of 275 mg ethylene oxide/m³ either prior to mating and throughout gestation or only during various stages of gestation resulted in reduced fetal body weight and crown-to-rump length, as well as reduced skeletal ossification (Hackett et al., 1982; Hardin et al., 1983). Fetal body weights were reduced in Fischer 344 rats following exposure, only during the period of organogenesis, to 183 mg ethylene oxide/m³, a concentration having no overt toxic effects on the dams (Snellings et al., 1982a). Repeated brief exposures of pregnant Sprague-Dawley rats during gestation to 1464 or 2196 mg ethylene oxide/m³ produced a decline in fetal body weight (at both concentrations) and maternal toxicity (reduced body weight gain) at 2196 mg/m³ (Saillenfait et al., 1996); however, there was no evidence of teratogenicity.

In offspring of female hybrid mice exposed to 2196 mg ethylene oxide/m³ at various intervals shortly after mating, there was a range of congenital malformations, including omphalocoele, hydrops, eye defects, open thorax, cardiac defects, cleft palate, and tail and limb defects (Generoso et al., 1987; Rutledge & Generoso, 1989). There were also increases in the numbers of mid-gestational and late fetal deaths and offspring that did not reach weaning (Generoso et al., 1987; Rutledge & Generoso, 1989; Rutledge et al., 1992). In the offspring of female mice exposed to >1647 mg ethylene oxide/m³ for brief periods shortly after mating, skeletal ossification was reduced and the incidence of axial skeletal anomalies and cleft sternum was increased (Polifka et al., 1991, 1992).

The effect of exposure rate was assessed in pregnant C57BL/6J mice through inhalation exposure to ethylene oxide for 1.5, 3, or 6 h at 3800 or 4900 (mg/m³)-h on gestational day 7 (Weller et al., 1999). Animals with short, high exposures to ethylene oxide had increased adverse effects on fetal death and resorptions, malformations, crown-to-rump length, and fetal weight compared with those exposed to the same total (mg/m³)-h but at longer, lower exposures.

Effects on the nervous system have been observed frequently in laboratory animals exposed to ethylene oxide. The paralysis observed in some animals was reversed upon cessation of exposure (Hollingsworth et al., 1956). Poor coordination of the hindquarters was observed in rats and mice following exposure to 810 mg ethylene oxide/m³ for 7–8 weeks (Snellings, 1982). In subchronic or chronic studies in rats exposed to 458–915 mg ethylene oxide/m³, there was a range of neurological effects, including awkward or ataxic gait, paralysis, and atrophy of the muscles of the hindlimbs, accompanied in some cases by pathological evidence of axonal degeneration of myelinated fibres in nerves of the hind legs (Hollingsworth et al., 1956; Ohnishi et al., 1985, 1986; Matsuoka et al., 1990; Mori et al., 1990). Abnormal posture during gait and reduced locomotor activity were also observed in mice after exposure to ethylene oxide at concentrations ranging from 86 to 425 mg/m³ for 6 h/day, 5 days/week, for 10 or 11 weeks (Snellings et al., 1984a); effects on various reflexes (righting, tail pinch, toe pinch) were also noted at the highest concentration examined (i.e., 425 mg/m³).

Paralysis of the hind limbs and atrophy of the leg muscles have also been reported in rabbits and monkeys following exposure to >370 mg ethylene oxide/m³ (Hollingsworth et al., 1956).

Histological alterations in the axons and demyelination were reported in cynomolgus monkeys exposed to 92 or 183 mg ethylene oxide/m³ for 2 years (Sprinz et al., 1982; Lynch et al., 1984b).

It is likely that the carcinogenicity of ethylene oxide in laboratory animals arises primarily as a result of its direct alkylation of biological macromolecules (i.e., nucleic acids). In vivo exposure to ethylene oxide induced mutations (5- to 5.6-fold) at the Hprt locus in splenic T-lymphocytes in rats and mice (Walker et al., 1997a,b). A statistically significant (i.e., P < 0.05) increase (1.5-fold) in the frequency of lacI mutations was observed in the lungs of transgenic mice exposed to 366 mg ethylene oxide/m³ (Sisk et al., 1997); the frequencies of lacI mutations in the bone marrow and spleen of these animals (1.9- and 1.3-fold, respectively), although increased, were not statistically different from those of the unexposed controls. Currently, there is no clear evidence of a relationship between the mutagenic response observed at these two "indicator" loci and the species- and tissue-specific carcinogenicity of ethylene oxide. Molecular analysis of ethylene oxide-induced mutations at the HPRT locus in human diploid fibroblasts exposed in vitro revealed that a high proportion involved large deletions of this gene (Bastlová et al., 1993).

A potential role of the formation of 7-HEGua in the carcinogenic response has been the focus of many studies, this adduct having been identified in both humans and laboratory animals. In reports by Walker et al. (1992) and Wu et al. (1999b), F344 rats and B6C3F₁ mice were exposed (via inhalation for 6 h/day, 5 days/

week, for 4 weeks) to concentrations of ethylene oxide similar to those used in previous carcinogenicity bioassays involving these strains (Lynch et al., 1984a,b; Snellings et al., 1984b; Garman et al., 1985; Garman & Snellings, 1986; NTP, 1987). Slightly higher levels of 7-HEGua were measured in tissues (lung, spleen, brain, liver) from rats than from mice; within each species, similar levels of the adduct were measured in the lung, spleen, brain, and liver. Since an increased incidence of brain tumours has been observed in rats but not in mice exposed to ethylene oxide and an increased incidence of lung tumours has been observed in mice but not in rats exposed to this substance, the results provided by Walker et al. (1992) and Wu et al. (1999b) point to no obvious relationship between the overall level of 7-HEGua within various tissues and the observed species-specific carcinogenic response. The potential roles of this and other ethylene oxide-induced DNA adducts, as well as other factors, in mediating the carcinogenicity of ethylene oxide have not been defined.

9.1.1 Irritation and sensitization

Exposure to ethylene oxide vapour can cause irritation of the eyes and respiratory tract (Thiess, 1963; ATSDR, 1990). Mild irritation of the skin has been reported after contact with aqueous solutions of ethylene oxide as low as 1% (Sexton & Henson, 1949). Dermal injury is characterized by oedema and erythema, occurring 1–5 h after exposure, followed by the formation of vesicles. Dermal irritation has also been observed after contact with ethylene oxide-sterilized materials and clothing (Royce & Moore, 1955; Marx et al., 1969; Hanifin, 1971; Biro et al., 1974; LaDage, 1979; Bommer & Ritz, 1987; Fisher, 1988; Lerman et al., 1995).

Ethylene oxide is a sensitizing agent. Type I (anaphylaxis) and Type IV (contact dermatitis) hypersensitivity reactions have been observed in individuals exposed to ethylene oxide. Anaphylactic reactions (ranging from mild to severe) have been noted among patients undergoing dialysis involving equipment sterilized by exposure to ethylene oxide (reviewed in Bommer & Ritz, 1987). Asthmatic reactions may occur either alone or in combination with anaphylactic events; case reports of occupational asthma attributed to ethylene oxide exposure have appeared (Dugue et al., 1991; Verraes & Michel, 1995). Reports of contact dermatitis attributed to ethylene oxide are not uncommon; these may be due to allergic reaction or to the irritative effects.

9.1.2 Reproductive effects

Hemminki et al. (1982) determined the incidence of spontaneous abortion among Finnish hospital staff who had used ethylene oxide, glutaraldehyde, and formaldehyde for instrument sterilization. Sterilizing staff employed in Finnish hospitals in 1980 were included in the analysis, with a total of 1443 pregnancies (545 workers exposed during pregnancy). No measurements of exposure were taken specifically as part of this study (Hemminki et al., 1982). However, independent measurements carried out in 24 Finnish hospitals between 1976 and 1981 revealed 8-h TWA exposures ranging from 0.2 to 0.9 mg ethylene oxide/m³, with peak concentrations up to 458 mg/m³ (Hemminki et al., 1982, 1983), although concentrations of ethylene oxide may have been higher prior to 1976. Nurses from auxiliaries in the same hospitals having no exposure to sterilizing agents, anaesthetic gases, or X-rays served as controls. Information on exposure to sterilizing agents was obtained from the supervising nurses. Information on pregnancy outcome was obtained via a questionnaire and confirmed using a hospital discharge register for all of Finland between 1973 and 1979. The adjusted (for age, parity, decade of reported pregnancy, coffee and alcohol consumption, and smoking) rate of spontaneous abortion for the sterilization staff as a whole (9.7%) was similar to the rate in the control group (10.5%). However, when the pregnancies of the sterilizing staff were analysed according to employment at the time of conception, the rate of spontaneous abortion was significantly (P < 0.001) increased in the exposed (15.1%) versus the unexposed group (4.6%). When the associations between ethylene oxide and the different sterilizing agents were analysed, only exposure to ethylene oxide during early pregnancy was related to an increased frequency of spontaneous abortion (adjusted rate of 16.1% in exposed versus 7.8% in unexposed workers; P < 0.01). Hospital discharge records revealed a similar pattern, with spontaneous abortion rates of 22.6% (significantly higher than controls, P < 0.05), 9.9%, and 9.2% in sterilizing workers exposed to ethylene oxide, unexposed workers, and controls, respectively. In a subsequent analysis, only pregnancies that began during hospital employment were analysed in all groups, with controls chosen from the same hospitals (Hemminki et al., 1983). The rate of spontaneous abortion remained significantly higher (P < 0.05) among the pregnancies associated with exposure to ethylene oxide (20.4%) compared with controls (11.3%).

Rowland et al. (1996) examined the occurrence of spontaneous abortion and pre- and post-term delivery in relation to ethylene oxide exposure among 7000 randomly selected dental assistants (aged 18–39) identified from the 1987 dental assistant registry in California, USA. The most recent pregnancy outcome was chosen for analysis to maximize recall of pregnancy and exposure information, with 1320 women who provided information on age and ethylene oxide exposure contributing to the analysis. A total of 32 women reported ethylene oxide exposure during pregnancy; no quantitative measures or details on timing of exposure during pregnancy were available. The age-adjusted relative risk of spontaneous abortion among ethylene oxide-exposed women was 2.5 (95% confidence interval [CI] = 1.0–6.3); the relative risks of pre-term births (21–37 weeks) and post-term births (>42 weeks) were 2.7 (95% CI = 0.8–8.8) and 2.1 (95% CI = 0.7–5.9), respectively. Using a logistic model, ethylene oxide-exposed women were 2.7 times (95% CI = 1.2–6.1) more likely to have any of the three adverse pregnancy outcomes after adjusting for age. Adjustment for unscavenged nitrous oxide exposure, high amalgam use, and smoking yielded a relative risk of 2.1 (95% CI = 0.7–5.7).

In the only identified study in which the effect of paternal exposure to ethylene oxide on reproductive outcome was assessed, Lindbolm et al. (1991) reported a significantly (P < 0.05) increased risk of spontaneous abortion (odds ratio [OR] = 4.7; 95% CI = 1.2–18.4) among Finnish women whose partners had been exposed to ethylene oxide. In total, 99 186 pregnancies were included in the analysis. Paternal exposure to ethylene oxide was based upon the job and industry in which the men were employed; quantitative data on exposure were not available, and the numbers of spontaneous abortions (n = 3) and pregnancies (n = 10) in the paternal ethylene oxide-exposed group were small. Other potential confounding factors, such as previous abortions and alcohol and tobacco consumption, were not considered in the analysis.

9.1.3 Neurological effects

Sensorimotor polyneuropathy was reported for a number of cases following single or long-term exposure to ethylene oxide (exposure concentrations, when reported, ranged from 7.7 to >1281 mg/m³) (Gross et al., 1979; Finelli et al., 1983; Kuzuhara et al., 1983; Zampollo et al., 1984; Schroder et al., 1985; Fukushima et al., 1986; Ristow & Cornelius, 1986; Crystal et al., 1988). Amelioration of the symptoms following cessation of exposure has been commonly observed. In individuals exposed to >1300 mg ethylene oxide/m³, sural nerve biopsies revealed axonal degeneration with mild changes in the myelin sheath; muscle biopsies revealed degeneration atrophy (Kuzuhara et al., 1983). Effects on the central nervous system (e.g., seizures) have been observed following single exposure to 915–1281 mg ethylene oxide/m³ (Gross et al., 1979; Salinas et al., 1981).

9.1.4 Genetic effects

Increases in chromosomal aberrations in peripheral blood lymphocytes have been consistently reported in studies of workers exposed to concentrations of ethylene oxide of >9.2 mg/m³ (Table 4). Effects observed at lower concentrations (i.e., <9.2 mg/m³) have been mixed.

Significant increases in the frequency of sister chromatid exchange in peripheral blood cells have also been observed among individuals exposed to elevated levels of ethylene oxide (i.e., usually >9.2 mg/m³). Results of studies of individuals exposed to lower levels (i.e., <0.9 mg/m³) have been mixed. In some studies, increases in the frequency of sister chromatid exchange have been observed to persist after exposure had ceased. Effects have been related to the concentration or duration of exposure to ethylene oxide in a number of studies.

In some studies, the frequency of micronuclei in peripheral blood was increased in workers exposed to relatively high (3.7–60.4 mg/m³) levels of ethylene oxide (Tates et al., 1991; Ribeiro et al., 1994). However, in the majority of the studies involving exposures to lower levels, no effect on the frequency of micronuclei was observed. Apparent inconsistencies in the data could reflect the influence of peak exposures, differences in exposure duration, or smoking status.

Table 4: Cytogenetic effects in humans.^a