Ethylene oxide (EO) is mainly used as a chemical intermediate and as a fumigant and sterilizing agent. Through its manufacturing and intended uses, EO may be released into the environment. Therefore, an assessment of the environmental significance of those potential emissions was conducted. Data were collected describing pertinent physical properties, degradation and other loss mechanisms that control the fate of EO in the environment. Available aquatic and terrestrial ecotoxicity data were assembled and used as the basis of calculating critical toxicity values to characterize hazard. Environmental compartment concentrations of EO were calculated using Level III fugacity-based modeling. Six scenarios were developed to account for different climatic conditions in various portions of the US. Finally, potential regional-scale risks to aquatic and terrestrial wildlife were determined. In the conceptual model that was developed in this assessment, EO diffuses into air, partitions between environmental compartments, is transported out of the different compartments via advection, and undergoes abiotic and biological degradation within each compartment. All known emissions within the continental USA were assumed to enter a modeled region roughly the size of the State of Ohio. Organisms (receptors) were assumed to dwell in both terrestrial and aquatic compartments. Receptors were assumed to include small mammals, soil invertebrates, water column (pelagic) organisms, and sediment benthos. The goal of this assessment was to characterize any potential adverse risks to terrestrial and aquatic wildlife populations. Hazard Quotients (HQ) were calculated by dividing predicted concentrations of EO in air, water, sediment, and soil by their critical toxicity values. Maximum calculated HQ values in air were 1.52x10(-7), in water were 1.17x10(-5), in sediment were 2.25x10(-4), and in soil were 1.37x10(-5). The results of this assessment suggest that EO as currently manufactured and used does not pose adverse risks to aquatic or terrestrial wildlife. In all cases, the HQ values were much less than the maximum desired HQ value of 1.0 (4,400-6,600,000 times), indicating that the potential for diffuse emissions of EO to pose adverse environmental risks is quite low.