Ethylene Oxide (EtO) Air Dispersion Modeling and Cancer Risk Assessment for Community Exposure
Project Overview
This environmental forensic investigation evaluated long-term ethylene oxide (EtO) emissions from both medical sterilization facilities and industrial manufacturing operations to characterize community-wide exposure and estimate associated lifetime cancer risk. The investigation integrated atmospheric dispersion modeling, emissions inventory reconstruction, environmental chemistry, industrial hygiene, toxicology, regulatory science, exposure assessment, and quantitative human health risk assessment to reconstruct historical ambient EtO concentrations over multiple decades. Multiple independent emissions inventories, permitting records, regulatory databases, meteorological observations, engineering documentation, and peer-reviewed scientific literature were synthesized to evaluate cumulative airborne EtO concentrations resulting from numerous emission sources operating within a heavily industrialized region.
A major objective of the investigation involved reconstructing historical EtO emissions from both hospital sterilization operations and chemical manufacturing facilities using independently reported emissions data. Historical source characterization incorporated EPA Toxic Release Inventory (TRI) data, National Emissions Inventory (NEI) records, state environmental permitting information, facility operating histories, engineering source parameters, and regulatory documentation describing stack characteristics, emission controls, and source configurations. Where multiple regulatory datasets were available, independent emissions inventories were compared to evaluate consistency and improve the reliability of historical emissions reconstruction. The investigation further developed alternative source-term scenarios to evaluate uncertainty associated with historical reporting and facility operations.
Atmospheric dispersion modeling represented the central component of the investigation. Historical emissions were reconstructed using the EPA-approved AERMOD air dispersion model to estimate long-term annual ambient EtO concentrations throughout surrounding residential communities. Atmospheric modeling incorporated historical emission rates, stack parameters, fugitive emission sources, terrain characteristics, receptor grids, meteorological observations, and atmospheric stability data using regulatory guidance consistent with EPA modeling recommendations. Two independent modeling approaches were developed using alternative source inventories and engineering assumptions, allowing comparison of predicted concentrations under multiple scientifically defensible scenarios and providing a robust evaluation of modeling uncertainty.
The investigation also incorporated comprehensive cancer risk assessment using accepted EPA inhalation risk methodologies. Modeled annual EtO concentrations were converted to lifetime excess cancer risk using EPA inhalation unit risk values together with accepted exposure assumptions and Regional Screening Level (RSL) methodologies. Risk estimates were developed for both the surrounding community and representative long-term residential exposure scenarios, allowing evaluation of cumulative cancer risk resulting from emissions originating from multiple independent sources. Sensitivity analyses were further conducted to evaluate the influence of emissions assumptions, source characterization, and alternative modeling methodologies on estimated cancer risk. This integrated approach linked atmospheric dispersion modeling directly with quantitative human health risk assessment.
The investigation was conducted using accepted principles of air dispersion modeling, environmental chemistry, atmospheric science, industrial hygiene, environmental toxicology, regulatory science, exposure assessment, and quantitative human health risk assessment. Rather than relying upon any single emissions inventory or modeling scenario, the evaluation synthesized multiple regulatory databases, engineering documentation, meteorological observations, emissions inventories, atmospheric dispersion modeling, published toxicological literature, and EPA risk assessment methodologies into a multidisciplinary weight-of-evidence framework. This integrated environmental forensic methodology provides a scientifically robust approach for evaluating long-term community exposure to airborne ethylene oxide emissions and assessing associated lifetime cancer risks.
Chemicals of Concern
This investigation evaluated ethylene oxide (EtO) as the principal chemical of concern together with its role as a hazardous air pollutant emitted from medical sterilization facilities and chemical manufacturing operations. Ethylene oxide was evaluated because of its historical use as a sterilizing agent for temperature-sensitive medical equipment and as an intermediate in the production of numerous industrial chemicals, including ethylene glycol, surfactants, plastics, detergents, and specialty chemical products. The investigation integrated environmental chemistry, atmospheric science, industrial hygiene, toxicology, emissions reconstruction, regulatory science, and quantitative human health risk assessment to characterize long-term airborne EtO concentrations and associated lifetime cancer risks resulting from multiple emission sources.
Ethylene oxide (EtO) represented the primary contaminant evaluated throughout the investigation because of its well-established carcinogenicity and its widespread historical use in both industrial manufacturing and medical sterilization. Historical engineering records, emissions inventories, regulatory databases, facility operating histories, and environmental permitting information were reviewed to reconstruct annual EtO emissions over multiple decades. Toxicological evaluations incorporated guidance published by the U.S. Environmental Protection Agency (EPA), the National Institute for Occupational Safety and Health (NIOSH), the World Health Organization (WHO), and the International Agency for Research on Cancer (IARC), all of which have recognized EtO as a potent carcinogenic compound requiring careful evaluation of long-term inhalation exposure. These toxicological assessments provided the scientific basis for quantitative cancer risk calculations performed during the investigation.
The investigation also evaluated airborne EtO emissions originating from medical sterilization processes, where ethylene oxide is used to sterilize heat- and moisture-sensitive medical equipment that cannot be treated using conventional steam sterilization. Engineering evaluations examined sterilizer operation, aeration processes, emission control technologies, venting characteristics, stack parameters, and historical emission inventories reported through federal and state regulatory programs. Historical emissions associated with sterilization activities were reconstructed using independently reported emissions data and incorporated into atmospheric dispersion modeling to characterize long-term ambient air concentrations surrounding sterilization facilities.
In addition to medical sterilization, the investigation evaluated industrial manufacturing emissions of ethylene oxide associated with chemical production processes. Historical manufacturing records, engineering documentation, emissions inventories, permitting records, and regulatory filings were reviewed to characterize EtO releases associated with industrial operations. Environmental chemistry evaluations considered both point-source emissions and fugitive releases together with emission control technologies including scrubbers, catalytic oxidation systems, thermal oxidation, enclosed flares, and other engineering controls designed to reduce atmospheric emissions. Evaluation of multiple industrial emission sources provided a comprehensive characterization of cumulative community exposure resulting from long-term industrial activities.
An additional focus of the investigation involved evaluation of the environmental behavior of ethylene oxide following atmospheric release. Atmospheric science assessments examined emission source characteristics, atmospheric transport, plume dispersion, meteorological influences, terrain effects, and long-term ambient air concentrations predicted using EPA-approved air dispersion modeling methodologies. Because ethylene oxide behaves primarily as a gaseous contaminant, evaluation emphasized long-term inhalation exposure rather than accumulation within environmental media such as soil or groundwater. Integration of emissions reconstruction with atmospheric dispersion modeling provided a scientifically supported framework for estimating historical ambient EtO concentrations experienced by nearby residential communities.
Finally, the investigation recognized that long-term community exposure resulted from the combined contribution of multiple independent emission sources operating over extended periods rather than any single facility in isolation. Accordingly, the investigation integrated engineering analyses, emissions inventories, atmospheric dispersion modeling, environmental chemistry, industrial hygiene, toxicology, regulatory science, exposure assessment, and quantitative cancer risk assessment to evaluate cumulative inhalation exposure resulting from multiple medical and industrial sources. This multidisciplinary approach provides a scientifically reliable framework for evaluating airborne ethylene oxide contamination and demonstrates the importance of integrating atmospheric modeling, emissions reconstruction, and human health risk assessment when investigating long-term community exposure to hazardous air pollutants.
Methods
The investigation was conducted using accepted principles of air dispersion modeling, atmospheric science, environmental chemistry, industrial hygiene, emissions reconstruction, regulatory science, exposure assessment, toxicology, and quantitative human health risk assessment. Because ethylene oxide emissions originated from multiple facilities operating over several decades, the investigation integrated historical emissions inventories, engineering records, regulatory databases, meteorological observations, atmospheric dispersion modeling, published toxicological literature, and EPA risk assessment methodologies to reconstruct long-term ambient air concentrations and evaluate cumulative cancer risk. Rather than relying upon a single emissions inventory or modeling assumption, the investigation employed a multidisciplinary weight-of-evidence approach combining engineering analyses, atmospheric science, environmental chemistry, and quantitative risk assessment to characterize community exposure to airborne ethylene oxide.
A major component of the investigation involved reconstruction of historical ethylene oxide emissions using independently reported regulatory emissions inventories. Historical emissions data were compiled from the EPA National Emissions Inventory (NEI), engineering records, facility operating histories, state regulatory information, permitting documents, and publicly available emissions databases. Source characterization included evaluation of emission rates, stack parameters, release characteristics, source geometry, operating schedules, and historical changes in emissions over time. Where annual emissions were unavailable, scientifically supported interpolation methods were used to estimate emissions between reporting years while maintaining consistency with available regulatory data. Alternative source-term scenarios were also developed to evaluate uncertainty associated with historical emissions reporting.
Atmospheric dispersion modeling represented the principal analytical component of the investigation. Historical emissions were modeled using the EPA-preferred AERMOD air dispersion model to estimate long-term annual ambient ethylene oxide concentrations across surrounding residential communities. Model development incorporated historical meteorological observations, terrain characteristics, receptor grids, atmospheric stability, source release parameters, building influences, and regulatory modeling guidance consistent with EPA recommendations. Concentration isopleths were developed for individual emission sources and cumulative annual concentrations were estimated by combining modeled contributions from all facilities operating during the study period. The use of EPA-approved atmospheric dispersion modeling provided a scientifically accepted framework for reconstructing historical ambient EtO concentrations over multiple decades.
The investigation further incorporated comprehensive quantitative cancer risk assessment using established EPA methodologies. Modeled annual ethylene oxide concentrations were converted into lifetime excess cancer risk using EPA inhalation unit risk values together with accepted exposure assumptions and Regional Screening Level (RSL) methodologies. Because historical residence durations varied among community members, the investigation adjusted lifetime exposure estimates using time-weighted exposure durations to estimate cumulative cancer risk associated with long-term residential exposure. Risk estimates were developed for representative residential receptors as well as broader community receptor grids, allowing evaluation of both individual and community-wide cancer risks resulting from cumulative ethylene oxide emissions.
Exposure assessment also incorporated evaluation of multiple independent emissions scenarios to characterize uncertainty associated with historical emissions inventories and engineering assumptions. Alternative modeling scenarios compared emissions reported through different regulatory programs, evaluated varying source configurations, and assessed the influence of engineering assumptions on predicted ambient concentrations and cancer risk estimates. Comparison of alternative modeling approaches provided a scientifically supported sensitivity analysis demonstrating the robustness of modeled exposure estimates while identifying parameters having the greatest influence on predicted community risk. Independent comparison of multiple source inventories strengthened confidence in the overall exposure reconstruction.
Finally, the investigation employed accepted principles of uncertainty evaluation fundamental to environmental forensics and atmospheric dispersion modeling. Uncertainty associated with historical emissions reporting, source characterization, meteorological variability, facility operations, emission controls, atmospheric dispersion, exposure duration, and toxicity values was addressed through integration of multiple independent regulatory datasets, engineering documentation, atmospheric modeling scenarios, toxicological evaluations, and published scientific literature. Rather than relying upon a single emissions inventory or model output, the investigation synthesized engineering analyses, environmental chemistry, atmospheric science, industrial hygiene, exposure assessment, and quantitative risk assessment into a comprehensive multidisciplinary framework. This methodology provides a scientifically reliable basis for evaluating long-term community exposure to airborne ethylene oxide while remaining consistent with accepted principles of atmospheric dispersion modeling, environmental chemistry, industrial hygiene, environmental toxicology, regulatory science, and human health risk assessment.
Results
Review of the available engineering, regulatory, atmospheric, and toxicological documentation demonstrated that evaluating long-term community exposure to ethylene oxide (EtO) requires integration of emissions reconstruction, atmospheric dispersion modeling, environmental chemistry, industrial hygiene, toxicology, and quantitative human health risk assessment. Although emissions inventories provide important information regarding annual releases from individual facilities, no single dataset independently characterizes cumulative community exposure resulting from multiple emission sources operating over several decades. Accordingly, the investigation synthesized multiple independent lines of scientific evidence to reconstruct historical EtO emissions, estimate long-term ambient air concentrations, evaluate cumulative exposure, and quantify lifetime cancer risk using accepted principles of environmental forensics and exposure science. Collectively, these complementary sources of scientific information provided a comprehensive framework for evaluating community exposure to airborne EtO emissions.
One of the principal findings of the investigation was the strong agreement between independently reported emissions inventories, atmospheric dispersion modeling, engineering evaluations, and regulatory guidance regarding the occurrence of measurable long-term EtO exposure throughout surrounding residential communities. Historical emissions inventories documented intermittent releases from both medical sterilization facilities and industrial manufacturing operations over many years, while AERMOD simulations reconstructed the transport and dispersion of those emissions under representative meteorological conditions. Independent comparison of multiple emissions inventories and alternative modeling scenarios demonstrated that the estimated spatial distribution of EtO concentrations remained generally consistent despite differences in emissions assumptions, strengthening confidence in the overall exposure reconstruction.
The investigation further demonstrated the importance of evaluating cumulative emissions from multiple facilities rather than assessing individual emission sources independently. Atmospheric modeling estimated annual ambient EtO concentrations attributable to each facility and then combined those contributions to evaluate cumulative community exposure. This cumulative modeling approach demonstrated that relatively small emissions from several independent sources may collectively contribute to elevated long-term ambient concentrations and increased lifetime cancer risk. Evaluation of cumulative exposure therefore provided a more scientifically representative assessment of community risk than evaluation of individual facilities in isolation.
Evaluation of quantitative cancer risk also demonstrated the value of integrating atmospheric dispersion modeling with accepted EPA inhalation risk methodologies. Modeled annual EtO concentrations were converted into lifetime excess cancer risk using EPA inhalation unit risk values together with exposure-duration adjustments and Regional Screening Level (RSL) methodologies. Cancer risk estimates were developed for representative residential receptors as well as broader community receptor networks, allowing comparison of localized exposure with community-wide risk. Integration of atmospheric modeling with quantitative risk assessment strengthened interpretation of modeled concentrations by relating estimated ambient air levels directly to recognized health-based cancer risk metrics used throughout environmental regulation and public health investigations.
The investigation additionally demonstrated the importance of evaluating uncertainty through multiple independent modeling approaches. Alternative emissions inventories, engineering assumptions, source configurations, and regulatory datasets were incorporated into sensitivity analyses to evaluate their influence on predicted ambient concentrations and lifetime cancer risk. Comparison of alternative modeling scenarios showed that, although numerical risk estimates varied depending on source assumptions, the overall spatial patterns of exposure and the identification of areas receiving the highest cumulative EtO concentrations remained generally consistent. Evaluation of uncertainty using multiple scientifically defensible scenarios substantially strengthened the reliability of the overall assessment.
Another important finding involved the successful integration of atmospheric dispersion modeling, emissions reconstruction, environmental chemistry, industrial hygiene, toxicology, regulatory science, exposure assessment, and quantitative cancer risk assessment into a unified environmental forensic framework. No individual emissions inventory, engineering analysis, atmospheric model, or toxicological evaluation independently characterized the complexity of long-term community exposure to airborne EtO. However, when emissions inventories, engineering documentation, AERMOD simulations, regulatory guidance, toxicological evaluations, exposure reconstruction, and cancer risk assessment were collectively reviewed, they provided a scientifically supported assessment of cumulative community exposure and lifetime cancer risk. This multidisciplinary, weight-of-evidence approach represents a fundamental principle of modern environmental forensics and provides a technically reliable framework for evaluating complex airborne hazardous air pollutant exposures involving multiple emission sources operating over extended periods.
Conclusions
The available scientific and technical documentation provides a comprehensive foundation for evaluating long-term community exposure to ethylene oxide (EtO) using accepted principles of atmospheric dispersion modeling, environmental chemistry, industrial hygiene, emissions reconstruction, environmental toxicology, regulatory science, exposure assessment, and quantitative human health risk assessment. Although historical EtO emissions originated from numerous facilities operating over several decades, integration of engineering records, regulatory emissions inventories, atmospheric dispersion modeling, meteorological analyses, toxicological evaluations, and EPA risk assessment methodologies provides a scientifically reliable framework for reconstructing ambient air concentrations and estimating lifetime cancer risk. This multidisciplinary approach is consistent with accepted scientific practice within the fields of environmental engineering, air quality modeling, industrial hygiene, and public health.
The investigation demonstrated that evaluation of long-term EtO exposure is most scientifically reliable when emissions reconstruction, atmospheric dispersion modeling, engineering analyses, environmental chemistry, toxicology, exposure assessment, and regulatory science are considered collectively rather than independently. This weight-of-evidence approach reduces uncertainty by integrating complementary sources of scientific information and provides a technically supported basis for reconstructing historical emissions, evaluating atmospheric transport, estimating community exposure, and quantifying lifetime cancer risk. Independent agreement among emissions inventories, engineering documentation, atmospheric modeling, and regulatory methodologies substantially strengthens confidence in the overall scientific assessment while recognizing the inherent uncertainties associated with historical emissions reconstruction.
The investigation further demonstrated the importance of evaluating cumulative community exposure resulting from multiple independent emission sources rather than focusing on individual facilities in isolation. Atmospheric dispersion modeling showed that long-term ambient EtO concentrations reflected the combined influence of numerous medical sterilization and industrial emission sources operating under varying meteorological conditions over extended periods. Comprehensive investigations integrating emissions inventories, engineering analyses, atmospheric science, industrial hygiene, environmental toxicology, and quantitative risk assessment therefore provide a more technically accurate understanding of community exposure than evaluations based solely upon individual emissions inventories or isolated monitoring events. Consideration of cumulative emissions is essential for developing scientifically defensible evaluations of long-term exposure to hazardous air pollutants.
The investigation also demonstrated the value of integrating EPA-approved AERMOD atmospheric dispersion modeling with accepted cancer risk assessment methodologies. Historical emissions inventories and engineering source parameters established the basis for estimating long-term ambient EtO concentrations, while inhalation unit risk values, Regional Screening Level (RSL) methodologies, and exposure-duration adjustments translated modeled concentrations into estimates of lifetime excess cancer risk. Evaluation of alternative emissions inventories, engineering assumptions, and multiple modeling scenarios further strengthened the assessment by characterizing uncertainty and demonstrating the robustness of modeled exposure estimates. Integration of atmospheric modeling with quantitative risk assessment therefore provided a substantially more comprehensive evaluation than reliance upon emissions inventories or toxicological information alone.
More broadly, this investigation demonstrates how modern environmental consulting integrates air dispersion modeling, emissions reconstruction, atmospheric science, environmental chemistry, industrial hygiene, environmental toxicology, regulatory science, exposure assessment, and quantitative human health risk assessment to evaluate complex airborne hazardous air pollutant investigations. These multidisciplinary evaluations provide scientifically supported assessments of long-term community exposure while remaining grounded in accepted atmospheric modeling principles, engineering analyses, peer-reviewed scientific literature, regulatory guidance, emissions inventories, and objective technical evaluation. By combining emissions reconstruction, atmospheric dispersion modeling, environmental chemistry, toxicological interpretation, and quantitative cancer risk assessment, environmental forensic investigations are able to develop reliable assessments of community exposure that are consistent with recognized scientific practice and contemporary environmental health methodologies.
This investigation was conducted under the direction of Dr. Paul Rosenfeld, an environmental scientist, air dispersion modeler, environmental forensic expert, and exposure assessment specialist with more than twenty-five years of experience evaluating hazardous air pollutants, ethylene oxide, atmospheric emissions, industrial sterilization operations, chemical manufacturing facilities, environmental chemistry, industrial hygiene, environmental toxicology, quantitative cancer risk assessment, and regulatory compliance. Throughout his career, Dr. Rosenfeld has conducted investigations involving ethylene oxide, volatile organic compounds, PFAS, petroleum hydrocarbons, particulate matter, combustion byproducts, hazardous air pollutants, industrial emissions, and numerous other environmental contaminants affecting residential communities, occupational environments, and industrial facilities. His professional experience includes emissions reconstruction, EPA AERMOD modeling, exposure assessment, regulatory review, environmental site investigations, contaminant fate and transport analysis, and expert witness testimony involving complex environmental contamination, toxic tort litigation, industrial emissions, and human health risk assessment.
References
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International Agency for Research on Cancer (IARC). (2012, updated). Ethylene Oxide. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans.
National Institute for Occupational Safety and Health (NIOSH). (1977). Special Occupational Hazard Review: The Use of Ethylene Oxide as a Sterilant in Medical Facilities.
National Institute for Occupational Safety and Health (NIOSH). (1981). Current Intelligence Bulletin 35: Ethylene Oxide (EtO): Evidence of Carcinogenicity.
U.S. Environmental Protection Agency (EPA). (2024). Hazardous Air Pollutants: Ethylene Oxide (EtO).
U.S. Environmental Protection Agency (EPA). (2024). Integrated Risk Information System (IRIS): Ethylene Oxide.
U.S. Environmental Protection Agency (EPA). (2024). National Emissions Inventory (NEI).
U.S. Environmental Protection Agency (EPA). (2024). Guideline on Air Quality Models (40 CFR Part 51, Appendix W).
U.S. Environmental Protection Agency (EPA). (2024). Regional Screening Levels (RSLs) User’s Guide.
World Health Organization (WHO). Environmental Health Criteria for Ethylene Oxide.