Environmental Forensic Investigation of Ethylene Oxide Sterilization Emissions, Air Dispersion Modeling, and Community Cancer Risk Assessment
Project Overview
RoseyFields conducted an environmental forensic investigation involving historical ethylene oxide (EtO) emissions from a commercial medical sterilization facility and the resulting potential community and plaintiff-specific inhalation exposure. The investigation was designed to evaluate whether historical sterilization operations released EtO at levels capable of increasing long-term cancer risk to nearby residents, and to quantify that risk using a combination of source-term reconstruction, atmospheric dispersion modeling, and inhalation risk assessment. Because commercial sterilization facilities can emit EtO through chamber exhaust vents, aeration room vents, and associated process releases, retrospective evaluation of community exposure requires a technically rigorous framework that integrates facility operations, emissions estimation, air dispersion modeling, and risk characterization rather than relying solely on regulatory reporting databases or isolated environmental measurements (U.S. EPA, 2017; U.S. EPA, 2024a).
The RoseyFields analysis evaluated historical EtO emissions from a sterilization facility located within a developed residential setting and focused on both community-scale exposure and plaintiff-specific excess lifetime cancer risk. The project involved reconstructing annual EtO emissions over the relevant exposure period, evaluating the adequacy of publicly reported emissions inventories, identifying applicable control technologies and historical control practices, and applying the U.S. EPA-preferred air dispersion model AERMOD to estimate annual average ambient EtO concentrations at nearby receptors. Modeled concentrations were then integrated with U.S. EPA inhalation risk methodology to estimate excess lifetime cancer risk under multiple source-term scenarios. This multidisciplinary approach combined environmental engineering, atmospheric modeling, GIS-based source-to-receptor analysis, exposure science, and regulatory risk assessment to evaluate both the magnitude and spatial extent of potential community exposure (American Meteorological Society & U.S. EPA, 2019; ATSDR, 2022).
A central objective of the investigation was to move beyond a generic “facility emitted EtO” narrative and instead quantify the relationship between facility operations, annual EtO throughput, modeled ambient air concentrations, and plaintiff-specific cumulative exposure. To do this, the analysis incorporated facility-specific source characterization, historical EtO purchasing and usage information, source-term scaling across multiple operating years, annual average concentration modeling, and scenario-based cancer risk calculations. The resulting framework allowed RoseyFields to evaluate not only whether the facility emitted a carcinogenic hazardous air pollutant into a nearby residential area, but also whether the magnitude of those emissions and the absence of earlier controls were sufficient to elevate modeled cancer risk above EPA risk-management benchmarks. In that sense, the case required an integrated environmental forensic reconstruction of emissions, transport, exposure, and risk rather than a simple compliance review or screening-level air toxics exercise (U.S. EPA, 1989; ASTM International, 2021).
The investigation also included a standard-of-care and control-technology component relevant to the historical management of EtO emissions from sterilization operations. RoseyFields evaluated the availability and performance of EtO emission controls such as acid-water scrubbers, catalytic oxidation systems, and related treatment technologies, as well as the regulatory and scientific history establishing EtO as a hazardous air pollutant and human carcinogen. This component of the project helped place the modeled emissions and risk results in the broader context of what was technically feasible, what was known about EtO hazards during the relevant years of operation, and how the facility’s historical emission-control practices compared to the available body of engineering and regulatory guidance for commercial sterilization facilities (U.S. EPA, 1994; U.S. EPA, 2024b).
Chemicals of Concern
The principal chemical of concern in this investigation was ethylene oxide (EtO), a highly reactive, volatile cyclic ether used as a low-temperature sterilant for heat- and moisture-sensitive medical devices and healthcare products. EtO has long been used in commercial sterilization because it can diffuse through packaging materials and sterilize complex equipment without damaging sensitive plastics, electronics, or composite medical products. At the same time, EtO is also a hazardous air pollutant and a carcinogenic inhalation toxicant, making the reconstruction of historical off-site air exposure a central issue in environmental forensic evaluations involving sterilization facilities. Its toxicological and regulatory significance is heightened by the fact that community exposure is driven primarily by airborne transport, not by deposition into soil or groundwater, so the relevant analytical framework is atmospheric dispersion and inhalation risk rather than multimedia environmental sampling alone (IARC, 2012; U.S. EPA, 2016).
From an environmental fate and transport perspective, EtO is important because it is a gas under typical ambient conditions and is released to the atmosphere during sterilization and aeration operations. Depending on facility design and process configuration, EtO may be emitted through sterilization chamber exhaust vents, back vents, aeration room vents, fugitive building losses, or other process-related release points. Once released, ambient concentrations are governed by source height, release rate, stack or vent parameters, meteorological conditions, terrain, and source-to-receptor distance. Accordingly, community exposure cannot be inferred solely from the fact that EtO was used onsite; it must be reconstructed through source-term development and atmospheric modeling that accounts for the physical characteristics of the release and the surrounding environment (American Meteorological Society & U.S. EPA, 2019; U.S. EPA, 2017).
EtO is also scientifically significant because its health evaluation sits at the intersection of hazard identification, dose-response assessment, exposure assessment, and risk characterization. It has been identified over time by NIOSH, IARC, and EPA as a carcinogenic agent of concern, and EPA’s modern inhalation risk framework for EtO is based on low-dose cancer potency estimates that have become central to commercial sterilizer risk assessments. In practical terms, this means that relatively low annual average ambient concentrations can still produce non-trivial modeled lifetime cancer risk when evaluated over multi-year residential exposure periods using EPA’s inhalation unit risk approach. As a result, even where measured or modeled concentrations appear numerically small in absolute terms, their interpretation requires toxicological and regulatory context rather than a simplistic concentration-only comparison (U.S. EPA, 2016; U.S. EPA, 2024a).
In addition to EtO itself, the broader causation and exposure context for cases like this can include background ambient EtO, endogenous EtO formation, and other carcinogenic air toxics relevant to lymphoid cancer risk or cumulative inhalation exposure context. In a technically rigorous risk assessment, those background and alternative exposure pathways do not replace the facility-specific analysis, but they can be important for distinguishing incremental facility-attributable cancer risk from total environmental and endogenous dose. That distinction matters because environmental forensic and toxic tort evaluations often require both a source-specific incremental risk estimate and an understanding of how that source-specific contribution fits within broader ambient and biologic exposure conditions. Accordingly, the ACS investigation was fundamentally an EtO case, but one that also implicated background exposure context, exposure apportionment, and the proper interpretation of regulatory cancer risk metrics in a plaintiff-specific setting (ATSDR, 2022; Kirman & Hays, 2017).
Methods
RoseyFields applied a retrospective environmental forensic and risk-assessment methodology designed to quantify historical EtO emissions, estimate ambient air concentrations attributable to those emissions, and calculate plaintiff-specific and community-scale cancer risk. The analytical framework combined facility source characterization, historical emissions reconstruction, air dispersion modeling, exposure scenario development, and cancer risk calculation. Rather than treating regulatory emissions inventories as complete and definitive, the analysis evaluated multiple lines of facility-specific evidence—including historical EtO purchasing and usage records, deposition testimony regarding EtO consumption, facility source parameters, and engineering information regarding emission control practices—to construct defensible source terms for the modeling period. This approach is consistent with environmental forensic practice when reported emissions are incomplete, delayed, or otherwise insufficient to characterize historical exposure (ASTM International, 2021; U.S. EPA, 2017).
A key methodological component was reconstruction of annual EtO emissions over the relevant exposure period. In sterilization operations, most EtO introduced into the sterilization process remains unreacted and is later vented unless captured and treated by effective emission controls. Because the facility did not historically provide a complete public record of emissions for the full exposure period, RoseyFields evaluated alternative source-term approaches to estimate annual EtO releases. One scenario used historical EtO purchasing records as a surrogate for annual facility throughput and treated 99% of purchased EtO as released to the atmosphere, reflecting an upper-bound “purchasing scenario” for uncontrolled emissions. A second scenario used facility testimony regarding the quantity of EtO actually used from each tank and estimated emissions on the basis of 90% usage combined with 99% release of that used mass, creating a somewhat lower but still facility-specific “reported usage scenario.” Scenario-based source-term development is particularly important in retrospective air toxics cases because it allows the analyst to bracket likely historical emissions and assess whether risk conclusions remain robust across technically plausible emissions assumptions (U.S. EPA, 2024b; ASTM International, 2021).
The source-term reconstruction also evaluated the relationship between uncontrolled emissions and a hypothetical pollution-control scenario representing the effect of appropriate EtO controls. In this case, RoseyFields evaluated a retroactive control scenario in which modeled emission rates were reduced by 99%, consistent with the order-of-magnitude control efficiencies historically associated with established EtO control technologies such as acid-water scrubbers and certain oxidation-based systems when properly designed and operated. This scenario was not intended to suggest that those controls had been in place during the relevant years; rather, it served as a comparative analytical tool to estimate how substantially modeled exposure and risk could have been reduced if effective emission controls had been implemented earlier in the operating history. This type of counterfactual control scenario is a useful environmental forensic method because it links the engineering question of control feasibility to the exposure question of avoidable risk (U.S. EPA, 1989; U.S. EPA, 1994).
To translate annual EtO emissions into off-site ambient concentrations, RoseyFields used AERMOD, EPA’s preferred near-field air dispersion model for regulatory and site-specific air toxics applications. AERMOD estimates ground-level concentrations at discrete receptors by combining source-specific emission rates and release parameters with meteorological conditions, atmospheric stability, terrain, and receptor location. In this investigation, AERMOD was used to estimate annual average EtO concentrations at the plaintiff residence and across the surrounding community under the historical emissions scenarios described above. The model setup included facility source characterization, receptor grid development, meteorological data processing, and terrain considerations consistent with EPA modeling guidance for site-specific risk assessments (U.S. EPA, 2017; American Meteorological Society & U.S. EPA, 2019).
Source characterization for the AERMOD analysis involved development of a point-source representation for facility EtO releases, using source location and stack or vent parameters derived from facility-specific information. These parameters included source coordinates, release characteristics, and other physical inputs necessary to simulate plume rise, atmospheric dispersion, and ground-level interception at residential receptors. Because the reliability of modeled concentrations depends heavily on the quality of source inputs, RoseyFields evaluated the available engineering and facility correspondence to identify representative source locations and source-term parameters. Where a parameter was not explicitly documented, professional judgment informed by experience with sterilization facilities and similar emission systems was applied to develop a technically reasonable modeling input, an accepted practice in forensic modeling when direct historical measurements are unavailable but the underlying process and facility type are known (U.S. EPA, 2017; ASTM International, 2021).
The air-modeling analysis was designed to evaluate both plaintiff-specific exposure and community-wide cancer risk contours. Receptor locations included the plaintiff residence as well as a broader receptor field sufficient to generate community cancer-risk isopleths. Because the plaintiff’s relevant exposure period spanned multiple years rather than a single short-term release event, RoseyFields focused on annual average ambient concentrations rather than acute hourly peaks. Annual average modeling is appropriate in this context because the relevant health endpoint—excess lifetime cancer risk from chronic inhalation exposure—is driven by long-term average exposure rather than short-term excursion concentrations. The analysis also used the linear scaling properties of AERMOD to extend a representative modeled year to other years in the exposure period based on year-specific emissions estimates, allowing annual average concentrations and cumulative risk to be reconstructed over the historical operating period without rerunning an entirely new model for every single year (American Meteorological Society & U.S. EPA, 2019; U.S. EPA, 2024a).
This temporal-scaling approach is technically important because the plaintiff-specific exposure assessment depended on multi-year residential occupancy and year-to-year changes in EtO throughput. Rather than assuming emissions were constant over the entire exposure period, RoseyFields evaluated annual EtO emissions and scaled modeled receptor concentrations to reflect the relative magnitude of emissions across the relevant years. That produced a more refined estimate of long-term exposure than would have been possible using a single “steady state” emissions assumption. It also allowed the analysis to estimate cumulative plaintiff exposure under multiple source-term scenarios and to compare facility-attributable exposure during years of higher and lower EtO throughput. For environmental forensic purposes, that type of temporal differentiation can materially improve the realism of the exposure reconstruction, especially where the exposure period spans a decade or more (ATSDR, 2022; U.S. EPA, 2017).
Modeled EtO concentrations were converted to excess lifetime cancer risk using EPA-style inhalation risk assessment methods. Specifically, RoseyFields applied the inhalation unit risk (IUR) framework to convert annual average ambient EtO concentrations into plaintiff-specific and community-level cancer risk estimates under a residential exposure scenario. The general structure of the risk calculation incorporated the modeled air concentration, exposure frequency, exposure duration, averaging time, and age-adjustment considerations applicable to mutagenic carcinogens. This framework is consistent with EPA and ATSDR risk assessment practice for chronic inhalation carcinogens and provides a quantitative means of estimating the incremental lifetime cancer risk associated with long-term exposure to facility-attributable EtO concentrations (ATSDR, 2022; U.S. EPA, 2024a).
The ACS analysis specifically evaluated the plaintiff’s residential exposure duration, used annual average modeled EtO concentrations at the residence, and applied EPA risk values to estimate incremental facility-attributable excess lifetime cancer risk. Importantly, the resulting risk estimate represented the cancer risk attributable to the facility’s modeled emissions rather than the plaintiff’s total background lifetime cancer risk from all possible sources. That distinction matters in both environmental forensics and toxic tort because the purpose of the source-specific calculation is to quantify the incremental contribution of the facility under investigation, not to estimate all possible contributors to the plaintiff’s lifetime cancer burden. At the same time, RoseyFields also evaluated broader community cancer-risk isopleths to determine whether elevated modeled risk extended beyond the plaintiff residence into the surrounding neighborhood, thereby placing the plaintiff-specific result in a broader source-to-receptor context (U.S. EPA, 1989; U.S. EPA, 2024a).
Finally, RoseyFields evaluated the historical standard of care for EtO sterilization emissions in the context of available control technologies, regulatory developments, and the long-standing recognition of EtO as a carcinogenic hazardous air pollutant. This component of the work considered the availability and historical use of control technologies such as acid-water scrubbers, catalytic oxidizers, and related systems designed to reduce EtO emissions from sterilization operations. It also considered the evolution of EPA and occupational health understanding of EtO hazards and the role of risk-management thresholds such as the 1-in-1,000,000 de minimis cancer risk benchmark in interpreting modeled results. Although regulatory thresholds are not equivalent to a plaintiff-specific causation determination, they remain technically important in an environmental forensic context because they provide a risk-management benchmark against which modeled facility-attributable cancer risk can be compared (U.S. EPA, 1994; U.S. EPA, 2024b).
Results
The RoseyFields investigation concluded that retrospective air dispersion modeling and source-term reconstruction provided a scientifically defensible basis for quantifying both plaintiff-specific and community-wide exposure to historical EtO emissions from the sterilization facility. By integrating annual EtO throughput data, source characterization, AERMOD concentration estimates, and inhalation risk calculations, the analysis was able to move from general allegations of EtO exposure to a quantified estimate of facility-attributable excess lifetime cancer risk. This is a critical distinction in environmental forensic practice: rather than simply showing that EtO was used at the facility or that the plaintiff lived nearby, the analysis quantified how much EtO was likely emitted, how those emissions dispersed into the surrounding residential area, and what incremental cancer risk those modeled concentrations implied under a residential exposure scenario (American Meteorological Society & U.S. EPA, 2019; U.S. EPA, 2024a).
One of the most important findings was that the magnitude of modeled cancer risk remained elevated across multiple source-term scenarios, even when the analysis did not rely on a single emissions estimate. Under the upper-bound purchasing-based emissions scenario, the plaintiff’s modeled excess lifetime cancer risk was approximately 4.29 in one million, while under the alternative reported-usage scenario it was approximately 3.86 in one million. Although these values are numerically small in an absolute sense, they exceed EPA’s commonly referenced 1-in-1,000,000 de minimis benchmark and therefore occupy a risk range that EPA has historically treated as relevant in hazardous air pollutant risk management. The significance of these values in the case study is not that they represent a certainty of disease, but that they quantify a modeled incremental facility-attributable cancer risk above the benchmark typically used by EPA to flag potentially meaningful inhalation cancer risk from chronic air toxics exposure (U.S. EPA, 2024a; U.S. EPA, 2017).
The analysis also demonstrated that emission controls could have dramatically reduced modeled risk. When RoseyFields applied a control scenario reflecting a 99% reduction in EtO emissions, the plaintiff’s modeled excess lifetime cancer risk fell to approximately 0.04 in one million, below the de minimis benchmark. This result is important because it links three technical questions that often remain separated in EtO litigation: how much EtO was emitted, how those emissions translated into off-site ambient concentrations, and how much risk reduction would have been achieved if effective controls had been implemented earlier. In other words, the control scenario served as a form of sensitivity analysis demonstrating that the elevated risk estimates were not simply an unavoidable consequence of sterilization operations; they were strongly influenced by the historical absence of effective emissions controls during the relevant years of operation (U.S. EPA, 1994; U.S. EPA, 2024b).
The community-scale modeling similarly indicated that the plaintiff’s risk estimate did not occur in isolation. RoseyFields prepared community cancer-risk isopleths showing that modeled annual average EtO concentrations produced elevated excess lifetime cancer risk extending beyond a single residential receptor and into the surrounding neighborhood. This matters from both a technical and forensic standpoint. Technically, it shows that the modeled source-receptor relationship was not an anomalous result tied only to one point receptor; rather, the plume geometry and risk contours indicated a broader zone of residential impact consistent with the facility’s location within a developed community. Forensically, it supports the interpretation that the plaintiff’s modeled exposure arose within a larger pattern of community air toxics impact rather than a highly isolated or implausible micro-scale receptor condition (American Meteorological Society & U.S. EPA, 2019; U.S. EPA, 2017).
Another important result of the investigation was the identification of deficiencies in relying solely on public emissions inventories for retrospective exposure reconstruction. Public inventories such as the National Emissions Inventory and Toxic Release Inventory can be useful screening tools, but the ACS investigation showed why they are not always sufficient for historical plaintiff-specific air toxics reconstruction. When a facility fails to report, underreports, or reports only during a portion of the relevant exposure period, the inventory record becomes an incomplete representation of historical emissions. By contrast, the use of facility-specific purchasing records, usage testimony, and source-term scaling allowed RoseyFields to construct a more comprehensive estimate of annual EtO emissions over the relevant years. This methodological distinction is significant because it demonstrates why a robust environmental forensic analysis often requires primary facility records and engineering reconstruction, not just reliance on secondary public databases (ASTM International, 2021; U.S. EPA, 2024b).
The results also highlight the importance of distinguishing facility-attributable incremental risk from total exposure context. The plaintiff-specific cancer risk values generated in the RoseyFields analysis represented the incremental risk attributable to the sterilization facility’s modeled emissions and did not attempt to represent the plaintiff’s total lifetime cancer risk from all sources, endogenous EtO formation, or other ambient carcinogens. That distinction is scientifically important because it preserves the proper scope of the air-modeling and risk-assessment exercise: the purpose of the model was to quantify the risk increment attributable to the facility under investigation, not to collapse all environmental and biologic risk into a single undifferentiated value. At the same time, the broader causation record in cases like this may require evaluation of background EtO, other air toxics, occupational history, and alternative risk factors. RoseyFields’ contribution was to isolate and quantify the facility-specific airborne EtO component of that larger causation and exposure picture (ATSDR, 2022; Kirman & Hays, 2017).
Finally, the ACS investigation showed that retrospective AERMOD-based cancer risk assessment can be a powerful environmental forensic tool when paired with robust source-term reconstruction and engineering context. The case did not depend on a single monitoring sample, a single emissions inventory number, or a generalized statement that EtO is hazardous. Instead, it integrated source characterization, annual throughput reconstruction, multi-scenario emissions estimation, annual average concentration modeling, receptor-specific exposure estimation, and inhalation cancer-risk calculations into a unified technical analysis. That integration is what allowed RoseyFields to produce a defensible estimate of plaintiff-specific and community-wide risk attributable to the facility’s historical EtO emissions, while also evaluating the likely risk reduction that would have occurred under earlier emissions control (U.S. EPA, 2017; ASTM International, 2021).
Conclusions
This environmental forensic investigation demonstrated that retrospective evaluation of EtO sterilization emissions requires more than a generic air toxics screening analysis. It requires facility-specific emissions reconstruction, source-term development, AERMOD-based atmospheric dispersion modeling, receptor-specific exposure estimation, and inhalation cancer-risk characterization performed within a coherent scientific framework. By integrating those disciplines, RoseyFields was able to quantify the relationship between historical sterilization operations and long-term residential EtO exposure rather than simply inferring risk from proximity to a sterilization facility. That approach is consistent with accepted environmental forensic practice because it emphasizes reconstruction of actual emissions, transport, and exposure rather than reliance on incomplete secondary reporting or generalized assumptions about facility risk (ASTM International, 2021; U.S. EPA, 2017).
A principal conclusion of the investigation was that historical EtO emissions from the sterilization facility were capable of producing modeled excess lifetime cancer risk above EPA’s de minimis benchmark at the plaintiff residence and across portions of the surrounding community under multiple source-term scenarios. That conclusion remained directionally consistent whether annual emissions were estimated using a purchasing-based upper-bound scenario or a somewhat lower usage-based scenario. The importance of this finding lies not merely in the numerical risk values themselves, but in the fact that they were derived through a facility-specific, multi-year reconstruction that accounted for historical EtO throughput, source characteristics, residential exposure duration, and EPA risk methodology. In environmental forensic terms, the analysis demonstrated a quantifiable, facility-attributable incremental cancer-risk signal associated with historical uncontrolled EtO emissions (U.S. EPA, 2024a; American Meteorological Society & U.S. EPA, 2019).
The investigation also demonstrated that control technology matters materially to risk. By modeling a scenario representing the application of effective EtO emissions controls, RoseyFields showed that plaintiff-specific excess cancer risk could have been reduced by roughly two orders of magnitude to a level below the EPA de minimis benchmark. That result is analytically important because it connects the engineering question of control feasibility to the exposure and risk question of avoidable harm. In practical terms, it shows that the elevated modeled risk was not an immutable feature of EtO sterilization, but was strongly influenced by whether the facility historically operated with effective emissions controls. This is precisely the type of technical linkage that makes environmental forensic modeling useful in complex air toxics matters: it can evaluate not only what risk occurred, but how that risk changed under alternative operating and control assumptions (U.S. EPA, 1994; U.S. EPA, 2024b).
More broadly, this case study highlights the role of environmental forensic science in evaluating historical sterilization emissions, community air toxics exposure, plaintiff-specific dose reconstruction, and excess lifetime cancer risk. RoseyFields’ work integrated environmental engineering, emissions estimation, atmospheric dispersion modeling, GIS-based receptor analysis, risk assessment, and source-specific exposure reconstruction to produce a technically defensible evaluation of EtO exposure from a commercial sterilization facility. These types of analyses are increasingly important in matters involving hazardous air pollutants, community exposure to industrial emissions, and retrospective evaluation of long-term inhalation cancer risk because they provide a quantitative framework for translating facility operations into modeled human exposure and risk outcomes (ATSDR, 2022; ASTM International, 2021).
This investigation was conducted under the direction of Dr. Paul Rosenfeld, an environmental scientist and environmental forensic expert with extensive experience in ethylene oxide investigations, medical sterilization emissions, air dispersion modeling, inhalation risk assessment, hazardous air pollutant exposure reconstruction, source-term development, and toxic tort litigation support. Dr. Rosenfeld’s work includes complex analyses of historical industrial emissions, community exposure to airborne carcinogens, AERMOD sensitivity analyses, environmental risk assessment, standard-of-care evaluations, and source attribution involving hazardous air pollutants and long-term residential exposure. Through RoseyFields LLC, Dr. Rosenfeld provides environmental forensic analysis, air quality and exposure modeling, risk assessment, regulatory evaluation, remediation and due-diligence support, toxic tort consulting, and expert witness testimony in matters involving ethylene oxide, industrial air toxics, historical community exposure, and environmental health risk. His work is grounded in the integration of engineering evidence, atmospheric science, toxicology, exposure assessment, and environmental forensic methodology to evaluate whether industrial emissions have materially contributed to human exposure and elevated health risk.
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