Environmental Forensic Investigation of an Indoor Carbon Monoxide Exposure Incident
HVAC System Evaluation, Indoor Air Quality Assessment, Exposure Reconstruction, and Human Health Risk Characterization
Project Overview
Carbon monoxide releases within occupied buildings represent one of the most significant indoor air quality emergencies because the gas is colorless, odorless, tasteless, and non-irritating, allowing dangerous concentrations to accumulate without warning. When combustion equipment malfunctions or building ventilation systems inadvertently distribute combustion gases throughout occupied spaces, exposures can rapidly affect large numbers of occupants. Environmental forensic investigations of these incidents require a multidisciplinary approach that integrates engineering, industrial hygiene, environmental chemistry, exposure science, atmospheric transport, and toxicology to reconstruct the release, evaluate contaminant migration, estimate occupant exposures, and assess potential health risks (U.S. Environmental Protection Agency [EPA], 2025; Agency for Toxic Substances and Disease Registry [ATSDR], 2012).
RoseyFields conducted a comprehensive environmental forensic investigation following an acute indoor carbon monoxide exposure incident at a large institutional building served by multiple interconnected heating, ventilation, and air conditioning (HVAC) systems. Following reports of occupants experiencing symptoms consistent with carbon monoxide exposure, emergency responders initiated environmental monitoring, building evacuation, and ventilation measures to protect building occupants. The subsequent forensic investigation focused on determining how combustion gases generated within the building’s mechanical equipment migrated through the HVAC system, reconstructing the sequence of events leading to the release, identifying contaminant transport pathways, estimating carbon monoxide concentrations experienced throughout the building, and evaluating potential human exposures using established scientific and regulatory guidance (National Research Council, 2010; American Industrial Hygiene Association [AIHA], 2023).
The investigation incorporated a broad range of environmental forensic disciplines, including historical document review, evaluation of mechanical system design and operation, HVAC airflow analysis, reconstruction of indoor contaminant migration, review of emergency response records, interpretation of environmental monitoring data, indoor air exposure modeling, and comparison of estimated concentrations with health-based exposure guidelines. Rather than relying solely on measurements collected after emergency response activities began, the investigation integrated engineering analyses with exposure science to characterize conditions that likely existed before the building was evacuated. This multidisciplinary approach provided a scientifically supported reconstruction of contaminant generation, transport, and occupant exposure while demonstrating how environmental forensic investigations can help explain complex indoor air quality incidents involving combustion-related contaminants and mechanical system failures (ASHRAE, 2023; EPA, 2025).
Chemicals of Concern
Carbon monoxide (CO) was the primary chemical of concern evaluated during this environmental forensic investigation. Carbon monoxide is a colorless, odorless, tasteless gas generated during the incomplete combustion of carbon-containing fuels. Because it cannot be detected by human senses, hazardous concentrations may accumulate indoors without warning, particularly when combustion equipment malfunctions or exhaust gases enter occupied spaces through defective ventilation systems. Indoor carbon monoxide incidents have been associated with improperly functioning boilers, furnaces, water heaters, generators, and other combustion appliances, making them an important focus of environmental investigations involving indoor air quality and occupant health (ATSDR, 2012; U.S. Environmental Protection Agency [EPA], 2025).
Unlike many airborne contaminants that produce irritation before reaching hazardous concentrations, carbon monoxide exerts its toxicity by binding to hemoglobin with an affinity approximately 200 to 250 times greater than oxygen, forming carboxyhemoglobin (COHb) and reducing the blood’s oxygen-carrying capacity. As COHb concentrations increase, oxygen delivery to vital organs—including the brain and heart—is impaired, producing symptoms that may include headache, dizziness, fatigue, nausea, impaired judgment, confusion, and, at higher exposures, loss of consciousness or death. Individuals with underlying cardiovascular disease, respiratory disease, anemia, pregnant women, children, and older adults may be more susceptible to adverse health effects because of increased oxygen demands or reduced physiological reserve (ATSDR, 2012; World Health Organization [WHO], 2021).
The environmental behavior of carbon monoxide differs substantially from many hazardous air pollutants because it does not readily adsorb to building materials, soils, or dust and is not persistent within indoor environments once the source has been eliminated. Instead, indoor concentrations are largely controlled by the rate of contaminant generation, building ventilation characteristics, HVAC operation, room-to-room airflow, and air exchange with the outdoor environment. Consequently, evaluating indoor carbon monoxide incidents requires reconstruction of the timing of emissions, contaminant transport through ventilation systems, and changing concentrations throughout occupied spaces rather than relying solely on measurements collected after emergency response activities have begun. Environmental forensic investigations frequently integrate engineering analyses, airflow evaluations, and exposure modeling to estimate concentrations that may have existed before monitoring instruments were deployed (ASHRAE, 2023; AIHA, 2023).
Health-based exposure guidelines published by the U.S. Environmental Protection Agency, the National Research Council, the World Health Organization, and occupational health organizations provide important benchmarks for evaluating potential exposures. These guidelines account for both concentration and duration of exposure because carbon monoxide toxicity depends on the cumulative formation of carboxyhemoglobin over time. Accordingly, environmental forensic exposure assessments often compare reconstructed concentrations with Acute Exposure Guideline Levels (AEGLs), ambient air quality criteria, and other health-based benchmarks to evaluate the potential for adverse health effects under the conditions present during an indoor release (National Research Council, 2010; WHO, 2021; EPA, 2025).
Methods
The environmental forensic investigation employed a multidisciplinary methodology to reconstruct the indoor carbon monoxide release, evaluate contaminant migration throughout the building, estimate occupant exposures, and assess potential health risks. Rather than relying on a single line of evidence, the investigation integrated engineering analyses, industrial hygiene principles, environmental chemistry, exposure science, and indoor air quality modeling to develop a comprehensive understanding of the incident. This approach allowed independent datasets to be evaluated collectively, improving confidence in the reconstruction of conditions that existed before emergency response personnel arrived and before ventilation activities altered indoor contaminant concentrations (American Industrial Hygiene Association [AIHA], 2023; U.S. Environmental Protection Agency [EPA], 2025).
The investigation began with an extensive review of historical records, including engineering documents, building mechanical drawings, maintenance records, incident reports, emergency response documentation, environmental monitoring data, witness statements, and publicly available technical literature relevant to indoor carbon monoxide releases. Building plans and mechanical system drawings were evaluated to identify the configuration of combustion equipment, mechanical rooms, HVAC components, supply and return air pathways, and potential routes through which combustion gases could migrate into occupied portions of the building. Operational information regarding HVAC scheduling, equipment startup, and ventilation system performance was incorporated into the reconstruction of the release chronology.
A detailed engineering evaluation of the building’s heating and ventilation systems was conducted to determine how combustion gases generated within the mechanical system could have entered occupied spaces. This evaluation considered the physical layout of the mechanical equipment, air handling units, ventilation pathways, pressure relationships, and building airflow characteristics. Particular emphasis was placed on understanding how contaminants released within one portion of the building could be transported through interconnected HVAC systems into multiple occupied areas. Fundamental principles of ventilation engineering and contaminant transport were applied to evaluate how air movement influenced the distribution of carbon monoxide throughout the facility (ASHRAE, 2023).
Indoor air quality data collected during the emergency response were reviewed and interpreted within the context of the evolving incident. Because environmental monitoring typically begins after occupants report symptoms and emergency personnel arrive, measured concentrations may not represent peak exposure conditions. Accordingly, the investigation evaluated the timing of monitoring activities relative to the progression of the release, building evacuation, shutdown of combustion equipment, and subsequent ventilation efforts. This temporal analysis provided critical context for interpreting field measurements and identifying potential differences between measured concentrations and earlier exposure conditions.
Environmental forensic exposure modeling was performed to estimate indoor carbon monoxide concentrations throughout the building during the release. Modeling incorporated engineering information regarding emission timing, contaminant generation, building volume, ventilation characteristics, HVAC operation, and indoor air mixing. The reconstructed exposure scenario was compared with available field measurements to evaluate consistency between observed conditions and modeled contaminant transport. Exposure estimates were then evaluated using established toxicological benchmarks, including Acute Exposure Guideline Levels (AEGLs) and other health-based exposure criteria published by federal agencies and scientific organizations. These comparisons provided a scientifically supported framework for evaluating potential occupant exposures under varying concentration and exposure duration scenarios (National Research Council, 2010; EPA, 2025).
Finally, the findings from each component of the investigation were integrated using a weight-of-evidence approach. Engineering analyses, environmental monitoring results, building airflow evaluations, exposure modeling, and toxicological guidance were collectively evaluated to develop a scientifically defensible reconstruction of the incident. By combining multiple independent lines of evidence rather than relying on any single dataset, the investigation provided a comprehensive assessment of contaminant generation, transport, exposure potential, and the environmental factors that influenced the distribution of carbon monoxide within the building (AIHA, 2023; ATSDR, 2012).
Results
The environmental forensic investigation demonstrated that the indoor carbon monoxide release was consistent with the generation of combustion gases within a mechanical system that subsequently migrated through interconnected HVAC components, allowing contaminants to be distributed throughout multiple occupied areas of the building. The available engineering information, environmental monitoring data, and building configuration collectively supported a release mechanism in which carbon monoxide entered the ventilation system before being transported beyond the mechanical equipment room into occupied spaces. Rather than remaining confined to the point of origin, the contaminant followed normal airflow pathways established by the building’s heating and ventilation system, increasing the potential for widespread occupant exposure (ASHRAE, 2023; U.S. Environmental Protection Agency [EPA], 2025).
Review of emergency response records indicated that occupants began experiencing symptoms consistent with acute carbon monoxide exposure before the source of contamination was identified. Emergency responders subsequently documented elevated carbon monoxide concentrations in multiple portions of the building, demonstrating that the contaminant had migrated well beyond the mechanical equipment area prior to building evacuation. The spatial distribution of measured concentrations was consistent with airflow patterns predicted from the HVAC system configuration and building layout, supporting the conclusion that mechanical ventilation played a significant role in transporting carbon monoxide throughout occupied areas (Agency for Toxic Substances and Disease Registry [ATSDR], 2012).
Evaluation of the incident chronology indicated that combustion equipment likely operated for a period of time before occupants recognized symptoms and before environmental monitoring commenced. Because emergency response activities, equipment shutdown, and building ventilation began shortly after the release was identified, field measurements likely reflected conditions after contaminant concentrations had already begun to decline. Consequently, reliance solely on measured concentrations obtained during emergency response could underestimate peak occupant exposures. The environmental forensic reconstruction therefore integrated engineering analyses, operational information, and exposure modeling to estimate conditions that likely existed before mitigation measures altered indoor air quality (National Research Council, 2010; AIHA, 2023).
Exposure modeling demonstrated that contaminant concentrations varied throughout the building as a function of HVAC airflow patterns, ventilation characteristics, occupancy location, and duration of exposure. Areas located closer to the source and directly connected to affected ventilation pathways were estimated to have experienced the highest concentrations, while more distant portions of the building experienced lower but still potentially significant exposures. These findings illustrated the importance of evaluating both contaminant transport pathways and time-dependent exposure scenarios when assessing indoor air quality incidents involving mechanical ventilation systems.
Comparison of reconstructed exposure concentrations with established health-based guidance demonstrated that portions of the building likely experienced carbon monoxide levels capable of producing adverse health effects during the release. Because carbon monoxide toxicity depends upon both concentration and exposure duration, the investigation evaluated estimated exposures using Acute Exposure Guideline Levels (AEGLs) and other recognized toxicological benchmarks. The reconstructed exposure scenarios were generally consistent with the symptoms reported by building occupants and with the elevated concentrations documented during emergency response activities, providing multiple independent lines of evidence supporting the exposure reconstruction (National Research Council, 2010; WHO, 2021).
Overall, the investigation demonstrated the value of integrating engineering analyses, environmental monitoring, indoor air quality modeling, and exposure science to reconstruct complex indoor contamination events. By combining multiple sources of technical evidence, the environmental forensic evaluation provided a scientifically supported reconstruction of contaminant generation, migration, and human exposure that extended beyond what could be inferred from field measurements alone. This multidisciplinary approach illustrates how environmental forensic investigations can provide a comprehensive understanding of indoor air quality incidents involving combustion-related contaminants while supporting scientifically defensible exposure assessments and risk evaluations (EPA, 2025; AIHA, 2023).
Conclusions
Indoor carbon monoxide releases represent complex environmental forensic investigations because contaminant concentrations can change rapidly as building ventilation systems distribute combustion gases through occupied spaces. The investigation demonstrated that reconstructing these incidents requires considerably more than reviewing emergency response measurements. A comprehensive understanding of the release depended upon integrating engineering analyses, HVAC system evaluation, indoor airflow characterization, environmental monitoring data, exposure modeling, and toxicological assessment to develop a scientifically supported reconstruction of contaminant generation, migration, and occupant exposure (ASHRAE, 2023; American Industrial Hygiene Association [AIHA], 2023).
The multidisciplinary evaluation demonstrated that carbon monoxide generated within a building mechanical system was transported through interconnected HVAC components, allowing contaminants to migrate beyond the immediate source area into occupied portions of the building. Review of the building configuration, operational information, emergency response documentation, and environmental monitoring data supported the conclusion that mechanical ventilation significantly influenced the distribution of carbon monoxide and the resulting exposure patterns. The investigation further demonstrated that contaminant concentrations likely evolved over time and varied throughout the building based on ventilation pathways, occupant location, and duration of exposure.
An important finding of the investigation was that emergency response measurements alone may not fully characterize occupant exposure during rapidly developing indoor air quality incidents. Because environmental monitoring often begins after symptoms are recognized, emergency responders arrive, combustion equipment is shut down, and ventilation measures are initiated, measured concentrations may underestimate peak conditions that existed earlier during the release. Integrating exposure modeling with engineering analyses and field measurements provided a more complete and scientifically defensible reconstruction of potential exposures than any individual dataset could provide independently (National Research Council, 2010; U.S. Environmental Protection Agency [EPA], 2025).
This investigation highlights the critical role environmental forensics plays in evaluating complex indoor air quality incidents involving combustion-related contaminants. By combining expertise in environmental chemistry, industrial hygiene, mechanical engineering, exposure science, atmospheric transport, and toxicology, environmental forensic investigations can reconstruct contaminant migration pathways, estimate historical exposure conditions, and evaluate human health risks using accepted scientific methodologies. These multidisciplinary approaches provide valuable technical insight for regulatory investigations, environmental consulting, facility evaluations, and expert witness services involving indoor air quality, mechanical system failures, and environmental exposure assessment.
This investigation was conducted under the direction of Dr. Paul Rosenfeld, an environmental scientist and environmental forensic expert with extensive experience in indoor air quality investigations, carbon monoxide exposure assessment, HVAC-related contaminant migration, exposure reconstruction, human health risk assessment, and environmental litigation support. Dr. Rosenfeld’s work focuses on evaluating complex indoor environmental contamination events involving combustion byproducts, mechanical system failures, building ventilation pathways, airborne chemical exposures, and occupant health complaints in commercial, institutional, residential, and industrial settings. His expertise includes environmental forensics, industrial hygiene, air quality assessment, contaminant fate and transport, toxic tort and personal injury matters, regulatory and compliance investigations, and expert witness testimony involving acute and chronic chemical exposure scenarios. Through RoseyFields LLC, Dr. Rosenfeld applies engineering, exposure science, toxicology, and environmental risk assessment to reconstruct historical indoor air conditions, identify contaminant pathways, evaluate potential health impacts, and provide technically rigorous opinions in matters involving indoor air contamination, HVAC system failures, and hazardous exposure events.
References
Agency for Toxic Substances and Disease Registry. (2012). Toxicological profile for carbon monoxide. U.S. Department of Health and Human Services.
American Industrial Hygiene Association. (2023). Recognition, evaluation, and control of indoor air quality problems. AIHA Press.
ASHRAE. (2023). ASHRAE handbook—Fundamentals. American Society of Heating, Refrigerating and Air-Conditioning Engineers.
National Research Council. (2010). Acute exposure guideline levels for selected airborne chemicals: Volume 8. National Academies Press.
Occupational Safety and Health Administration. (2024). Carbon monoxide poisoning. U.S. Department of Labor.
Raub, J. A., Mathieu-Nolf, M., Hampson, N. B., & Thom, S. R. (2000). Carbon monoxide poisoning—A public health perspective. Toxicology, 145(1), 1–14.
U.S. Environmental Protection Agency. (2025). Integrated Science Assessment (ISA) for Carbon Monoxide (Final Report). U.S. Environmental Protection Agency.
World Health Organization. (2021). WHO global air quality guidelines: Particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide. World Health Organization.
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