U.S. Cities Face Escalating Threat from Compound Heat and Air Pollution Events
Urban environments across the United States are confronting an intensifying public health challenge that transcends the usual concerns about heat waves or pollution alone. Recent investigations conducted by researchers at the University of Oklahoma reveal that “compound events” — simultaneous occurrences of extreme heat and elevated air pollution — are increasing in both frequency and severity in metropolitan areas nationwide. These overlapping environmental hazards carry profound implications for urban populations, exacerbating health risks and exposing the vulnerabilities of cities already grappling with climate change impacts and air quality challenges.
Extreme heat remains the deadliest weather-related threat in the United States, accounting for more annual fatalities than any other climatic hazard, according to the National Weather Service. However, the convergence of heat waves with harmful air pollutants creates a compounded threat that goes largely underappreciated in conventional assessments. Chenghao Wang, Ph.D., who holds appointments in the School of Meteorology and the Department of Geography and Environmental Sustainability at OU, earned a NASA Early Career Investigator Award to analyze how such compound phenomena manifest in urban settings and affect public health outcomes. His Sustainable Urban Futures (SURF) Lab has published seminal research exposing the complexity and rising intensity of these joint hazards.
Compound heat and air pollution events are characterized by the concurrence of intense heat and high concentrations of pollutants such as ozone and fine particulate matter. Wang’s team’s research, published in Urban Climate, highlights that urban heat waves not only occur more frequently than in surrounding rural areas but are also longer-lasting and more intense. Interestingly, ozone levels, a key pollutant linked to respiratory distress, tend to be higher in rural locations. Yet when heat and ozone peaks align, nearly 89 percent of surveyed cities exhibit greater cumulative pollutant and heat intensities than do these rural counterparts, signaling an urban amplification of public health risks.
Building upon the ozone-focused study, further work published in Environmental Research scrutinizes a comprehensive 23-year dataset encompassing overlapping heat waves and fine particulate matter (PM₂.₅) pollution episodes. PM₂.₅, microscopic airborne particles capable of infiltrating deep into lung tissue and the bloodstream, are strongly correlated with a range of deleterious health effects, including respiratory ailments and cardiovascular disease. Under conditions where heat waves and PM₂.₅ peaks coincide, virtually all cities—approximately 98 percent—experience compound events that are more frequent and intense than individual hazards. Moreover, over half of these cities report events of extended duration, thereby magnifying exposure risks.
Spatial analyses reveal that the patterns of compound heat-PM₂.₅ episodes closely mirror the distribution of PM₂.₅ pollution itself, underscoring air pollution as the dominant driver of these synergistic events. Notably, the western United States has witnessed marked increases in days characterized by wildfire smoke-associated PM₂.₅ and concurrent heat, exasperating the frequency and severity of compound weather-pollution events. This is a critical finding given that wildfire-related particulate matter is frequently excluded from official air quality evaluations under the EPA’s Exceptional Events Rule, potentially obscuring the true scale of health risks tied to these episodes. SURF Lab researchers advocate for greater inclusion of wildfire smoke data in regulatory frameworks to more accurately reflect and address urban health hazards.
The urban heat island effect plays a pivotal role in intensifying these compound phenomena. Cities tend to exhibit elevated temperatures compared to their rural surroundings due to a confluence of anthropogenic factors—extensive impervious surfaces like asphalt and concrete, sparse vegetation, and densely packed buildings that trap and radiate heat long after sunset. This nocturnal thermal retention ensures that city dwellers endure prolonged heat stress, unlike rural residents who benefit from more rapid nighttime cooling. Such sustained high temperatures exacerbate the combined health effects when accompanied by elevated air pollution levels.
Simultaneously, urban emissions from transportation networks, industrial processes, and energy consumption contribute to heightened pollution concentrations within city boundaries. These elevated pollution levels, when combined with intensified heat, produce a hazardous interaction that amplifies risks for respiratory and cardiovascular disease, particularly among vulnerable populations such as the elderly, children, and individuals with preexisting health conditions. This intersection of environmental stressors challenges existing public health infrastructures and calls for nuanced, multi-faceted mitigation strategies.
Addressing these emerging compound threats demands targeted, place-based approaches that recognize the distinct drivers of heat and pollution in urban versus rural contexts. For cities, interventions might focus on expanding urban forestry programs, installing green roofs, and utilizing reflective building materials to reduce ambient temperatures. These strategies can mitigate the urban heat island effect and concurrently improve air quality by enhancing pollutant dispersion and absorption. Contrastingly, rural areas, less burdened by impervious surfaces, require alternative policies to manage pollutant sources and protect public health from episodic yet severe compound events.
This body of research not only deepens scientific understanding but also highlights the importance of integrating climate adaptation efforts with air quality management. Ensuring that wildfire-derived PM₂.₅ is accounted for within regulatory air quality frameworks represents a key policy recommendation. Such integration would better align environmental regulations with real-world health risks, fostering more effective protective measures for populations exposed to these dangerous compound hazards.
An integral dimension of the research process is the involvement of future scientific leaders through programs like the National Weather Center’s Research Experiences for Undergraduates (REU). This NSF-supported initiative pairs undergraduates with mentors to explore pressing issues related to meteorology, climate, and sustainability. Training emerging scholars in the complexities of compound heat and pollution events equips the next generation to develop innovative solutions that safeguard urban environments and public well-being in the face of escalating climate challenges.
The growing evidence from University of Oklahoma’s SURF Lab accentuates that compound heat and air pollution events represent a multifaceted threat demanding urgent attention. As urban populations expand and climate change drives further temperature extremes, comprehensively addressing these overlaps is key to reducing morbidity and mortality related to extreme weather and poor air quality. Policymakers, scientists, and urban planners alike must convene around adaptive, equitable strategies to navigate the intertwined challenges of future urban sustainability and health.
Subject of Research: Compound heat and air pollution events in U.S. urban environments and their increasing frequency and public health impacts.
Article Title: [Not provided]
News Publication Date: 12-Aug-2025
Web References:
- National Weather Service Hazards Statistics
- NASA Early Career Investigator award info
- SURF Lab website
- Environmental Research article DOI
- Urban Climate article DOI
- NWC REU program
- NSF-related training article
References:
- Wang et al., Urban Climate, DOI: 10.1016/j.uclim.2025.102511
- Leffel et al., Environmental Research, DOI: 10.1016/j.envres.2025.122508
Keywords: Heat waves, Extreme weather events, Air pollution, Air quality, Cities
