Air Quality

Christopher G. Nolte; J. Jason West; Michelle L. Bell; Arlene M. Fiore; Panos G. Georgopoulos; Jeremy J. Hess; Loretta J. Mickley; Susan M. O'Neill; Jeffrey R. Pierce; Robert W. Pinder; Sally Pusede; Drew T. Shindell; Sacoby M. Wilson

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Climate change can worsen air pollution, including by increasing wildfire smoke and pollen, impacting human health and hampering efforts to reach air quality goals. Air pollution disproportionately affects communities of color and low-income communities, and actions can be focused to increase equity despite climate hazards. Coordinated actions can sharply reduce greenhouse gas emissions while greatly improving air quality and health.

Good air quality is vital to human health and the environment. Ozone and fine particulate matter (PM_2.5) are air pollutants with widespread health and environmental effects that derive from emissions from a variety of natural and human-caused sources, including industry, power plants, vehicles, and agriculture. Ozone is a colorless gas that forms in the atmosphere from emissions of other compounds. At ground level, ozone is a powerful oxidant that, when inhaled, affects the respiratory and cardiovascular system, causing a wide range of health outcomes including lung damage and premature mortality.¹^,² It also damages crops and natural vegetation.¹^,³ PM_2.5 is defined as airborne particles with a diameter of 2.5 micrometers and smaller—about 30 times smaller than the width of a human hair. These small particles can be inhaled into the lungs, leading to health problems including cardiovascular disease, adverse birth outcomes, neurological disease, and increased risk of death.⁴^,⁵^,⁶^,⁷^,⁸^,⁹^,¹⁰ PM_2.5 is a complex mixture of solid and liquid substances,¹¹ including particles emitted directly from combustion and those formed in the atmosphere from gases emitted from natural and human sources. PM_2.5 also contributes to regional haze, which can impair enjoyment of scenic vistas, including in national parks.

Authors

Federal Coordinating Lead Author: Christopher G. Nolte, US Environmental Protection Agency
Chapter Lead Author: J. Jason West, University of North Carolina at Chapel Hill
Chapter Authors: Michelle L. Bell, Yale University; Arlene M. Fiore, Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; Panos G. Georgopoulos, Rutgers University; Jeremy J. Hess, University of Washington; Loretta J. Mickley, Harvard University; Susan M. O'Neill, USDA Forest Service, Pacific Northwest Research Station; Jeffrey R. Pierce, Colorado State University; Robert W. Pinder, US Environmental Protection Agency; Sally Pusede, University of Virginia; Drew T. Shindell, Duke University; Sacoby M. Wilson, University of Maryland

Contributors

Technical Contributors: William R. L. Anderegg, University of Utah; Bhargavi Chekuri, University of Colorado School of Medicine; Nina Gabrielle G. Domingo, Yale University; Isabella M. Dressel, University of Virginia; Stuart Illson, University of Washington; Jean-Francois Lamarque, National Center for Atmospheric Research; Yuhao Pan, unaffiliated; Robbie M. Parks, Columbia University; Muye Ru, Morgan Stanley; Cascade Tuholske, Montana State University
Review Editor: Neal Fann, US Environmental Protection Agency
USGCRP Coordinators: Leo Goldsmith, US Global Change Research Program / ICF; Allyza R. Lustig, US Global Change Research Program / ICF

Recommended Citation

West, J.J., C.G. Nolte, M.L. Bell, A.M. Fiore, P.G. Georgopoulos, J.J. Hess, L.J. Mickley, S.M. O'Neill, J.R. Pierce, R.W. Pinder, S. Pusede, D.T. Shindell, and S.M. Wilson, 2023: Ch. 14. Air quality. In: Fifth National Climate Assessment. Crimmins, A.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, B.C. Stewart, and T.K. Maycock, Eds. U.S. Global Change Research Program, Washington, DC, USA. https://doi.org/10.7930/NCA5.2023.CH14

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Ground-level ozone and PM_2.5 have declined in the US due to programs that lowered emissions. From 2000 to 2020, extreme ozone levels (98th percentile) declined by 18%,¹² and annual average PM_2.5 concentrations declined by 41%.¹³ Continued reductions in human-caused emissions are projected to bring still cleaner air in the US.¹⁴^,¹⁵

Despite these improvements, in 2021 nearly 102 million people lived in areas where pollution levels exceeded health-based air quality standards.¹³ Estimates of annual US deaths from exposure to ambient ozone and PM_2.5 range from about 60,000¹⁶—more deaths than from either motor vehicle accidents, kidney disease, breast cancer, or prostate cancer—to 260,000¹⁷^,¹⁸^,¹⁹ or more,²⁰ valued at $750 billion to $3 trillion (in 2022 dollars).²¹^,²² Air pollution damages to US crops are estimated at approximately $12 billion annually (in 2022 dollars).²³ The negative impacts of air pollution are not distributed equally, with communities of color and low-income communities disproportionately burdened.²⁴^,²⁵

Climate change, driven mainly by human greenhouse gas (GHG) emissions that are not harmful to breathe at typical atmospheric levels, affects air pollutant concentrations through multiple pathways (KM 14.1) including wildfire smoke (KM 14.2) and affects aeroallergens (KM 14.4), with effects on health. Air pollutants also affect climate (KM 3.1), and the main sources of air pollutants are also the main sources of GHG emissions, suggesting that there is opportunity to address climate and air quality goals simultaneously (KM 14.5). Current inequities in air pollution exposure may be alleviated or worsened by the impacts of climate change and actions to reduce GHG emissions (KM 14.3).

Climate Change Will Hamper Efforts to Improve US Air Quality

Climate change is projected to worsen air quality in many US regions , thereby harming human health and increasing premature death . Extreme heat events, which can lead to high concentrations of air pollution, are projected to increase in severity and frequency , and the risk of exposure to airborne dust and wildfire smoke will increase with warmer and drier conditions in some regions . Reducing air pollution concentrations will unequivocally help protect human health in a changing climate.

Air pollution concentrations are determined by natural and human-caused emissions and by atmospheric conditions, including temperature, humidity, and winds. Climate change is projected to worsen air quality in many regions, harming human health. Some of the largest increases in PM_2.5 and ozone exposure are expected in heat- and drought-prone regions (Figure 14.1) and in areas where vulnerable populations live (KM 14.3). For example, increasing heat and drought already contribute to more frequent wildfires and associated smoke episodes (KMs 14.2, 7.1). Severe climate change, with a US average warming of 9°–14°F, would increase annual US air pollution–related deaths by about 25,000 in 2100, relative to 2000.²⁶^,²⁷ This estimate assumes population growth but no change in emissions, including wildfire smoke. Given that wildfires and smoke PM_2.5 are projected to increase in a warmer climate (KM 14.2), this mortality rate may be an underestimate.

Climate change is expected to alter meteorology over the US in several ways that will directly degrade air quality (Figure 14.1). For example, ozone levels are higher on warm, sunny days because the chemical reactions that produce ozone speed up with temperature and sunlight. Exposure to these short-term ozone episodes has been linked to increased mortality.²⁸ Some gases that produce ozone and PM_2.5 come from soils and vegetation, and these emissions are sensitive to temperature and rainfall. Such processes typically lead to higher pollution levels during heatwaves, when exposure to PM_2.5 appears to be especially harmful.²⁹^,³⁰^,³¹^,³²

Local air pollution events are also strongly tied to large-scale weather patterns.³³^,³⁴^,³⁵^,³⁶ For example, cold fronts sweep clean air across the eastern US, clearing the air of pollution.³⁷ How climate change will affect these large-scale patterns is not well known. In the eastern US, the largest and most persistent pollution events often co-occur with extreme heat.³⁸ Air stagnation events, when weak winds provide little ventilation near the ground, promote pollution accumulation. Co-occurrences of heat and air stagnation are projected to increase with climate change.³⁹ Air pollution is also expected to worsen as the warm season lengthens, with greater pollution during the spring and autumn.⁴⁰^,⁴¹ Other meteorological changes accompanying climate change may improve air quality. For example, increasing humidity may reduce ozone through chemical reactions, while increasing precipitation may remove PM_2.5 from the atmosphere (Figure 14.1).

Methane, a key GHG that contributes to near-term warming (KM 14.5), is a source of global background ozone when it undergoes chemical oxidation in the presence of nitrogen oxides.⁴²^,⁴³ Continued growth in methane emissions from wetlands and human activities would raise background ozone levels, including in winter (KM 3.1),⁴⁴^,⁴⁵ increasing the potential for a longer ozone season that begins earlier in the spring.⁴⁶ As with ozone episodes, long-term exposure to background ozone also increases mortality.²^,⁴⁷

The response of ozone and PM_2.5 to climate change—and their associated impacts on health—will vary regionally, reflecting the net balance of several complex chemical, meteorological, and small-scale processes, which vary spatially and over time (Figure 14.1).⁴⁸^,⁴⁹^,⁵⁰ Across the Midwest and Northeast, year-round ozone is expected to increase by 2035 under a very high scenario (RCP8.5).⁵¹ In California and the Northeast, increasing temperatures under a moderate scenario (RCP4.5) would double the number of severe ozone episodes by the 2050s relative to the early 2000s,⁵² with further increases in summer average ozone in these regions by 2100.⁵³ Projecting future PM_2.5 is complicated, as different types of PM_2.5 are expected to respond differently to changing climate.⁵¹^,⁵⁴ Wildfires are expected to increase smoke PM_2.5 in the West and Alaska (KM 14.2). The rugged western topography makes it particularly susceptible to PM_2.5 increases, especially in winter when mountain valleys trap polluted air.⁵⁵ Declines in lake area in some areas of the mountainous West, driven mainly by human water use but also by changing climate, have exposed lakebeds and increased dust emissions.⁵⁶^,⁵⁷^,⁵⁸ These declines in lake area are projected to continue as temperatures rise and snowpack diminishes (KM 4.1), with further increases in dust.⁵⁹^,⁶⁰^,⁶¹ In the arid Southwest, dust concentrations are expected to double by 2100, compared to 2010, due to warmer and drier conditions (KMs 6.1, 28.3, 28.4).⁶²^,⁶³ Multiple studies agree that climate change is expected to increase PM_2.5 concentrations in the Northeast.⁴⁰^,⁴⁹^,⁶⁴

A table-like figure illustrates how climate change is projected to affect air concentrations of ozone and fine particulate matter (abbreviated PM 2.5), as described in the text and caption. Icons accompanied by descriptive text are shown for each climate-related change. Changes that are expected to increase both ozone and PM 2.5 follow: 1) wildfires: increasing wildfires will degrade air quality; 2) heatwaves: high temperatures and clear skies can increase pollution; 3) temperatures: overall, pollution concentrations will increase as temperatures rise; 4) drought: drought will decrease uptake of ozone by vegetation and increase dust PM 2.5; and 5) biogenic emissions: warmer temperatures will increase pollutant sources from vegetation and soil. Precipitation is projected to have little change on ozone but decrease PM 2.5; its description reads “higher precipitation may wash out PM 2.5.” Changes that have uncertain effects on ozone and PM 2.5 follow: 1) regional transport: transport of pollution may change, but the trends are unclear; and 2) stagnation: pollutants accumulate during stagnant periods, but trends in stagnation are uncertain. Finally, higher humidity will decrease ozone but increase PM 2.5.

Climate change will have varying effects on ozone and fine particulate matter (PM_2.5) concentrations, including through impacts on weather-sensitive emissions.

Figure 14.1. Climate change is projected to alter concentrations of two key US air pollutants, ozone and PM_2.5, through several processes. Red icons signify increased ozone and PM_2.5, and the blue icon denotes decreased PM_2.5. Plus and minus signs indicate the expected pollutant response to climate-driven changes in meteorology. Question marks and purple icons denote uncertainty in either the response or in how the meteorological process will change with climate change. Given uncertainties and regional differences in pollution responses, the magnitude of these responses is not presented. Key Messages 14.1 and 14.2 provide more detailed descriptions of the mechanisms involved. Adapted from The Royal Society 2021⁶⁵ [CC BY 4.0].

The adverse effect of climate change on the air we breathe is known as the climate penalty on air quality, in which climate change counteracts some of the benefits expected from emissions reductions.⁶⁶ Figure 14.2 illustrates how air quality can vary under different scenarios of air pollution sources and GHGs in future decades. In general, climate change is expected to worsen air quality, although the actions that policymakers and communities take today could counteract this outcome. Steeper reductions in the human-caused emissions that contribute to ozone and PM_2.5 are expected to lessen this climate penalty and limit adverse health effects.¹⁵^,⁶⁴^,⁶⁷^,⁶⁸

Two time series graphs show simulated historical (1980 to 2014) and projected (2015 to 2100) changes in fine particulate matter (abbreviated PM 2.5) and ozone, as described in the caption. The top graph shows changes in annual average PM 2.5. Y-axis values, ranging from negative 3 to positive 6, indicate change relative to 2015 to 2024 in units of micrograms per cubic meter. For the historical period, the multimodal average PM 2.5 (black line) decreases from about 3 to 0.5 micrograms per cubic meter. Projections show PM 2.5 steadily increasing to nearly 1.0 by 2100 for the SSP5-8.5 scenario (brown red line), little change for SSP3-7.0 (red), and decreases to negative 0.5 for SSP2-4.5 (yellow) and just under 0.5 for SSP1-2.6 (dark blue). The bottom graph shows changes in summer (June through August) average daily maximum 8-hour average ozone. Y-axis values, ranging from negative 10 to positive 6, indicate change relative to 2015 to 2024 in parts per billion (abbreviated ppb). For the historical period, the multimodal average shows year to year variability between about 0 in the 1980s and an average of around 1.0 more recently. Projections for SSP5-8.5 show an increase to about 2 between around 2050 and 2080, followed by a decrease back to about zero by 2100. SSP3-7.0 shows a similar pattern, increasing to about 1 before declining back to about zero. For SSP2-4.5, values decline fairly steadily to about negative 3 by 2100 and to about negative 5 for SSP1-2.6.

Reductions in human-caused emissions that contribute to ozone and fine particulate matter (PM_2.5) are expected to improve air quality in a changing climate.

Figure 14.2. Future air quality depends on both air pollution control measures and climate change. Modeled pollutant concentrations are shown averaged over the contiguous US, with the historical period in black and projections in various colors, for (a) annual average PM_2.5 and (b) summer (June–August) average daily maximum 8-hour average ozone, a metric of ozone pollution. Trends are shown relative to the 2015–2024 average value. Historical air quality improvements reflect clean air policies. Thick lines are multimodel average values. Thin lines show individual model simulations, indicating uncertainties from modeled processes and natural weather variability for each scenario. The focus on the contiguous states reflects the stronger influence from domestic emissions compared to other US regions (Alaska, Hawaiʻi and the US-Affiliated Pacific Islands, and the US Caribbean), where the balance of processes contributing to pollution and responses to climate change are expected to differ. These projections do not include the expected strong influence of climate change on wildfire smoke. Model simulations are described by Turnock et al. 2020.¹⁵ Figure credit: Massachusetts Institute of Technology. See figure metadata for additional contributors.

Increasing Wildfire Smoke Is Harming Human Health and Catalyzing New Protection Strategies

Wildfires emit gases and fine particles that are harmful to human health, contributing to premature mortality, asthma, and other health problems . Climate change is contributing to increases in the frequency and severity of wildfires, thereby worsening air quality in many regions of the contiguous US and Alaska . Although large challenges remain, new communication and mitigation measures are reducing a portion of the dangers of wildfire smoke .

Large wildfires have become more frequent in the western US in recent decades. While wildfires occur naturally, climate change and other human influences have increased their likelihood (Focus on Western Wildfires; KM 28.5; Figure A4.14).⁶⁹ Wildfires are projected to increase in many regions over the coming century (KM 27.2).⁷⁰^,⁷¹^,⁷²^,⁷³ Smoke pollutants emitted by wildfires negatively impact human health, visibility, and solar energy generation.⁷⁴^,⁷⁵ Wildland fires are the largest contributors to PM_2.5 concentrations in some parts of the western US⁷⁴^,⁷⁶^,⁷⁷ and impact air quality across the US (Figure 14.3). These concentrations could increase, particularly in the western US, by the end of the century,⁷⁸ offsetting improvements from reduced human-caused air pollutant emissions.⁷¹^,⁷⁹

Art × Climate

Lorraine Woodruff-Long
San Francisco Air Quality Fall 2020
(2020, fiber)

Artist’s statement: This quilt was made as the fires raged in Northern California from September 3 through the first rains of the season on November 8, 2020. Each four-inch square was modeled from the PurpleAir.com outdoor Air Quality Index (AQI) of San Francisco. The higher the AQI value, the greater the level of air pollution and the greater the health concern. Air quality ranged from purple/red (hazardous/unhealthy), to orange/yellow (unhealthy for sensitive groups/moderate) to green (good/satisfactory.) San Francisco’s microclimates cause variety across the city, as indicated by the varied confetti and bar colors.

View the full Art × Climate gallery.

Artworks and artists’ statements are not official Assessment products.

Wildfires emit PM_2.5 and other air pollutants, including volatile organic compounds (VOCs), nitrogen oxides (which contribute to ozone generation in plumes), and toxic gaseous and particulate species.⁷⁴^,⁷⁷ Since publication of the Fourth National Climate Assessment in 2018, studies have revealed factors influencing the smoke pollutant mixture, including the following: 1) smoke enhancements to ozone may be amplified when smoke mixes with urban pollution;⁸⁰^,⁸¹ 2) chemical reactions in plumes change the composition of smoke PM_2.5 but generally not its amount;⁸² and 3) hazardous VOC concentrations generally decrease with plume age due to chemical losses,⁷⁷ but structures burning in wildfires could emit additional toxic material, increasing health risks in the wildland–urban interface.⁷⁴^,⁸³^,⁸⁴ Finally, microbes emitted by fires and transported in smoke suggest that the region biologically affected by fires is more extensive than previously thought.⁸⁵^,⁸⁶^,⁸⁷

Human exposure to smoke pollutants is associated with mortality, asthma, and other respiratory problems, as well as worse outcomes for birth, COVID-19 infection rates (Focus on COVID-19 and Climate Change), and emotional well-being.⁸⁸^,⁸⁹^,⁹⁰^,⁹¹^,⁹²^,⁹³^,⁹⁴^,⁹⁵ Smoke exposure in the US presently contributes to 1,000–9,000 hospital and emergency department visits and 6,000–30,000 deaths annually.⁹⁶^,⁹⁷ Smoke can disproportionately impact certain racial, ethnic, occupational, and age-related subpopulations in both urban and rural areas (KM 22.2),⁷⁶^,⁹⁸^,⁹⁹^,¹⁰⁰ but the most impacted subpopulations are not consistent across studies. As future wildfire activity increases in some US regions, mortality rates and respiratory hospitalizations attributable to wildfires are also expected to increase (KM 27.5).⁷¹^,¹⁰¹

Fire is a natural part of many ecosystems. Land managers use prescribed fire to promote ecosystem health and to reduce the vulnerability to severe fires (KMs 7.3, 28.5),¹⁰² especially in a changing climate.¹⁰³^,¹⁰⁴ Indigenous communities have long used fire to steward their environments (KM 16.3).¹⁰⁵^,¹⁰⁶ Prescribed fire emissions vary greatly by region and season¹⁰⁷ but are typically much lower per acre than those from wildfires.⁷⁴ Prescribed fire activity could increase in some regions as land managers attempt to reduce the frequency, intensity, and spread of wildfires in a changing climate (KM 7.3).¹⁰³^,¹⁰⁴ Although air quality and health impacts are associated with prescribed fire smoke (KM 22.2),¹⁰⁸ well-designed prescribed fires targeted for specific locations have the potential to reduce overall smoke exposure¹⁰⁹ and health impacts of subsequent wildfires.¹¹⁰^,¹¹¹

Advances in remote sensing and improved smoke prediction systems,¹¹²^,¹¹³^,¹¹⁴^,¹¹⁵^,¹¹⁶ combined with better communications strategies,¹¹⁷ are helping protect the public from unhealthy smoke conditions (Figure 14.3). Smoke exposure reduction techniques, including masks and portable air filters, can help people limit the amount of PM_2.5 that is inhaled during a smoke event,¹¹⁷^,¹¹⁸^,¹¹⁹^,¹²⁰^,¹²¹ as well as pollen and other particulate air pollution. Smoke forecasters synthesize modeled, satellite, and monitoring data to create daily forecasts¹²² that reach the general public, including underserved communities—for example, through Spanish translations. Communication of these forecasts and techniques to reduce smoke exposure occurs through interagency federal,¹¹⁷^,¹²³ state, and Tribal programs, as well as social media. However, people tend to take protective actions, such as staying indoors and using air filters, in response to symptoms from exposure rather than take preventive measures.¹²⁴ More work would be needed to quantify and communicate the benefits of exposure-reduction actions.¹²⁵^,¹²⁶

A screenshot from the website tool described in the caption shows air quality index (abbreviated AQI) levels and other information for September 13, 2020 across most of the US. Circles, triangles, and squares show the location of permanent, temporary, and low-cost particulate matter sensors, respectively, with the color of these symbols indicating AQI: green for good, yellow for moderate, orange for unhealthy for sensitive groups, red for unhealthy, purple for very unhealthy, and brown for hazardous. Gray circles indicate no data. AQI values ranging from moderate to unhealthy for sensitive groups to hazardous are shown across much of the Northwest and Southwest and in the western portions of the Northern Great Plains. Moderate AQI values appear in isolated portions of the remainder of the contiguous United States (abbreviated CONUS), predominantly areas within smoke plume extents (gray shading) that cover much of the northern part of the CONUS and a band extending from the Southwest upward across the central part of CONUS toward the Northeast. Good AQI is shown across most of the eastern half of CONUS, as well as Hawai’i and Alaska. Active fires appear in locations in Washington, Oregon, Idaho, California, Montana, Wyoming, South Dakota, Utah, Arizona, Texas, Florida, and Alaska. Satellite fire detection symbols appear in locations scattered across CONUS and Alaska.

Wildfire smoke affects air quality across the country.

Figure 14.3. Wildfire smoke can affect the daily lives of people across the country, as communicated in real time to the public on September 13, 2020, on the AirNow Fire and Smoke Map (https://fire.airnow.gov/). Monitors measuring particulate matter are color-coded by air quality index from green for good air quality to brown for hazardous. Here, unhealthy to hazardous air quality conditions are shown at multiple monitors (circle, triangle, and square icons) across the western US, and satellite imagery (gray) shows smoke extending across much of North America. On this day, the US Caribbean was free of smoke, and monitor or sensor data were not yet available, so the region is not shown. Data are not available for US-affiliated Pacific Islands. Adapted from EPA 2022.¹²⁷ Base map: Copyright © 2022 Esri and its licensors. All rights reserved.

Air Pollution Is Often Worse in Communities of Color and Low-Income Communities

Communities of color, people with low socioeconomic status, and other marginalized populations are disproportionately harmed by poor air quality . In the coming decades, these same communities will, on average, face worsened cumulative air pollution burdens from climate change–driven hazards . Decision-making focused on the fair distribution of air quality improvements, rather than on overall emissions reductions alone, is critical for reducing air pollution inequities .

Air pollution disproportionately affects people of color and people with low socioeconomic status in both cities and rural places.¹²⁸^,¹²⁹^,¹³⁰^,¹³¹ While air quality has improved over recent decades, air pollution disparities have persisted.¹³²^,¹³³^,¹³⁴^,¹³⁵^,¹³⁶^,¹³⁷ There is a clear pattern of more air pollution sources being located in communities of color and low-income neighborhoods. Diesel traffic exhaust is among the largest sources of air pollution inequalities in urban areas,¹³⁸ while other emitters, including industrial facilities,²⁵^,¹³⁹ prescribed agricultural burns,¹⁴⁰ concentrated animal feeding operations,¹⁴¹^,¹⁴²^,¹⁴³^,¹⁴⁴^,¹⁴⁵ power generation,¹⁴⁶ and oil and gas infrastructure,¹⁴⁷^,¹⁴⁸ contribute to air pollution disparities in cities and rural environments. Racism in historical practices and policies has contributed to ongoing inequities, protecting White areas from pollution and disinvesting in and off-loading those costs onto communities of color, for example, through redlining and housing segregation.¹⁴⁹^,¹⁵⁰^,¹⁵¹

The health impacts of the unequal distribution of air pollution are magnified by factors including reduced access to nutrition, social and institutional support, and healthcare, as well as psychosocial stress from racism and poverty.¹⁵² As a result, a given level of air pollution can cause more harm to people of color and those with lower socioeconomic status.³⁰^,¹⁵²^,¹⁵³^,¹⁵⁴ Environmental inequalities often overlap, such as exposure to both poor air quality and higher-than-average urban heat (KM 21.3).¹⁵⁵^,¹⁵⁶ Exposure to air pollution and high air temperatures in combination can worsen health outcomes.²⁹^,³⁰^,¹⁵⁷^,¹⁵⁸ Environmental inequalities also often compound in ways that exacerbate negative impacts; for example, reduced tree cover, common in urban communities of color,¹⁵⁹ intensifies urban heat (KM 12.2) and affects air quality (KM 14.1). Disparities in air-conditioning access¹⁶⁰^,¹⁶¹ and other housing differences may increase infiltration of outdoor air pollution and wildfire smoke into homes and schools in communities of color and lower-income neighborhoods,¹⁶² and low-income households may have less ability to adopt in-home air filtration.

A 3.6°F (2°C) increase in average global temperatures relative to the 1986–2005 average is projected to worsen PM_2.5-related premature mortality for African Americans over age 65 by 40%–60% more than for people of other racial and ethnic groups.¹⁵⁵ This same temperature change is projected to cause substantially higher rates of PM_2.5-related asthma for African American children and smaller, but still disproportionate, increased rates for Latino, Asian, Pacific Islander, and American Indian and Alaska Native children. In New York City and Newark, New Jersey, projected trends in air stagnation are expected to worsen inequalities in concentrations of nitrogen dioxide (NO₂),¹⁶³ an air pollutant associated with asthma.¹⁶⁴^,¹⁶⁵ The impact of climate change on air quality–related inequalities may differ depending on the sources of pollution and whether pollutants are emitted directly or formed through chemistry (KM 14.1). However, climate change can increase cumulative and unequal air quality–related health burdens, such as from the combined effects of air pollution and temperature, even if air pollution itself does not worsen.²⁹^,³⁰^,¹⁵⁷^,¹⁵⁸

Actions to address climate change through GHG regulation will also affect air quality, with the distribution of benefits dependent on the mitigation approach. Programs focusing on GHG sources with the lowest mitigation costs have had mixed impacts on air pollution equity.¹⁶⁶^,¹⁶⁷ In California, GHG regulation through carbon cap-and-trade increased emissions of combustion-related air pollutants in communities of color and low-income neighborhoods.¹⁶⁸ Approaches focused on lowering aggregate emissions across a large geographic region, or from a single emissions category, have been shown to be less effective than interventions aimed at reducing air pollution inequalities for a specific location.¹⁶⁹ Solutions can be designed to reduce disparities and overcome the challenges associated with GHG regulation.¹⁷⁰^,¹⁷¹

Box 14.1. Environmental Justice, Air Pollution, and Climate Change: Houston, Texas

Houston’s Ship Channel region is a patchwork of chemical refineries, freeways, homes, and playgrounds (Figure 14.4; Box 26.1). Air pollution levels along this busy industrial waterway, connecting downtown Houston to Galveston Bay, are among the highest in the city (Figure 14.5). Flares and odors are commonplace,¹⁷²^,¹⁷³^,¹⁷⁴ and community concerns about health impacts are often ignored. Many of Houston’s African American, Latino, and working-class families live in the neighborhoods of the Ship Channel, where they are more likely to breathe harmful cancer-causing air pollution from diesel trucks and refineries.¹³⁸^,¹⁷⁵^,¹⁷⁶^,¹⁷⁷^,¹⁷⁸^,¹⁷⁹^,¹⁸⁰ Communities living at the fenceline of the petrochemical industry face ongoing vulnerabilities, such as dual exposure to air pollution and heat and endangerment from damages to petrochemical facilities caused by stronger hurricanes (KMs 9.2, 15.2). In 2017, Hurricane Harvey triggered widespread industrial releases of hazardous air pollutants throughout the Houston Ship Channel.¹⁸¹^,¹⁸²^,¹⁸³ Houston is also the stage for foundational scholarship on environmental justice by Dr. Robert Bullard (KM 20.3), where community organizations lead work to reduce air pollution and make communities more resilient to climate change.

Industries expose people living near the Ship Channel—often African American, Latino, and low-income residents—to harmful air pollution.

Figure 14.4. Nighttime industrial flaring exposes residents to air pollution near the Houston Ship Channel in the Deepwater community in Pasadena, Texas, a primarily African American, Latino, and low-income neighborhood. Photo credit: ©Cassandra Casados-Klein, Air Alliance Houston.

Four maps of Houston, Texas, show the racial and ethnic makeup of the city, as well as the inequitable distribution of air pollution, cancer risk, and higher summer temperatures. Panel (a), far left, shows the largest racial or ethnic groups in the city’s neighborhoods: African American (blue), Latino (green), Asian (orange); each color is shown as light, medium, or dark, corresponding to 40 to 50%, 50 to 60%, or 60 to 80% of the population. A black box at center right of the map shows neighborhoods surrounding the Ship Channel, many of which are 60 to 70% Latino. Neighborhoods to the left and above the box are predominantly Latino as well, while those to the left and below are predominantly African American. Asian neighborhoods are primarily in the bottom left quadrant of the map. Map (b), center left, shows nitrogen dioxide (abbreviated N O 2) concentrations from 2 (dark blue) to 4 (dark brown) times 10 to the 15th power molecules per square centimeter. The highest concentrations of N O 2 are in the Latino neighborhoods around the Ship Channel, with slightly lower concentrations in the nearby Latino and African American neighborhoods, and concentrations dropping to around 2 in the outer areas of the area shown. Panel (c), center right, shows lifetime cancer risk associated with air pollution ranging from 20 (purple) to 60 (dark orange) cases per million people exposed. Highest risk is on the right side of the Ship Channel area, an area that is 40 to 60% Latino. Panel (d), far right, shows average summer air temperature from 83 (dark blue) to 84.5 (dark red) degrees Fahrenheit. Temperatures are highest in the predominantly Latino Ship Channel neighborhoods, as well as in the neighborhoods immediately above, below, and to the west, many of which are also predominantly Latino.

Air pollution, its health impacts, and temperatures are unequally distributed across Houston, Texas.

Figure 14.5. Air quality and temperatures vary across Houston, Texas (urbanized area outlined in black). (a) For each neighborhood, the largest racial or ethnic group is shown: African American (blue), Latino (green), and Asian (orange). Higher-than-average levels of (b) nitrogen dioxide (NO₂; in 2019), (c) lifetime cancer risks associated with chronic air pollution exposure per million equally exposed people (2018), and (d) summer (June–August) air temperatures (2020) are found in neighborhoods that are primarily African American and Latino, especially those surrounding the Ship Channel (black box). There is variability in time and at very fine spatial scales that may not be captured here. Figure credit: University of Virginia, Columbia University, and Montana State University.

Climate Change Is Worsening Pollen Exposures and Adversely Impacting Health

Increased allergen exposure damages the health of people who suffer from allergies, asthma, and chronic obstructive pulmonary disease (COPD) . Human-caused climate change has already caused some regions to experience longer pollen seasons and higher pollen concentrations , and these trends are expected to continue as climate changes . Increasing access to allergists, improved diagnosis and disease management, and allergy early warning systems may counteract the health impacts of increasing pollen exposure .

Allergic airway disease, including allergic rhinitis and asthma, is widespread in the US, is becoming more prevalent, and imposes a burden of several billion dollars in healthcare costs and lost productivity annually.¹⁸⁴ Exposure to allergenic pollens and molds (aeroallergens) triggers allergic disease development.¹⁸⁵^,¹⁸⁶^,¹⁸⁷ Co-exposure to aeroallergens and pollutants like ozone, nitrogen oxides, and PM_2.5 can exacerbate allergic airway disease symptoms.¹⁸⁸^,¹⁸⁹^,¹⁹⁰ Aeroallergen exposure can compromise the body’s antiviral defenses, possibly increasing susceptibility to respiratory viral infections in both allergic and nonallergic people.¹⁸⁶^,¹⁹¹ It is also probable that pollen exposure is associated with COPD mortality.¹⁹² Pollen can also transport viruses.¹⁹³

Local climate affects emissions of allergenic tree and grass pollens and fungal spores. Climate change is altering pollen season characteristics for allergen-producing trees during spring and for grasses and weeds during summer and fall.¹⁹⁴ Rising atmospheric carbon dioxide (CO₂) can increase pollen allergenicity.¹⁹⁵^,¹⁹⁶^,¹⁹⁷

Multiple US regions have experienced longer, more intense pollen seasons, with earlier start dates and increased emissions and airborne loads over the past 30 years, increasing the potential for exposures (Figure 14.6; KM 22.2).¹⁸⁷^,¹⁹⁴^,¹⁹⁶^,¹⁹⁸^,¹⁹⁹^,²⁰⁰^,²⁰¹ For example, the season for ragweed pollen, a significant allergen, has lengthened since the 1990s (Figure A4.13), and its range has expanded northward;²⁰² ragweed grows faster, flowers earlier, and produces more pollen in high-CO₂ areas.¹⁹⁶^,²⁰³ With climate change, ragweed pollen is projected to increase in most regions (Figure 14.6) and to co-occur with high ozone more frequently.²⁰⁴^,²⁰⁵ Likewise, the number of days with total pollen concentrations exceeding thresholds for triggering allergies is projected to increase in most US regions.²⁰⁴^,²⁰⁶^,²⁰⁷^,²⁰⁸

Increasing frequency and intensity of heatwaves, storms, and floods associated with climate change can also intensify aeroallergen exposures. Mold proliferation is increased by floods. Thunderstorms can exacerbate respiratory allergy and asthma in patients with hay fever, and similar phenomena have been observed for molds.²⁰⁹

At left: a map of the continental United States shows observed trends in annual total pollen from 1990 to 2018 as described in the caption. Colored circles at various locations in the contiguous US (abbreviated CONUS), plus one location in Alaska and two in Canada, show the percent change per year, with the legend ranging from negative 25 (dark purple) to positive 25 (brownish red). Larger circles indicate stations with more years of data. Stations with longer records and the largest increases are in Texas, Oklahoma, Kansas, Nebraska, Missouri, Canada, and Alaska. Other increases are shown in the Southeast and in Mid-Atlantic states. Other stations in the Southeast show a decrease, as do some stations in the Northeast and California, and single stations in Oklahoma, Colorado, and Utah. At right: a map of CONUS shows projected changes in ragweed pollen concentrations in 2047 compared to 2004. A legend shows percent concentration change per year from negative 1.5 percent (dark purple) to greater than 3 percent (dark brown). The largest increases in ragweed pollen concentrations are projected for the coastal Northwest, southeast California, southwest Arizona, southern Vermont and New Hampshire, and the Appalachians from northwest Georgia to central Virginia. The largest decreases are projected for eastern Texas, central Mississippi, western New York and Pennsylvania, and across most of the Midwest region except for portions of Wisconsin and Michigan. The Northern Great Plains show a mix of small increases and decreases.

Pollen has been increasing in many US regions and is projected to continue to increase as climate changes.

Figure 14.6. (a) Observed long-term pollen increases are shown as the linear trend of total annual pollen at 60 stations (1990–2018). (b) Modeled projected changes in average airborne ragweed pollen concentrations in 2047, relative to 2004, are shown for climate change conditions under a very high scenario (RCP8.5). Yellow and red shades indicate increases in pollen concentrations, and circle size in panel (a) reflects the number of years of data at each station. Observations are not available for many US states and affiliated territories, and the modeled projection does not include non-contiguous US states and territories. There is a net increase in concentration overall, with marked increases in certain areas and declines in others. (a) Adapted from Anderegg et al. 2021¹⁹⁴ [CC BY 4.0]; (b) adapted from Ren et al. 2022²¹⁰ [CC BY 4.0].

Allergic airway disease is underdiagnosed, and many therapies are underutilized.²¹¹ Increasing access to allergists and diagnostic tests can help clarify what exposures drive allergies for individuals and aid in developing therapeutic plans including medical and immune therapies.²¹² Staying indoors and wearing masks to reduce exposure, as well as avoidance of allergens through early warning systems²¹³ and other public health campaigns, can also reduce impacts.²¹⁴ Understanding of climatic influences on pollen exposures can inform diagnosis and disease management, but it remains unclear whether these and other advances can blunt the health impact of increased aeroallergen exposures as the climate warms.

Policies Can Reduce Greenhouse Gas Emissions and Improve Air Quality Simultaneously

Substantial reductions in economy-wide greenhouse gas emissions would result in improved air quality and significant public health benefits . For many actions, these benefits exceed the cost of greenhouse gas emission controls . Through coordinated actions emphasizing reduced fossil fuel use, improved energy efficiency, and reductions in short-lived climate pollutants, the US has an opportunity to greatly improve air quality while substantially reducing its climate impact, approaching net-zero CO₂ emissions .

Fossil fuel energy use is responsible for 92.1% of US CO₂ emissions²¹⁵ and the majority of PM_2.5-induced deaths.²⁰^,²¹⁶ Consequently, actions to control GHGs, including reductions in energy demand or shifts toward cleaner energy sources, typically reduce air pollutant emissions from the same sources, benefiting air quality and health.

By contrast, actions that have substantially improved US air quality since 1990 generally did not reduce GHG emissions, as they focused on technologies that remove air pollutant emissions from power plants, industrial facilities, and vehicles but do not reduce fossil fuel consumption—and some actions increased fossil fuel use and GHG emissions (Figure 14.7).²¹⁵^,²¹⁷^,²¹⁸ In the past decade, fuel switching from coal toward renewables (wind and solar) and lower-emitting sources (fossil gas) has reduced emissions of both GHGs and air pollutants.²¹⁹^,²²⁰

To further improve air quality, more stringent smokestack and tailpipe controls on fossil fuel sources may be chosen. Alternatively, GHG mitigation scenarios that meet the long-term temperature goal of the Paris Agreement and approach net-zero emissions this century replace fossil fuels with cleaner energy sources and reduce overall energy use (Figure 14.7; KM 32.2).²²¹^,²²²^,²²³ This clean energy transition would provide air quality²²⁴ and health benefits²²⁵ beyond what smokestack and tailpipe controls can provide.

A grid with four quadrants shows the potential for emissions reduction actions to achieve air quality and climate benefits, as described in the caption. Items to the left of the vertical axis are detrimental to the climate, while those to the right are beneficial. Items above the horizontal axis have air quality health benefits, while those below have health detriments. Distance along the vertical and horizontal axes indicate magnitude of benefits or detriments. Items shaded in blue are traditional air-quality-focused actions, as follows: controls on motor vehicle tailpipes and power plant smokestacks have air quality benefits but are mostly detrimental to the climate, while switching from coal to fossil gas has air quality benefits and climate benefits and detriments. Items shaded in orange are actions targeted at short-lived climate pollutants, as follows: air-quality actions targeting black carbon have air quality benefits and mostly climate benefits but some detriments; plant-based diets and methane reductions both have relatively large climate benefits as well as air quality benefits. Items in white boxes outlined in gray mostly fall into the quadrant indicating both climate and air quality benefits, as follows: renewable and nuclear energy and electric vehicles, with large benefits, and energy efficiency, decarbonization of industrial processes, building electrification, fire reduction, and agricultural nitrogen management with progressively smaller benefits, respectively. One item—increased biomass for energy—has climate benefits but air quality detriments. Carbon capture and storage has climate benefits but has both benefits and detriments for air quality. Only one item—transportation biofuel—is situated in the center of the diagram straddling all four quadrants, with both benefits and detriments for both climate and air quality.

Many emissions-reduction actions can achieve multiple benefits for climate, air quality, and health.

Figure 14.7. Environmental policies to mitigate emissions will affect both air quality and climate change, and actions can be coordinated to address both problems simultaneously. Blue boxes show mitigation actions aimed at conventional air pollution controls; orange boxes show actions targeted at short-lived climate pollutants; and white boxes show other types of actions. Emissions-reduction actions in the upper right have greater air quality and climate benefits. Box position indicates the relative potential of actions, from most detrimental to most beneficial, and should not be interpreted quantitatively (e.g., that one action has twice the potential of another). The size of the boxes indicates some uncertainty, with actions in boxes straddling an axis being uncertain in the direction of the effect. Addressing climate change requires moving to the actions on the right-hand side of the figure, where many options simultaneously improve air quality. Figure credit: EPA, University of North Carolina at Chapel Hill, and Duke University.

Economy-wide GHG reductions are expected to decrease emissions of air pollutants emitted from the same sources, resulting in benefits for air quality and health (KMs 13.3, 32.4).²²⁶^,²²⁷^,²²⁸^,²²⁹^,²³⁰ Each metric ton of CO₂ reduced is estimated to bring about health benefits²³¹ that are valued in 26 US studies from $8 to $430 (in 2022 dollars), with a median of $100 per ton of CO₂ (see Traceable Accounts for details on relevant studies), mainly from avoided premature death. These health benefits can significantly offset or exceed implementation costs for many GHG mitigation measures (Figure 14.8). Since health benefits exceed costs in most studies, these GHG reductions are economically beneficial, even without accounting for other benefits of slowing climate change. Estimates of these benefits vary across many studies because of differences in mitigation actions considered, methods of assessing emissions, pollutant concentrations and health impacts, and mortality valuation.²³² Most studies have typically evaluated mortality while neglecting morbidity impacts, such as preterm births, restricted activity days, and hospitalizations,²³³ and therefore may underestimate the full health benefits of GHG reductions. However, some individual actions, including biomass energy and carbon capture and storage, may provide small air quality benefits or even worsen air quality (Figure 14.7; KM 5.3).²³⁴ Lastly, GHG mitigation policies may alleviate or worsen inequities in air pollution exposure, depending on their design (KMs 14.3, 32.4).

Air quality health benefits alone exceed or significantly offset the costs of greenhouse gas reductions.

Figure 14.8. Controls on greenhouse gas (GHG) emissions also reduce air pollutant emissions from the same sources (often fossil fuel combustion), improving air quality and saving lives. Each circle denotes the results from a study in the US during 2013–2022. These studies find that the value of health benefits significantly offset or in most cases exceed the GHG emissions control costs, apart from other benefits of slowing climate change. Figure credit: EPA, University of North Carolina at Chapel Hill, and Duke University.

The air quality benefits of GHG controls by reducing co-emitted air pollutants occur mainly locally and regionally and nearly immediately following emissions reductions.¹⁹^,²³⁵ By contrast, benefits of slowing climate change, including lessening the impacts of climate change on air quality (KM 14.1), are long term and distributed globally. Recognizing these air quality health benefits strengthens incentives for local, state, and national actions to reduce GHG emissions.²³⁶

Indoor air quality can also be affected by GHG reduction actions, as some methods for improving building energy efficiency decrease ventilation, which can increase mold and degrade indoor air quality.²³⁷ Newer approaches to building design improve energy efficiency while meeting temperature control and indoor air quality needs.²³⁸ More widespread application of these approaches can reduce energy use, mitigate GHG emissions, and improve indoor air quality (KM 12.3).

Climate mitigation actions focused on short-lived climate pollutants (SLCPs) can also improve local air quality. Reducing SLCPs, including methane, black carbon, and ozone, directly improves air quality and reduces the near-term rate of warming, affecting climate more quickly than reductions in long-lived GHGs like CO₂.²³⁹^,²⁴⁰ Methane directly contributes to warming and increases ozone air pollution globally.⁴²^,²⁴¹ The social cost of methane is estimated at around $2,200 (in 2022 dollars) per metric ton²⁴² when accounting for impacts via climate change. Other estimates that also include health impacts of ozone are higher (about $4,600 to $9,200 per metric ton in 2022 dollars), with over half of that from ozone health impacts.²⁴³^,²⁴⁴^,²⁴⁵ VOCs and carbon monoxide (CO) form ozone in the atmosphere, and reducing their emissions benefits both climate and air quality. Nitrogen oxides also contribute to ozone but have a net cooling influence by shortening methane’s lifetime and forming PM_2.5.²⁴⁰^,²⁴⁶ Together, global emissions of methane, VOCs, CO, and black carbon have contributed about 1.5°F to global average warming in 2019, compared to about 1.4°F from CO₂ increases (KM 3.1).²⁴⁷

Art × Climate

Lindsy Halleckson
Sky Parameters: Carbon Monoxide
(2019, acrylic on canvas)

Artist’s statement: My minimal paintings reference sky and weather. I find inspiration at the edge of day. The quiet, liminal, and changing space is full of possibility. Most recently, my work has been infused with atmospheric data. This piece depicts a subset of air quality data from Hennepin County, Minnesota, where I live and work. Using data from the 2014 State and County Emissions Sources published by the EPA, this piece portrays sources of Carbon Monoxide emissions, which are roughly: Mobile 89% (blue); Miscellaneous 6% (white); Fuel Combustion 5% (ocher).

View the full Art × Climate gallery.

Artworks and artists’ statements are not official Assessment products.

Most forms of PM_2.5 cool the climate, and removing them exacerbates climate warming (KMs 2.1, 3.1), as seen from historical sulfur dioxide reductions to improve air quality.²⁴⁸^,²⁴⁹^,²⁵⁰^,²⁵¹ If PM_2.5 reductions are undertaken together with CO₂ and SLCP reductions, this short-term warming may be outweighed, leading to a net cooling.²⁵²^,²⁵³ Carbon particles, mostly from fires and burning fossil fuels, cause a mix of warming and cooling effects.²⁴⁰ Of these, black carbon is the component that contributes most to warming, and actions targeting sources that emit relatively more black carbon, like diesel engines, are expected to best reduce warming while improving air quality. Ammonia, which contributes to PM_2.5 and is growing in relative importance as a PM_2.5 source, comes mostly from agriculture.²⁵⁴ Agricultural ammonia and methane emissions can be reduced by more efficient use of fertilizer²⁵⁵^,²⁵⁶ and adopting healthier plant-based diets.²⁴⁴^,²⁵⁷ Finally, air pollutants can influence regional climate such as through changes in clouds and precipitation, and black carbon can increase snowmelt, which affects water resources (KM 4.1).²⁵⁸

Process Description

Authors were selected to provide diversity in topical focus areas and to align expertise with the anticipated topics for the chapter, as well as for geographic and racial diversity. All authors are recognized experts in climate change and air quality, including in the focus areas of the chapter.

The author team met online roughly every two weeks to discuss the organization of topics, main points to emphasize, and the many logistical questions related to writing the chapter. The author team agreed on five key topics as the focus of the chapter, reflected in the Zero Order Draft (ZOD). The ZOD was made publicly available, and a public engagement workshop was held on January 18, 2022, where the author team gathered public comments on the ZOD. All written public comments on the ZOD were reviewed by the author team, and responses were provided for each. Similarly, the author team responded to comments received on multiple drafts that followed.

Key Messages were developed by small author teams, who were responsible for developing the content of each topic area, and discussed among all authors. The team achieved consensus on the wording of the Key Messages for the Third Order Draft through group meetings to discuss this text specifically. Following comments on drafts of the Fourth Order Draft, the team made small revisions to the Key Messages, and these were discussed among authors to again achieve consensus.

KEY MESSAGES

KEY MESSAGE 14.1

Climate Change Will Hamper Efforts to Improve US Air Quality

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Description of Evidence Base

An extensive literature base documents air quality modeling of the response of ozone and fine particulate matter (PM_2.5) to future climate change. Comparison across studies, however, is challenging due to the use of different scenarios, time periods, metrics, and process representations in the modeling systems. The chemistry of both ozone and PM_2.5 is complex, which adds to the difficulty of predicting the influence of climate change on air quality. Source gases of ozone and PM_2.5 include methane, carbon monoxide, nitrogen oxides, non-methane volatile organic compounds, sulfur dioxide, ammonia, and dimethyl sulfide; types of PM_2.5 directly emitted into the atmosphere include black carbon, organic carbon, mineral dust, sea salt, pollen, and spores.

The literature using observations to infer process-level relationships between air pollutants and climate is growing and includes links with temperature, precipitation, winds, and near-surface mixing.³⁹^,²⁵⁹^,²⁶⁰ However, observational records are relatively short (a few decades at best), and isolating responses to meteorology requires disentangling air pollution responses to large emissions perturbations over the observing period to reveal the influence of climate change and variability. Air pollution trends in recent decades in some urban areas and at the regional scale are well established based on high-quality monitoring.¹³^,²⁶¹ A large literature base employs a wide range of methods to attribute observed trends and variability to anthropogenic emissions versus meteorological variability. Highly resolved spatial distributions needed to assess community-level exposure are sparse but growing, and new observations from satellites and low-cost sensors will prove useful in this regard. For example, the Tropospheric Emissions: Monitoring of Pollution (TEMPO) satellite instrument, launched in April 2023, promises to provide hourly, fine-spatial information about US pollution.²⁶²^,²⁶³

Many processes involving interactions between climate and air quality have been the foci of major lab, field, and modeling efforts (e.g., wildfires) or represent fundamental physics (e.g., the increase in water vapor as temperatures rise), and new work since the Fourth National Climate Assessment (NCA4) was published in 2018 further strengthens this deep evidence base. Such processes and their impacts on air pollution in a changing climate are illustrated in Figure 14.1. Wildfires are a key example of how feedbacks from the biosphere are expected to increase air pollution in future years (KM 14.2).²⁶⁴ An increased frequency of heatwaves will also lead to more extreme levels of ozone and PM_2.5 (KM 2.2),³⁸^,²⁶⁵^,²⁶⁶ while warmer average temperatures will increase seasonal mean daily maximum 8-hour average (MDA8) ozone and PM_2.5 concentrations.⁴⁹^,⁵¹^,²⁶⁰ The source gases of ozone and PM_2.5 from plants and soils are expected to increase with warmer and drier conditions,²⁵⁹^,²⁶⁷^,²⁶⁸^,²⁶⁹ thus degrading air quality. In addition, as plants wither and die during drought, ozone that would otherwise be deposited on leaves may accumulate in the atmosphere,²⁷⁰^,²⁷¹ although this process is less well studied. Other processes may lead to lower pollution in a warmer climate. Some studies project that annual average precipitation, which removes PM_2.5, will increase across much of the United States by 2100,²⁷² but not all studies agree.²⁷³ Basic physics explains why atmospheric humidity will rise with temperature, and the chemical reactions governing ozone destruction will increase with humidity, reducing ozone in unpolluted regions.⁶⁸^,²⁷⁴ In contrast, greater humidity is expected to worsen PM_2.5 air quality in some regions.²⁷⁵ Finally, future trends in the regional transport of pollution or in the frequency of weather patterns like stagnation will have consequences for US air pollution, but these trends are not well established across the US.²⁷⁶^,²⁷⁷^,²⁷⁸

Efforts to model the net response of US air quality to climate change have taken two main approaches, with some studies focusing on the impact from climate change alone²⁷^,⁴¹^,⁴⁹^,⁵⁰^,⁵¹^,⁵²^,⁶⁸^,²⁷⁹ and other studies including the influences of both climate change and changing emissions from human sources of ozone and PM_2.5, such as fossil fuel combustion.²⁶^,³⁹^,⁴⁵^,⁶⁷ Some studies compare the combined effects of emissions and climate change with climate change alone.⁴⁴^,⁴⁶ There is general agreement across these studies that climate change will degrade US air quality in many regions with high concentrations of pollutants. Summertime average surface ozone is expected to increase across much of the northern and eastern United States²⁶^,⁵¹ and during heatwaves in populous areas already affected by pollution.⁵³ Surface PM_2.5 is also projected to increase in areas prone to wildfires (KM 14.2) or dust events,⁶³ but there is less agreement on the response of PM_2.5 elsewhere.⁵⁰^,⁵¹^,⁵⁴^,²⁸⁰

Many epidemiological health studies have identified a wide range of adverse health outcomes following exposure to wildfire smoke and dust, as well as to ozone and particulate matter. Such adverse outcomes are expected to generally increase in response to ongoing climate change.²⁶

Major Uncertainties and Research Gaps

Uncertainties remain in how meteorology will respond to climate change in different regions of the United States and how these meteorological responses, in turn, will trigger changes in different air pollutants. While it is well established that rising methane will increase background ozone at the surface, there is uncertainty in the spatial patterns of this response tied to nitrogen oxides emissions, including from ship plumes.⁴²^,²⁸¹ Climate variability tends to dominate the uncertainty in shorter-range projections (thin lines in Figure 14.2).²⁸²^,²⁸³^,²⁸⁴ Health responses to the combined impacts of exposure to multiple pollutants and other climate change impacts (heat, flooding) are not well quantified. Extensive research into the relative toxicity of PM_2.5 mixtures has not consistently shown that any particular source or component is more strongly related to health effects than total PM_2.5 mass.²⁸⁵

The lack of systematic information available from chemistry–climate models for US air quality complicates the assessment of future change. For example, Figure 14.2 makes use of the most comprehensive set of coordinated simulations with international climate models that include the atmospheric chemistry necessary for projections of future air quality. There are different numbers of models with simulations available for each scenario. Specifically, seven models simulated PM_2.5 for both the historical simulations and four future air pollutant emissions and climate scenarios during 2015–2100 (see Table 3 in the Guide to the Report). In contrast, for ground-level ozone, fewer models (one to five depending on scenario) archived the hourly ground-level ozone needed to calculate the MDA8 metric used to assess compliance with the National Ambient Air Quality Standards. In Figure 14.2, thick lines show the average of all available model simulations for each scenario, with each simulation shown individually by the thin lines. A list of the individual models that produced each scenario in Figure 14.2, together with the simulated fields, are available in the metadata. Models and simulations are further described by Turnock et al. (2020).¹⁵

In more recent studies, progress is being made in quantifying different sources of uncertainty in emissions scenarios and future projections for US air quality, including separately determining the uncertainty associated with model mechanisms and with naturally arising climate variability.²⁵⁹^,²⁸⁶^,²⁸⁷

Description of Confidence and Likelihood

The overall assessment of medium confidence that climate change is projected to worsen US air quality in many US regions reflects uncertainty in the net ozone and PM_2.5 responses to climate change across different regions.⁴⁸^,⁴⁹^,⁵⁰^,⁵¹^,⁵⁴^,⁶⁸^,²⁸⁰ The evidence for air pollution impacts on health is well established from epidemiological and toxicological studies,⁴^,⁷^,⁹^,¹⁰ supporting a very likely, high confidence assessment. There is very high confidence and it is very likely that climate change will increase the intensity and frequency of extreme heat (KM 2.2).²⁴⁷ Observational evidence, theoretical understanding, and modeling studies all support an assessment of high confidence that increasing frequency of warmer and drier conditions will very likely raise the risk of exposure to airborne dust and wildfire smoke in some regions.⁶²^,⁶³^,⁶⁹^,²⁸⁸

KEY MESSAGE 14.2

Increasing Wildfire Smoke Is Harming Human Health and Catalyzing New Protection Strategies

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Description of Evidence Base

This section was based on a review of the recent peer-reviewed literature. Many studies detail the harmful health effects of wildfire smoke on human health. A growing weight of evidence indicates that wildfires and associated air quality impacts will increase in the future with a warming climate, but the interactions are complex and regionally driven. Our understanding of smoke exposure and health impacts has been aided by combinations of surface and satellite-based observations, as well as model simulations.²⁸⁹^,²⁹⁰ Smoke prediction (forecast) systems are a useful mitigation tool,¹²¹ and the number of them online, along with many science improvements, has grown in recent years across North America.¹¹²^,¹¹⁴^,¹¹⁵^,²⁹¹^,²⁹²^,²⁹³^,²⁹⁴

Since NCA4, particularly impactful wildfire smoke years have driven the development of new communication and smoke mitigation measures. The authors highlight the growing base of information on how the public can protect itself before and during a wildfire, such as that found in the EPA Smoke-Ready Toolbox (https://www.epa.gov/smoke-ready-toolbox-wildfires), as well as the development of wildfire smoke mitigation programs by many states and Tribes, in addition to federal programs.²⁹⁵^,²⁹⁶^,²⁹⁷^,²⁹⁸ Evidence shows that social media plays an important role in communicating mitigation measures. For example, smoke blogs in many western states are a nexus of information.²⁹⁹^,³⁰⁰^,³⁰¹^,³⁰²

Major Uncertainties and Research Gaps

Uncertainties in future smoke exposure are intrinsically tied to the uncertainties in future wildfires. Hence, improvements in future wildfire projections will reduce uncertainties in future smoke exposure. Related to this is the uncertainty regarding how future use of prescribed fire as a management tool for wildfire mitigation and ecosystem health will affect smoke at regional and national extents. Finally, quantification of how Indigenous fire practices influence smoke both historically and into the future will also reduce this uncertainty.

Uncertainties remain in our understanding of the health effects of smoke-specific particulate matter and the impacts of cumulative smoke exposure over many years. Research investigating indoor concentrations during wildfire smoke events is preliminary, and there is a specific need to understand how indoor concentrations vary between socioeconomic groups during wildfire smoke events. Research quantifying the effectiveness of smoke mitigation measures and other health protection interventions is limited, and relying on personal interventions such as wearing face masks, filtering indoor air, and staying indoors can have limitations.³⁰³^,³⁰⁴

Description of Confidence and Likelihood

There is very high confidence that wildfires emit gases and fine particulate matter that are harmful to human health based on epidemiological and toxicological studies.⁷⁴^,⁷⁷^,⁸⁸^,⁸⁹^,⁹⁰^,⁹¹^,⁹²^,⁹³^,⁹⁴^,⁹⁵ Many studies document the effects of short-term acute exposures on respiratory healthcare outcomes (Liu et al. 2015; Reid et al. 2016⁹²^,⁹³ and references therein). Less quantified but also of concern are the effects of long-term lower-level exposure.⁹²^,⁹³ A growing weight of evidence supports the likely, high confidence assessment that with a warming climate, wildfires and associated air quality impacts will increase in the future in many regions of the contiguous US and Alaska, but the fire-climate interactions are complex and regionally driven, and the extent to which human management actions will influence future wildfire activity is unknown (Ch. 7). Since NCA4, particularly impactful wildfire smoke years have driven the development of new communication and smoke mitigation measures.¹¹⁷^,¹¹⁸^,¹¹⁹^,¹²⁰^,¹²¹ Advancements in the science in models and observational data are also leading to products to help inform the public.¹¹²^,¹¹³^,¹¹⁴^,¹¹⁵^,¹¹⁶^,¹⁵⁴ However, these developments may not be enough to substantially reduce exposure, especially for all demographic groups.¹²⁵^,¹²⁶ This uncertainty in exposure reduction leads to the assessment of medium confidence in the efficacy of these measures and the conclusion that challenges remain.

KEY MESSAGE 14.3

Air Pollution Is Often Worse in Communities of Color and Low-Income Communities

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Description of Evidence Base

This section is based on a review of peer-reviewed scientific literature, focusing on work published in the last decade. It has been repeatedly shown that communities of color, low-income communities, and other marginalized groups are disproportionately exposed to and harmed by air pollution.²⁵^,³⁰^,¹³²^,¹³³^,¹³⁴^,¹³⁵^,¹³⁶^,¹³⁷^,¹³⁸^,¹³⁹^,¹⁴⁰^,¹⁴¹^,¹⁴²^,¹⁴³^,¹⁴⁴^,¹⁴⁵^,¹⁴⁶^,¹⁴⁷^,¹⁴⁸^,¹⁵²^,¹⁵³^,¹⁵⁴^,¹⁵⁵ Over the last 10 years, there has been an emphasis on developing and applying new measurements and models to describe air pollution inequalities and, in some cases, on deepening commitments to community-engaged scholarship. Improved monitoring and modeling have advanced tools for distinguishing pollutant differences within and between neighborhoods, whereas research over previous decades was largely based on analyses of source proximity and/or health impacts. A new generation of sensors, costing a few hundred dollars each, is supporting collaborative air quality and exposure research and producing actionable results.³⁰⁵^,³⁰⁶^,³⁰⁷^,³⁰⁸^,³⁰⁹^,³¹⁰^,³¹¹ In addition, recent advances in satellite remote sensing are enabling more detailed observations of neighborhood-level pollution inequalities, with satellite measurements being used directly in the case of nitrogen dioxide (NO₂)¹³⁸^,¹⁶³^,¹⁷⁷^,³¹² and in combination with models for PM_2.5 and NO₂,¹³³^,¹³⁴^,¹³⁵^,³¹³^,³¹⁴ with additional information, especially on daytime temporal variability, anticipated with the launch of TEMPO. Machine learning and regression models are filling observational gaps and improving estimates of unequal exposures.¹²⁹^,¹³⁴^,³¹³^,³¹⁵ Current understanding of air pollution health impact disparities is also improving through neighborhood-level datasets on disease rates.¹³³^,³¹⁴ Chemical transport models, which are standard research and air quality decision-making tools that account for key chemical and physical processes, have only begun to be used for neighborhood-level environmental justice applications because of model resolution challenges.³¹⁶^,³¹⁷ That said, neighborhoods are typically larger than the spatial gradients of primary pollutants, and emissions sources are often clustered in overburdened communities. As a result, models with very-fine-scale spatial resolution (hundreds of meters) may not always be needed to describe neighborhood-level inequalities,¹⁶³^,³¹⁸ further opening the range of tools applicable to describing and understanding air pollution inequalities. As air pollution datasets evolve, they reinforce what communities with environmental justice concerns have been saying for decades.

Major Uncertainties and Research Gaps

While patterns of inequities related to air pollution sources, exposure, and associated adverse health impacts are well established, we lack tools that fully describe neighborhood-level distributions of a wide variety of pollutants harmful to health, such as air toxins, and of pollutant mixtures. Air pollution exposures also occur in the home, in classrooms, and at work, and there is little research simultaneously considering outdoor, indoor, and occupational exposures. To date, researchers have largely focused on producing high spatial resolution air pollution maps, and as a result, there is far less knowledge of the temporal variability and source patterns driving air pollution inequalities. Without also capturing this temporal variability, it is difficult to incorporate issues of inequalities in broader air quality and climate change decision-making.¹⁶³ Equity-related questions are not a common feature of air pollution–climate research, partly because of computational limitations on model spatial resolution and partly because of disciplinary and regulatory divides in the fields of air quality and environmental justice. There is limited research on how greenhouse gas (GHG) mitigation actions have differential impacts on air quality affecting different communities, but there is clear evidence that without considering equity, GHG regulations can adversely affect air quality in communities of color and communities with low-socioeconomic status.¹⁶⁸

Description of Confidence and Likelihood

There is very high confidence that communities of color, low-income communities, and other marginalized populations, on average, live in greater proximity to emissions sources, experience higher levels of air pollution, and are disproportionately harmed by poor air quality²⁵^,¹²⁹^,¹³⁸^,¹⁵²^,¹⁵³^,³¹⁹—this has been repeatedly shown for decades. The author team assigns very likely, high confidence to the statement that these same communities will disproportionately face worsened cumulative air pollution burdens from climate change–driven hazards. Regarding the likelihood, there are two facets to consider concerning how climate change will affect air pollution inequity: 1) how the amount and distribution of air pollution will differ in the future and 2) how the health impacts of air pollution exposures will vary with climate change. There is less research on how the amount and distribution of air quality (i.e., air pollution inequalities) will change in the future,¹⁵⁵^,¹⁶³ with varying effects possible depending on which control strategies are employed and whether pollutants are directly emitted into the atmosphere or formed in the atmosphere through chemistry. The likelihood and confidence statements are largely based on the second facet—because of well-documented inequalities in the distribution of other climate-sensitive environmental benefits and harms (KMs. 9.2, 12.2, 15.2) and because of other forms of structural racism affecting the impacts of air pollution on health and well-being,¹⁵²^,¹⁵⁶ hence the high confidence. The cumulative burdens of air pollution with other climate change-driven hazards are very likely to increase in the coming decades in the absence of equity-focused emission controls. The author team assigns high confidence to the statement that equity-focused decision-making is critical for reducing air pollution inequities, as it has been borne out over decades of improved air quality across the US that air pollution disparities persist.¹³²^,¹³³^,¹³⁴^,¹³⁵^,¹³⁶^,¹³⁷ Sector, market, and pollutant threshold-based controls have been shown to have smaller equity benefits than location-specific interventions,¹⁶⁹ with California’s GHG market serving as a real-world demonstration that GHG controls have the potential to worsen air pollution inequalities.¹⁶⁸

KEY MESSAGE 14.4

Climate Change Is Worsening Pollen Exposures and Adversely Impacting Health

Read about Confidence and Likelihood

Description of Evidence Base

This section was based on a review of the recent peer-reviewed literature. A large number of articles using new data and tools have been published in the past few years, and some have provided insight into the attribution of observed shifts in pollen metrics to anthropogenic climate change.

Recent developments have enhanced our understanding of climatic influences on pollen. These include improved understanding of plant phenology,²⁰³^,³²⁰^,³²¹^,³²²^,³²³ improved measurements of aeroallergen concentrations,¹⁹⁴^,²⁰¹^,³²⁴ new modeling platforms for pollen emissions and transport,²⁰⁴^,²⁰⁵^,²⁰⁷^,³²⁵^,³²⁶ novel analytics tools for recognizing pollen patterns,³²⁷^,³²⁸ automatic analysis of pollen types,³²⁹^,³³⁰ and remotely sensed data on meteorology, air quality, and phenology.³²¹^,³³¹^,³³² In addition to these methodological advances that allow for greater insight into factors influencing aeroallergen distribution and concentration, climatic influences are becoming clearer as the climate shifts further, and longer time series allow for greater confidence in the correlations observed.

Strategies for reducing the impact of allergic airway disease by avoiding and reducing pollen exposure,²¹³ which can be facilitated through public health campaigns²¹⁴ and taking medications to reduce immune response intensity,²¹² have been established for years. More recent literature has highlighted gaps in diagnosing and treating allergic airway disease.²¹¹

Major Uncertainties and Research Gaps

There are several papers suggesting overall trends in pollen season and concentrations for total pollen and ragweed, but there is limited evidence for specific taxa, and there is less literature on climate change impacts on indoor and outdoor mold exposure. There is also limited evidence linking changes in health impacts with changes in exposure; however, there is abundant evidence that allergic respiratory disease is driven by exposure, so there is a strong presumption of a link. There is relatively limited information on the health equity impacts of changes in pollen exposure and on the effectiveness of early warning systems in reducing symptom burden. Lastly, there is little information quantifying the likelihood that investments in adaptation can fully close the adaptation gap and negate climate change–attributable shifts in allergic airway disease.

Description of Confidence and Likelihood

There is very high confidence in the linkage between aeroallergen exposure and the development and intermittent exacerbation of allergic airway disease and, by extension, that increased aeroallergen exposure damages the health of people who suffer from allergic airway disease.¹⁸⁵^,¹⁸⁶^,¹⁸⁷^,¹⁸⁸^,¹⁸⁹^,¹⁹⁰^,¹⁹² There is high confidence and it is very likely that human-caused climate change, particularly warming, has already changed the patterns of pollen seasons based on both observational studies in North America as well as modeling studies assessing the influence of anthropogenic climate change compared against a counterfactual without anthropogenic climate forcing (Figure 14.6).¹⁸⁷^,¹⁹⁴^,¹⁹⁶^,¹⁹⁸^,¹⁹⁹^,²⁰⁰^,²⁰¹ This evidence demonstrates that shifts in pollen concentrations vary by region. There is high confidence and it is very likely that as the climate changes further, these trends will continue and that further shifts in aeroallergen concentrations and distribution will depend on the rate at which the climate changes and, in particular, the rate of warming in a given location (Figure 14.6).²⁰⁴^,²⁰⁶^,²⁰⁷^,²⁰⁸^,²¹⁰ Based on past experience with managing allergic airway disease, there is high confidence that the health impacts associated with increased pollen from climate change can be counteracted fully or in part through improvements including increasing access to allergists, improved diagnosis and disease management, and allergy early warning systems.²¹¹^,²¹²^,²¹³^,²¹⁴

KEY MESSAGE 14.5

Policies Can Reduce Greenhouse Gas Emissions and Improve Air Quality Simultaneously

Read about Confidence and Likelihood

Description of Evidence Base

The author team made use of the existing literature, emphasizing studies published since NCA4 but also referencing some classic papers published before 2018. The author team emphasizes here how decisions to control GHG emissions often have effects on air pollutant emissions. Similarly, decisions to control air pollutant emissions may influence GHG emissions. The author team therefore highlights the opportunity to control both types of emissions simultaneously through reductions in fossil fuels use, addressing both air pollution and climate change. Conclusions are informed by historical changes in emissions in the US and elsewhere, particularly the actions to implement air quality regulations through controls on smokestack emissions from power plants and large industries and controls on tailpipe emissions from motor vehicles. A fuller array of possible actions is presented in Figure 14.7, which emphasizes the capacity for actions to affect emissions of both air pollutants and GHGs in the near-term (targeting 2030), without explicit consideration for the cost-effectiveness of actions. Figure 14.7 does not present the potential for emissions reductions quantitatively, as the author team is not aware that this has been analyzed previously for the US. Rather the author team used information from several key sources to inform where boxes are placed in Figure 14.7, including US emissions inventories for GHGs²¹⁵ and air pollutants,³³³ which constrain the potential reductions of some actions. Estimates of the global GHG mitigation capacity from the Sixth Assessment Report (AR6) of the Intergovernmental Panel on Climate Change (IPCC) Working Group III²²¹ help quantify the capacity for reduction, although these estimates are not specifically for the US, and estimates specifically for US energy system actions are from Figure 32.22. Estimates of sector contributions to US air pollution–related deaths²¹⁶ are also used, as are qualitative estimates of the effects of GHG reductions on air pollution in the United Kingdom.⁶⁵ Using these sources of information, emissions-reduction actions are put in order separately along the two axes in Figure 14.7 and then plotted. In some cases, minor changes in the order are made to fit the boxes on the figure. The boxes themselves are intended to communicate that there is some uncertainty in the emissions reductions, including boxes that straddle an axis, indicating uncertainty in the sign of the influence. Box positions should not be interpreted quantitatively (e.g., inferring that emissions-reduction capacity for one action is twice that of another action). Actions considered include those emphasized in past emissions reductions and considered for future action in the US, and not all possible actions can be included here. The analysis also focuses on technology actions rather than policy approaches (cap-and-trade, incentives for clean technology) used to achieve these goals.

There are many studies of the air quality and human health benefits due to the co-pollutant emission reductions from GHG mitigation actions.²²⁶^,²²⁷^,²²⁸^,²²⁹^,²³⁰^,²³¹ The author team surveyed the literature and found 26 studies that either directly reported or contained enough information to quantify the monetary value of human health benefits from improved air quality per ton of mitigated GHG emissions. In some cases, it was necessary to contact the authors to ensure that the data were being interpreted correctly. These 26 studies form the basis of the range presented in the text ($8 to $430 in 2022 dollars, with a median of $100 per metric ton of CO₂). The estimates of human health benefits and costs from these studies span a range of two orders of magnitude because of different methods used, geographical scope, time periods analyzed, and GHG reduction actions considered. Figure 14.8 presents results from the subset of these studies that included both the air quality human health benefits and GHG mitigation costs. A complete list of the 26 studies and their reported values is available in the metadata for Figure 14.8.

Discussion of short-lived climate pollutants has a strong foundation in past research, as summarized in the IPCC AR6,²⁴⁰ although some significant uncertainties remain in the magnitude of global anthropogenic radiative forcing for some of these species and in the net effects on climate from reductions of short-lived climate pollutants²⁵² in the United States in particular.

On the subject of social costs, since this chapter is about the link between climate change and air quality, it seemed appropriate to use costs that include both climate change and air pollution.²⁴⁴ As the text states, “over half of [the value is] from ozone health impacts,” so it is clear that this differs from commonly used costs, such as those produced by the US Government’s Interagency Working Group on the Social Cost of Greenhouse Gases for use in regulations, which include only damages related to climate changes.³³⁴

Major Uncertainties and Research Gaps

Whereas there are new global modeling studies estimating air pollutant concentrations in future Shared Socioeconomic Pathway (SSP) scenarios, including the impacts of climate change on air quality, no study has yet downscaled these simulations to the United States for studying air pollution impacts. There is a gap in research that critically assesses how air pollution is projected to change in the US under scenarios that lead to decarbonization and approach net-zero emissions. There is also limited research in quantifying the effects of actions considered on both GHG and air pollutant emissions, as well as their costs and potential for emissions reductions, since much of the literature available focuses on GHG reductions without estimates of concurrent air pollutant emissions reductions.

Description of Confidence and Likelihood

There is high confidence and it is very likely that broad policies to reduce greenhouse gas emissions economy-wide in the United States will reduce air pollutant emissions and benefit air quality and health, although some individual actions may not achieve these benefits (Figure 14.7).²²⁷^,²³⁰^,²³¹ Many studies have estimated the air quality and human health benefits of greenhouse gas reduction actions, most of which have found that monetized benefits exceed the costs of greenhouse gas controls (see Figure 14.8 and associated metadata), when premature mortality is monetized using methods commonly used in the United States,²² such as those used by the EPA. Therefore, there is high confidence that monetized health benefits would exceed costs for many greenhouse gas reduction actions, and it is likely that many specific actions will also have health benefits exceeding costs.¹⁹^,²²⁶^,²²⁹ Based on several individual studies, there is high confidence that pursuing actions that emphasize reduced fossil fuel use, improved energy efficiency, and reductions in short-lived climate pollutants would not only put the United States on a trajectory that would substantially reduce GHG emissions and approach net-zero emissions (KM 32.4) but also substantially improve air quality and health.²²⁴^,²³¹

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Likelihood

Virtually Certain	Very Likely	Likely	As Likely as Not	Unlikely	Very Unikely	Exceptionally Unlikely
99%–100%	90%–100%	66%–100%	33%–66%	0%–33%	0%–10%	0%–1%

Confidence Level

Very High	High	Medium	Low
Strong evidence (established theory, multiple sources, well-documented and accepted methods, etc.) High consensus	Moderate evidence (several sources, some consistency, methods vary and/or documentation limited, etc.) Medium consensus	Suggestive evidence (a few sources, limited consistency, methods emerging, etc.) Competing schools of thought	Inconclusive evidence (limited sources, extrapolations, inconsistent findings, poor documentation and/or methods not tested, etc.) Disagreement or lack of opinions among experts

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INTRODUCTION

Climate Change Will Hamper Efforts to Improve US Air Quality

Increasing Wildfire Smoke Is Harming Human Health and Catalyzing New Protection Strategies

Air Pollution Is Often Worse in Communities of Color and Low-Income Communities

Climate Change Is Worsening Pollen Exposures and Adversely Impacting Health

Policies Can Reduce Greenhouse Gas Emissions and Improve Air Quality Simultaneously

TRACEABLE ACCOUNTS

Process Description

KEY MESSAGES

KM 14.1

Climate Change Will Hamper Efforts to Improve US Air Quality

Description of Evidence Base

Major Uncertainties and Research Gaps

Description of Confidence and Likelihood

KM 14.2

Increasing Wildfire Smoke Is Harming Human Health and Catalyzing New Protection Strategies

Description of Evidence Base

Major Uncertainties and Research Gaps

Description of Confidence and Likelihood

KM 14.3

Air Pollution Is Often Worse in Communities of Color and Low-Income Communities

Description of Evidence Base

Major Uncertainties and Research Gaps

Description of Confidence and Likelihood

KM 14.4

Climate Change Is Worsening Pollen Exposures and Adversely Impacting Health

Description of Evidence Base

Major Uncertainties and Research Gaps

Description of Confidence and Likelihood

KM 14.5

Policies Can Reduce Greenhouse Gas Emissions and Improve Air Quality Simultaneously

Description of Evidence Base

Major Uncertainties and Research Gaps

Description of Confidence and Likelihood

REFERENCES

Likelihood

Confidence Level