Effects of Human-Caused Greenhouse Gas Emissions on U.S. Climate, Health, and Welfare (2025)
Chapter: 5 Impacts on Human Health
5
Impacts on Human Health
5.1 KEY MESSAGES
Human-caused emissions of greenhouse gases and resulting climate change harm the health of people in the United States. Evidence since 2009 supports and strengthens EPA (2009a) conclusions and has deepened the understanding of how these risks affect health. Climate-related illnesses and deaths are increasing in both severity and geographic range across the United States.
Climate change intensifies risks to human health from exposures to extreme heat, ground-level ozone, wildfire smoke and other airborne particulate matter, extreme weather events, and airborne allergens, affecting incidence of cardiovascular, respiratory, and other diseases. Much evidence is now available on how heat affects morbidity and mortality, not only by directly causing heat exhaustion and heat stroke, but also by worsening effects on cardiovascular, respiratory, kidney, mental health, and other disorders. New evidence has deepened understanding of how climate-sensitive drivers increase ozone pollution, how long-term ozone exposure leads to health effects beyond those of short-term exposure, and how health outcomes are amplified by co-occurrence of ozone exposure with heat and particulate matter exposure.
Climate change has increased exposure to wildfire smoke and dust, which has been linked to adverse health effects. Since 2009, wildfire smoke exposure has increased, particularly in the U.S. West, and new evidence has linked wildfire smoke exposure to a wide range of adverse human health outcomes, including respiratory disease and premature death. New evidence has also shown that climate change has increased airborne soil dust and associated health effects, particularly for areas that are warmer and drier, such as the U.S. Southwest.
The increasing severity of some extreme weather events, such as wildfires and heavy precipitation events, has contributed to injury, illness, and death in affected communities. Although non-climate factors, including adaptation, can mitigate the negative effects of climate change on health, extreme events can overwhelm the ability to respond. Extreme events can impact human systems, including health care and food systems, power systems, and other critical infrastructure, adding to the risks faced by individuals and communities.
Health impacts related to climate-sensitive infectious diseases—such as those carried by insects and in contaminated water—have increased. An increase in the geographic distribution of tick-borne diseases, anticipated in the 2009 report, has been confirmed and attributed to climate warming. Dengue, a mosquito-borne viral disease, has increased in activity and geographic range since the 2009 report, now appearing in people who have not traveled (“non-travelers”) in Texas, Florida, Arizona, and Hawaii. Climate change is expanding the area of
endemicity of some fungal diseases. Heavy precipitation, drought, and warming temperatures are linked to a rise in waterborne disease outbreaks.
New evidence is developing about additional health impacts of climate change. Newer areas of evidence include potential impacts on mental health, nutrition, immune health, antimicrobial resistance, kidney disease, and negative pregnancy-related outcomes. In addition, research has grown showing that combined exposure to multiple climate-sensitive risk factors, either simultaneously or cumulatively over time, worsens health outcomes.
Groups such as older adults, people with preexisting health conditions or multiple chronic diseases, and outdoor workers are disproportionately susceptible to climate-associated health effects. New findings also point to elevated risks for pregnant people and children. Even as non-climate factors, including adaptation measures, can help people cope with harmful impacts of climate change, they cannot remove the risk of harm.
Weather and climate interact with many factors to shape the effects of climate-sensitive health outcomes in any given place and time (Ebi et al., 2020). The climate changes discussed in preceding chapters affect human health directly through events such as heat waves or wildfires, as well as indirectly, through pathways like air and water quality and nutrition, with these impacts further shaped by broader environmental, social, and public health conditions (WHO, 2018). The climate and health field has expanded considerably since EPA (2009a) was published, with substantial new data and research strengthening the evidence base, deepening understanding of how climate change affects health, and clarifying the pathways through which these impacts occur. This chapter explores health effects from exposure to extreme temperatures and events such as wildfires and hurricanes, the influence of climate on infectious and noncommunicable diseases, and health implications of changes to air quality. Examples illustrate areas where knowledge has grown or evolved, including areas of health research not discussed in EPA (2009a).
EPA (2009a) concluded that negative health effects from climate change are experienced disproportionately by some populations. Subsequent assessments of the impacts of human-caused greenhouse gas (GHG) emissions and resulting climate changes have also concluded that these changes are harming physical and mental health (see discussion of GHG emissions and climate effects in Chapters 2 and 3) (IPCC, 2022a; USGCRP, 2023). The evolution of understanding the health risks from climate change since 2009 is summarized in Table 5.1. Evidence has continued to accumulate about multiple health effects; none of the areas identified in EPA (2009a) showed weakened evidence of health effects. Although this report does not consider the impacts of adaptation or mitigation measures in reducing climate-associated health risks in detail, Box 5.1 highlights a few potential ways such adaptations may influence risk as well as some of the potential limitations of such measures.
5.2 TEMPERATURE EFFECTS
EPA (2009a) concluded that “[s]evere heat waves are projected to intensify in magnitude and duration over the portions of the United States where these events already occur, with potential increases in mortality and morbidity, especially among the elderly, young, and frail” (p. ES-4). Observations continue to show a warming trend in regions of the United States, particularly in Alaska, the West, and the Northeast, with hot extremes increasing with cold extremes decreasing (see Section 3.2). Studies and assessments of human health consequences continue to support the EPA (2009a) conclusion that changes in average temperatures and increased exposure to temperature extremes contribute to adverse health outcomes in many places in the United States.
Studies on ambient temperature and health have identified U-, V- or J-shaped patterns in which extremes of both hot and cold are associated with adverse effects, recognizing that duration, humidity level, extent of divergence from a location’s usual temperature, and other parameters influence these outcomes and that some groups are more susceptible to the effects of temperature than others (Burkart et al., 2021; Ye et al., 2012). For example, a 2023 study spanning 2000–2016 and involving 61.6 million Medicare beneficiaries aged 65 and older found that cardiovascular hospitalizations were higher in areas with hotter summers or colder winters (Klompmaker et al., 2023). USGCRP (2016) identified as a key finding that “[d]ays that are hotter than usual in the summer or colder than usual in the winter are both associated with increased illness and death [Very High Confidence]. Mortality effects are observed even for small differences from seasonal average temperatures [High Confidence]” (p. 6)
Health Effects Associated with Heat Exposure
Recent evaluations and assessments have highlighted the wide-ranging health consequences of exposure to increased temperature and heat (IPCC, 2022a; Lancet Countdown, 2024; USGCRP, 2023). Heat contributes to excess illness and death in the United States and globally, with an estimate of net increases in all-cause mortality risk associated with increased average annual temperatures from 0.1% to 1.1% per 1.8°F (1°C) (Cromar et al., 2022). According to data from the National Weather Service, heat is associated with more weather-related deaths than any other extreme weather event.1
Much evidence is now available on how heat affects morbidity and mortality, not only by directly causing heat exhaustion and heat stroke, but also by worsening effects on cardiovascular, respiratory, kidney, mental health, and other disorders (see Section 5.5 discussing climate-sensitive diseases). Heat waves are linked with higher rates of emergency department visits, hospitalizations, or deaths for such conditions (Khatana et al., 2022; Sun et al., 2021). Exposure to higher temperatures and heat stress are also linked to adverse pregnancy and birth outcomes (Baharav et al., 2023; Jiao et al., 2023; Khalili et al., 2025; Kuehn and McCormick, 2017; USGCRP, 2023; Weeda et al., 2024; Zhang et al., 2023).
Rising temperatures increase risks for workers in sectors such as construction, agriculture, transportation, warehousing, and waste management (USGCRP, 2023). Other outdoor workers, including those in landscaping, natural resources management, and firefighting, are also affected by outdoor temperatures, as are people who live in poorly insulated or unshaded homes and the unhoused. For example, a 2022 meta-analysis of occupational heat exposure examined 2,409 outdoor workers across 41 jobs in 21 countries, including the United States. The study’s findings suggest that occupational heat stress elevated workers’ core and skin temperatures, heart rate, and the concentration of dissolved chemicals and particles in urine (Ioannou et al., 2022). Among workers routinely exposed to heat stress (≥6 hours/day, 5 days/week, for ≥2 months annually), a study found that approximately 15% developed kidney disease or acute kidney injury (Flouris et al., 2018). Increased specific humidity in some heat-prone areas decreased evaporative cooling through sweat and was found to exacerbate heat stress, although results of epidemiological studies exploring the role of specific humidity in heat-related health outcomes have been mixed (Baldwin et al., 2023). With global warming, heat reaches unsafe thresholds for sustained labor earlier in the morning, making it harder to adapt shifts to safer hours (Parsons et al., 2021). Heat waves can also reduce the places where fans, rather than air conditioning, are able to provide sufficient cooling, amplifying the impacts on poorer communities (Parsons et al., 2023).
In the United States, recent studies have assessed excess heat-related deaths attributable to climate change, identifying impacts on mortality although finding that percentages attributable specifically to human-induced climate change remain relatively small. As noted for many health effects, a complex interplay of factors beyond a location’s recorded temperature affects health outcomes. For example, one study that generalized local epidemiological evidence across the contiguous United States found 12,000 (95% confidence interval [CI] [7,400, 16,500]) heat-related premature deaths annually in the United States averaged over 2010–2019 (Shindell et al., 2020). Using data from sites in the United States and around the globe, researchers estimated that 37.0% (20.5–76.3%) of warm-season heat-related deaths could be attributed to human-induced climate change (34.7% in the United States); this analysis assessed that a 0.30% increase (95% CI [0.01, 0.76]) of heat-related mortality in the United States is attributable to human-induced climate change (Vicedo-Cabrera et al., 2021). A recent analysis of U.S. mortality during 1999–2023 in which heat was an underlying or contributing factor observed an increasing trend; further research will be needed to understand whether such a trend is directly attributable to climate change (Howard, Androne et al., 2024).
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1 See Weather Related Fatality and Injury Statistics at https://www.weather.gov/hazstat (accessed September 8, 2025).
TABLE 5.1 Assessment of Change in Evidence of Risks from Climate Change Since EPA (2009a)
| Type of Effect | Areas Addressed in EPA (2009a) | New Areas of Evidence Since EPA (2009a) |
|---|---|---|
| Exposure to extreme heat |  |
|
| Exposure to ground-level ozone | |
|
| Exposure to airborne particulate matter | |
|
| Exposure to extreme weather events | |
|
| Exposure to vector-borne diseases | |
|
| Development or exacerbation of chronic diseases | |
|
| Exposure to airborne allergens | |
|
| Effects on mental health | |
|
| Effects on pregnancy and birth outcomes | |
|
| Effects on nutrition |  |
|
| Effects on immune health | |
|
| Effects on antimicrobial resistance | |
|
| Effects on metabolic diseases | |
NOTES: Large arrows indicate topics for which the evidence has continued to accumulate. Small arrows indicate areas for which a potential health effect has been identified and further studies are ongoing.
Health Effects Associated with Exposure to Cold and Differential Impacts of Heat Versus Cold on Mortality
EPA (2009a) found that “Some reduction in the risk of death related to extreme cold is expected” as a result of climate change, but “it is not clear whether reduced mortality from cold will be greater or less than increased heat-related mortality in the United States” (p. ES-4). Many factors are involved in evaluating cold-induced deaths, and cold-related mortality reductions with climate change have not been observed and remain unclear.
Factors important to assessments of temperature-related morbidity and mortality include not only the temperature maximum or minimum itself, but also whether the extreme event happens earlier or later in the season, its duration, how it relates to temperatures normally experienced in that region, the influence of seasonality on infectious disease transmission, intersections with a person’s age and the aging population, preexisting health conditions, and other physical and biological factors, including where they live and work (Healy et al., 2023; Liu et al., 2025). Some parts of the United States may experience increases in heat-related mortality and other locations
BOX 5.1
The Role of Adaptation in Reducing Risks to Human Health
Adaptation can help to reduce health risks from climate change. For example, heating and air conditioning can reduce risks from outdoor temperature extremes. A study of non-accidental death due to hot days found that this risk declined from 10.6% to 0.9% from the 1960s, associated with increases in air conditioning and controlling for parameters such as geographic location and mean summer temperature, while noting that “the remaining 20% of un-air-conditioned housing are not randomly located, but primarily in areas that have less summer heat, but where summer temperatures are likely to increase” (Nordio et al., 2015, p. 85).
The potential effectiveness of adaptation measures in reducing future climate-driven health risks is uncertain. Predicting their success requires assumptions about many factors that are unrelated to climate, including human behavior, government policies, and technological advances, which are not explored in detail in this report. Furthermore, there are limits to the ability to adapt to climate-associated health risks, benefits from available adaptations may be uneven and incomplete, and available adaptation approaches may differ in utility locally and regionally. In some instances, effective adaptation approaches are known but remain unimplemented or have been applied inconsistently. For other climate-related health risks, evidence-based adaptation strategies have yet to be identified.
Not all groups and communities have the same access to adaptations that mitigate health risks. For example, communication gaps can limit awareness of increased heat risks and readiness strategies to reduce such risks in places that have not historically dealt with high temperatures (Healy et al., 2023; Howe et al., 2019). Countries such as the United States show weaker temperature–mortality links than lesser developed countries, largely due to the availability of air conditioning, but not all communities have access to this option (Carleton et al., 2022). For example, outdoor workers exposed to extreme heat and wildfire smoke may not be able to take breaks indoors with air conditioning or consistently use protective equipment like masks. Finally, adaptation can be costly or resource intensive. For example, installing home air filtration systems to reduce risks from exposure to poor air quality or restricting development in the wildland–urban interfaces to reduce risks associated with wildfires require financial and policy resources.
might witness reductions in cold deaths (Lee and Dessler, 2023). On a population level, reductions in cold-related mortality could possibly be expected in a few regions under climate warming scenarios. Epidemiological studies that examine the number of deaths associated with cold have found conflicting results. For example, an analysis of U.S. mortality trends from 1999 to 2022 reported a 3.4% annual increase in age-adjusted cold-related mortality rates, with a sharp rise after 2017 (12.1% annually), suggesting that cold-related mortality has not uniformly declined over time (Liu et al., 2025). Methodological factors contribute to the complexity of assessing relative heat- and cold-related mortality data, including choices about the temperatures to use and the exposure-response relationship; for example, Alahmad et al. (2025) have noted that the area under the curve for heat is often smaller (for example only a third of the total area), contributing to observations of greater numbers of cold-associated death. The seasonality of cold-related deaths also corresponds to other factors beyond temperature, such as exposure to influenza, which increases in moderately cold and drier conditions, and to socioeconomic status (Ebi and Mills, 2013). Because multiple factors follow the same seasonal pattern as temperature, attribution of the observed increases in health impacts as temperature reach their coldest seasonal levels can be difficult. Evidence since EPA (2009a) strengthens association with adverse heat-related health consequences, while adding nuance around how changes to climate are anticipated to affect deaths from heat versus cold, suggesting a need for further research in this area to understand the balance of effects.
5.3 AIR QUALITY
Studies over the past 15 years have expanded the understanding of how climate change affects ozone and airborne particulate matter, and how exposure to these air pollutants negatively affects human health. In 2009, EPA concluded that “[t]he evidence concerning adverse air quality impacts provides strong and clear support for an endangerment finding” (EPA, 2009b, p. 66497). This conclusion was largely based on strong evidence that climate change is increasing ground-level (tropospheric) ozone concentrations. The report cited uncertainty in the directional effect on particulate matter. Since 2009, several assessments have documented the air quality impacts of climate change, with particular focus on ground-level ozone and particulate matter (see Section 3.5). Evidence supports the EPA (2009a) conclusion on ground-level ozone and has expanded understanding of the health impacts. For particulate matter, evidence now points to increases in atmospheric concentrations under climate change in some U.S. locations, especially in areas prone to wildfires and dust. Because exposure to these pollutants affects a range of health outcomes, including premature mortality, cardiovascular effects, and respiratory effects, climate-driven increases in ozone and particulate matter have deleterious health impacts.
Key authoritative assessments published since EPA (2009a) conclude that climate change worsens air pollution. For example, USGCRP (2023) concluded with medium confidence that air quality would worsen in many parts of the United States, with harm to human health and increased premature death being very likely (high confidence): “Extreme heat events, which can lead to high concentrations of air pollution, are projected to increase in severity and frequency (very likely, very high confidence), and the risk of exposure to airborne dust and wildfire smoke will increase with warmer and drier conditions in some regions (very likely, high confidence)” (p. 14–5). New research published since this assessment continues to support this conclusion. The sections below describe the current state of knowledge on ozone and particulate matter.
Many people are exposed simultaneously to multiple air pollutants, heat, pollen, and other climate-sensitive risk factors. In 2020, a systematic literature review found sufficient and moderate-quality evidence for synergistic effects of heat and air pollution (Anenberg et al., 2020). While limited evidence prevented conclusions from being drawn about synergistic effects from co-exposure to these risk factors with pollen, the authors concluded that “many disease states, including heart and lung disease, share a common pathway in which exposure to heat, air pollution, and pollen cause systemic and organ-specific inflammation and cellular damage.” Since that review was published, additional studies have found that co-exposure to heat and air pollution had larger effects beyond the sum of their individual effects (Chen et al., 2024; Rahman et al., 2022; Rai et al., 2023; Stafoggia et al., 2023).
Ground-Level Ozone
EPA (2009a) concluded that climate change is expected to worsen ozone pollution across broad regions of the United States, increasing risks of respiratory illness, premature death, and ecological harm, even as the effects on particulate matter remain uncertain. Recent literature supports the EPA (2009a) conclusion regarding ground-level (tropospheric) ozone and adds a fuller understanding of the multiple drivers of ozone increases under climate change as well as harmful interactions with other exposures like heat and particulate matter. Climate change contributes to increases in ozone exposure on both short-term and long-term time scales (see Chapter 3).
Ozone exposure is associated with respiratory effects (EPA, 2020; Holm and Balmes, 2022), pre-term birth (Rappazzo et al., 2021), and premature mortality from all causes and from cardiopulmonary disease (Jerrett et al., 2009; Lim et al., 2019; Turner et al., 2016). Studies show that ozone concentrations tend to be higher in areas of the United States that are suburban, exurban, or rural; in wealthier neighborhoods; and in areas where a higher fraction of the population is non-Hispanic Asian or non-Hispanic White (Collins et al., 2022; Liu et al., 2021).
USGCRP (2016) concluded: “Climate change will make it harder for any given regulatory approach to reduce ground-level ozone pollution in the future as meteorological conditions become increasingly conducive to forming ozone over most of the United States [Likely, High Confidence]. Unless offset by additional emissions reductions of ozone precursors, these climate-driven increases in ozone will cause premature deaths, hospital visits, lost school days, and acute respiratory symptoms [Likely, High Confidence]” (p. 9).
Evidence since 2009 shows that long-term exposure to ozone has health effects beyond those from short-term
exposure to ozone. Epidemiological studies increasingly show health effects associated with long-term ozone exposure, including chronic respiratory disease and premature mortality among adults and decreased lung function and lung function growth among children (Di et al., 2017; Hao et al., 2015; Holm and Balmes, 2022; Kazemiparkouhi et al., 2020; Lim et al., 2019; Turner et al., 2016). For example, a large prospective cohort of U.S. adults with 17 years of follow-up from 1995 to 2011 found that for each 10 parts per billion increase in the annual average 8-hour daily maximum ozone exposure, ischemic heart disease increased by 6% (95% confidence interval [2%, 9%]) and chronic obstructive pulmonary disease increased by 9% (95% confidence interval [3%, 15%]) (Lim et al., 2019).
Methane (CH4) emissions also contribute to long-term ozone concentrations (see Chapter 2, Figure 2.2 for information on annual CH4 emissions and Chapter 3 for discussion of climate effects on air quality) and to health impacts. An analysis led by the UN Environment Programme reported that every 10 million metric tons of CH4 emissions leads to approximately 430 (approximately 290 to 550) premature respiratory deaths and approximately 330 (approximately 110 to 540) premature cardiovascular deaths in the United States attributable to ozone in persons aged 30 and older, along with approximately 150 respiratory hospitalizations and approximately 1,500 asthma-related accident and emergency department visits in the United States due to ozone exposure (UNEP and CCAC, 2021).
Taken together with evidence of conditions favorable to creating more ozone pollution (see Chapter 3), this evidence supports the EPA (2009a) conclusion that GHGs and climate change exacerbate ozone pollution, adversely affecting health in the United States. Furthermore, the new evidence adds context to that understanding, including climate-sensitive drivers of ozone pollution, and shows that health outcomes are amplified by climate-driven increases in co-occurrence of ozone exposure with heat and particulate matter exposure, and by synergistic health effects from these multiple exposures.
Particulate Matter
EPA (2009a) found that “the directional effect of climate change on ambient particulate matter levels remains uncertain” (p. ES-5). Since 2009, evidence has shown that climate change is altering particulate matter levels across the United States, with climate-driven increases in the western United States due to increased wildfire smoke and dust.
Fine particulate matter, or PM2.5, referring to particles that are 2.5 microns or smaller in diameter, are solid and liquid particles that can penetrate deeply into the human lung and affect a wide range of biological systems. PM2.5 is a mixture of chemical components that varies geographically, mainly driven by local emission sources and atmospheric transport of air pollution regionally. Major components include black carbon, organic carbon, nitrate, sulfate, ammonium, and metals and other trace elements. The EPA has found PM2.5 to be causally associated with cardiovascular effects and mortality and likely to be causally associated with respiratory effects, nervous system effects, and cancer (EPA, 2022). Many studies show that PM2.5 is inequitably distributed, with communities with lower income and higher proportions of non-White populations most exposed (Colmer et al., 2020; Jbaily et al., 2022; Ma, Zang, Opara et al., 2023).
USGCRP (2016) found that climate change is expected to alter several meteorological factors that affect PM2.5. Factors that are expected to increase PM2.5 include increased humidity, increased stagnation events, and increased biogenic emissions. Factors that are expected to decrease PM2.5 include increases in precipitation and enhanced atmospheric mixing. The USGCRP report also found links between climate change and increased frequency and length of wildfires and wildfire seasons, with associated emissions and harmful impacts on health. It found that “wildfires may dominate summertime PM2.5 concentrations, offsetting even large reductions in anthropogenic PM2.5 emissions” (p. 77). Climate change has been projected to increase drought in some regions, which can also lead to more airborne dust exposure. USGCRP (2023) similarly found that most known climate-related drivers of PM2.5 increase concentrations, including wildfires, heat waves, temperature, drought, biogenic emissions, and humidity, and that more precipitation would lower PM2.5 levels, as precipitation is a main removal mechanism of PM2.5 from the air. However, to date there is no consensus on the impacts of climate-driven changes in regional transport and stagnation on PM2.5.
Wildfires
EPA (2009a) found that “In some regions, changes in the mean and variability of temperature and precipitation are projected to increase the size and severity of fire events, including in parts of the United States” (p. 86), noting that “Increase in wildfire frequency associated with a warmer climate has the potential to increase PM levels in certain regions” (p. 94). While EPA (2009a) addressed impacts of wildfires on welfare, it did not expand on the health impacts of wildfires in detail. Since 2009, a large body of literature has developed on climate change-driven variations in wildfire smoke exposure and associated health impacts in the United States. See Chapter 3 for a discussion of wildfire trends and variability, including an increase in burned acreage in the West.
The presence of more wildfires across the U.S. landscape and wildfire smoke in the skies is one condition that has affected the United States with more severity and scope than expected since 2009. Wildfires threaten health directly—through injuries, burns, and heat exposure—and indirectly, through smoke-related respiratory illness and trauma-related mental health harms (Gould et al., 2024; Lei et al., 2024; Ma, Zang, Liu et al., 2023).
These health impacts are felt in communities directly affected by the fire and first responders such as wildfire fighters, as well as in distant locations, as smoke can travel across state and even national boundaries. Wildfire smoke contributes to both short-term health outcomes from acute exposures and chronic health outcomes from long-term average PM2.5 concentrations. Wildfires affect air quality as extreme episodic events and as contributors to everyday exposures (Gould et al., 2024).
USGCRP (2016) concluded, “Wildfires emit fine particles and ozone precursors that in turn increase the risk of premature death and adverse chronic and acute cardiovascular and respiratory health outcomes (Likely, High Confidence). Climate change is projected to increase the number and severity of naturally occurring wildfires in parts of the United States, increasing emissions of particulate matter and ozone precursors and resulting in additional adverse health outcomes (Likely, High Confidence)” (p. 85). As the literature continued to build, the USGCRP (2023) concurred with the 2016 report that climate change contributes to more frequent and severe wildfires that worsen air quality in many regions, while noting that “Although large challenges remain, new communication and mitigation measures are reducing a portion of the dangers of wildfire smoke (medium confidence)” (p. 14–9).
Wildfires release a complex mix of particulate matter, carbon monoxide, nitrogen oxides, and volatile organic compounds that can harm human health. Wildfire-specific particulate matter likely has a different chemical profile compared with particulate matter originating from other sources, and exposure to PM2.5 from wildfire smoke has been reported to be more harmful to health than PM2.5 from other types of sources, although more research is needed to understand this differential toxicity (Aguilera et al., 2021; Alari et al., 2025; Gould et al., 2024). Wildfire smoke from fires in urban and industrial areas and the wildland–urban interface can also contain toxic metals such as lead and mercury, plasticizers, and other pollutants (NASEM, 2022). Wildfire smoke has also been documented to contain carcinogens such as benzene, benzo[a]pyrene, hexavalent chromium, and dibenz[a,h]anthracene (Naeher et al., 2007). As a recent review noted, “The amount and composition of pollution emitted from a specific fire vary depending on the fire’s size, temperature of combustion, materials burned (e.g., grasses, tree species, buildings, vehicles), distance the smoke has traveled, and environmental conditions like wind speed, temperature, and humidity” (Gould et al., 2024, p. 279; Montrose et al., 2022).Consistent with these prior assessments, new studies find that wildfires are offsetting reductions in anthropogenic PM2.5 emissions over the western United States and increasing daily cumulative smoke PM2.5 exposure for the average person in the United States compared with the 2006–2019 average (Lancet Countdown, 2024; Wei et al., 2023). The number of people in the United States who have experienced at least 1 day with wildfire smoke PM 2.5 >100 μg m−3 (micrograms per cubic meter) has significantly increased, with about 25 million people exposed in 2020 (Childs et al., 2022).
Observational studies indicate that exposure to wildfire smoke, like exposure to air pollution from other sources, is associated with a spectrum of adverse health outcomes. Substantial literature documents wildfire smoke’s impacts on respiratory health (Zhou et al., 2021). Gould et al. (2024) found that “same-day all-cause mortality increased by 0.15% (95% confidence interval [CI] 0.01–0.28%) per 1-μg m–3 increase in wildfire-specific PM2.5. There were robust positive associations between wildfire PM2.5 and same-day respiratory outcomes: Respiratory hospitalizations increased by 0.25% (95% CI 0.09–0.52%) and respiratory ED [emergency department] visits increased by 0.36% (95% CI 0.19–0.53%) per additional 1-μg m–3 increase in ambient wildfire
smoke PM2.5. [Gould et al.] found a non–statistically significant 0.06% (95% CI 0.00–0.12%) increase in same-day cardiovascular hospitalizations and no meaningful change in same-day cardiovascular ED visits (−0.03%; 95% CI −0.18–0.12%) per additional 1-μg m–3 increase in ambient wildfire smoke PM2.5. For all outcomes except respiratory hospitalizations and cardiovascular ED visits, there was evidence of heterogeneity in effects across studies (i.e., Q-statistic p <0.05)” (p. 282).
Research is also associating exposure to wildfires with negative pregnancy and birth outcomes, such as preterm birth, potentially through the effects of both maternal stress from experiencing the wildfire and exposure to wildfire smoke (Heft-Neal et al., 2022). Exposure to wildfire smoke has also been associated in recent literature with declines in mental health and to worsened cognitive outcomes (Eisenman and Galway, 2022; Xu et al., 2020). Wildfire smoke can spread spores and microbes that contribute to fungal disease and antimicrobial resistance (Mulliken et al., 2023; Salazar-Hamm and Torres-Cruz, 2024). New research also indicates that wildfire smoke exposure may contribute to skin diseases, eye conditions, and cancer.
One study estimated that of 164,000 estimated wildfire PM2.5-related deaths over 15 years in the United States, about 10% (or 15,000) were linked with climate change-driven increases in wildfire. These premature deaths from climate change-driven increases in wildfire smoke translated to a cumulative economic burden of $160 billion. More than one-third of the climate change-driven PM2.5 deaths occurred in 2020, with monetized damages of $58 billion (Law et al., 2025). Another study estimated that long-term exposure to carbonaceous PM2.5 from fire smoke led to 7,455 (95% CI [6,058, 8,852]) premature deaths across the continental United States each year, with monetized damages of $68.3 billion (95% CI [$31.9 billion, $104.0 billion]) (Jin et al., 2025).
Dust
EPA (2009a) found that “[particulate matter] and [particulate matter] precursor emissions are affected by climate change through physical response (windblown dust), biological response (forest fires and vegetation type/distribution), and human response (energy generation). Most natural aerosol sources are controlled by climatic parameters like wind, moisture, and temperature; thus, human-induced climate change is expected to affect the natural aerosol burden” (p. 94). Since 2009, evidence linking climate change with increased airborne soil dust and associated health effects has continued to build, particularly for areas that are warmer and drier, such as the Southwest United States (see Section 5.5 on climate sensitive diseases).
Dust is a component of particulate matter in the air and is a key contributor to PM10, or particles 10 microns or smaller in diameter (NASEM, 2025). Major emission sources include soil entrainment into the air and anthropogenic activities, such as road and vehicle tire wear. Climate change in other world regions can impact dust concentrations over the United States, as dust from African deserts reaches the Caribbean and dust from Asia reaches the western United States and Hawaii (Rosas et al., 2025; Yu et al., 2019). Dust exposure can lead to a variety of deleterious health outcomes, including premature death, allergies, asthma attacks, and respiratory infection. A large scoping review found that among 204 epidemiological studies, over 80% reported positive associations between dust and adverse health outcomes (Lwin et al., 2023).
USGCRP (2016) concurred with the EPA (2009a) statement, finding that “climate-driven changes in meteorology can also lead to changes in naturally occurring emissions that influence air quality (for example, wildfires, wind-blown dust, and emissions from vegetation)” (p. 71). USGCRP (2023) also concluded in Key Message 14.1 that “the risk of exposure to airborne dust and wildfire smoke will increase with warmer and drier conditions in some regions (very likely, high confidence)” (pp. 14–5). These findings for the United States are consistent with the 2024 Report of the Lancet Countdown on Health and Climate Change that has a global scope. As part of that effort, the report found that “The hotter and drier weather conditions are increasingly favoring the occurrence of sand and dust storms” (Romanello et al., 2024). Recent studies build on earlier findings around how climate change affects dust concentrations in parts of the United States and provide more information about contributions of different climate-sensitive drivers. In a historical, observational study, Achakulwisut et al. (2018) found that drought and soil dryness were key drivers of increased fine dust over the Southwest United States, with 0.22–0.43 μg m-3 increase in fine dust for each unit increase in the 2-month Standardized Precipitation-Evapotranspiration Index, an indicator of soil dryness.
Climate change can also lead to more entrainment of dust from exposed lakebeds into the air (NASEM, 2020, 2025; West et al., 2023). Changing precipitation patterns, higher temperatures, persistent droughts, less water inflow from reduced snowpack, and increased evaporation, among other climate-sensitive conditions, can result in lower water levels for lakes in parts of the United States, such as the Great Salt Lake (Baxter and Butler, 2020). With a larger area of exposed lakebed, more dust can become entrained into the air, exposing people in nearby communities and across a broader area (Grineski et al., 2024). Lakebed dust often contains metals, pathogens, and other health-harmful agents (Putman et al., 2025). More exposure to lakebed dust could result in a variety of health outcomes, with potentially higher risks for children (Putman et al., 2025). Evidence on the relative contributions of climate change and water management practices to declining lake levels is limited and differs across lakes.
Indoor Air
Impacts of climate change on indoor air quality were not directly addressed in EPA (2009a). Indoor air quality is affected by outdoor air coming in, contaminants generated indoors (e.g., mold, dust mites, volatile organic compounds and other chemicals off gassing from building materials, indoor combustion), indoor temperatures, and other factors (NASEM, 2024d). As Americans spend most of their time indoors (Klepeis et al., 2001), changes in indoor air quality can have important effects on public health.
USGCRP (2016) identified climate impacts on indoor air quality as an emerging issue. The report highlighted multiple pathways through which climate change can negatively affect indoor air quality, including worsening outdoor air pollution that infiltrates indoors; altered patterns of indoor–outdoor air exchange; and more favorable conditions for growth and spread of pests, infectious agents, and disease vectors. Evidence showing negative impacts of climate change on indoor air quality has continued to build since the 2016 report, particularly related to indoor air quality during wildfire smoke events, mold driven by building dampness, and climate sensitivity of pollutants originating indoors.
Indoor exposure to wildfire smoke is a concern, especially as common public health messaging during wildfire smoke events is to stay indoors. USGCRP (2023) found that “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” (pp. 14–23). Recent studies show that volatile organic compounds from wildfire smoke can persist indoors for days after the smoke event (Dresser et al., 2024; Li et al., 2023), and that smoke and other forms of outdoor air pollution increase indoor PM2.5 levels, particularly in lower income areas (Krebs and Neidell, 2024). Wildfire smoke exposure and increased indoor crowding to avoid outdoor smoke is also associated with increased risk of infection from viruses (Arregui-García et al., 2025; Mahendran et al., 2025; Orr et al., 2025), such as influenza and COVID-19.
Climate change can also influence indoor air quality from pollutants originating indoors. Rain, flooding, and humidity changes affect building dampness, leading to mold and other microbial agents that increase risk of allergic rhinitis, asthma, and other respiratory conditions (Eguiluz-Gracia et al., 2020; WHO, 2009). Increased temperature and humidity can alter rates of chemical off-gassing from building and furniture materials, as well as chemical reaction rates, both of which influence levels of indoor air pollutants (Abbat and Wang, 2020; Salthammer and Morrison, 2022).
These potential impacts of climate change on indoor air quality likely differ across households and other buildings, driven by geographic and building-specific factors such as building codes, building materials, presence of air filtration devices, and climate control, making it challenging to evaluate the combined impact of the effects described here for different locations (Mansouri et al., 2022).
Air Quality Impacts from Co-emitted Pollutants
Fossil fuel combustion leads to emissions of both carbon dioxide (CO2) and co-emitted particulate matter and ozone and particulate precursors that directly affect air quality (NASEM, 2024a; USGCRP, 2023). Information about air quality and health co-benefits from reduced fossil fuel combustion was building in 2009 (Bell et al., 2008), and since that time has expanded with many quantified and monetized estimates of co-benefits from reduc-
ing fossil fuel combustion in power, residential, transportation, and other sectors (Balbus et al., 2015; Buonocore et al., 2016; Garcia et al., 2023; Levy et al., 2016; Sergi et al., 2020). For example, a review of the health impacts during the first 6 years of the Regional Greenhouse Gas Initiative—a policy that reduced GHG emissions in the Northeast and Atlantic regions of the United States—found that “These benefits include hundreds of avoided cases of premature deaths, heart attacks, asthma attacks, and hospital admissions, and tens of thousands of avoided cases of other health symptoms, lost work days, and restricted activities” (Manion et al., 2017, p. 38). Several studies project that future economic benefits associated with avoiding health effects of co-emitted pollutants are substantial (e.g., McDuffie et al., 2023; Shindell et al., 2024; West et al., 2023).
5.4 ADDITIONAL EXTREME WEATHER EVENTS
Extreme weather events contribute to injury and illness, exacerbate chronic disease, and affect mental health (Ebi et al., 2021; USGCRP, 2016). The occurrence of extreme weather events can also disrupt critical infrastructure and health care systems, reducing access to care, disrupting supply chains, and contributing to mortality (Salas et al., 2024).
Chapter 3 discusses the ways that climate change is affecting extreme weather. This section addresses health effects associated with extreme weather events, noting that it is challenging to attribute health impacts from individual weather events to climate change. Determining how climate change influences any single weather or climate event requires accounting for multiple natural and human factors (NASEM, 2016), and the health effects associated with any event also can be affected by multiple factors.
Droughts
Droughts are projected to become more frequent, longer lasting, and more severe across some regions of the United States (Martin et al., 2020; Overpeck and Udall, 2020; Tripathy et al., 2023). Research has linked drought to health consequences including higher risks of respiratory, cardiovascular, and all-cause mortality in several U.S. regions, particularly among older adults, women, and rural residents (Abadi et al., 2022; Gwon et al., 2023, 2024, 2025; Salvador et al., 2023). Drought compounds the health risks of other extreme events such as heat waves and dust storms (Leeper et al., 2025) and combined drought–heat events have been associated with increased mortality in people with chronic lung disease (Rau et al., 2025). Beyond physical health, drought contributes to mental and occupational health risks, with studies showing greater stress among farmers during drought years (Berman et al., 2021). Climate change and drought also impact water quality (see discussion in Chapter 6).
Hurricanes
Globally, the share of hurricanes reaching the most intense categories has increased over the past four decades, and although landfalls in the United States have not increased, there is emerging evidence that U.S. hurricanes are moving more slowly at landfall, producing heavy rainfall, damaging wind, and coastal flooding (see discussion in Chapter 3). Several studies have analyzed the mortality associated with hurricanes. For example, an analysis of two approaches found that Hurricane Maria was responsible for 1,191 excess deaths using census population data, and 2,975 excess deaths (95% CI [2,658, 3,290]) from September 2017 to February 2018 when accounting for demographic shifts that had also taken place over that time (Santos-Burgoa et al., 2018). Effects can persist for a prolonged period after the event itself; a modeling study examining longer-term, indirect effects of tropical cyclones on mortality in the United States estimated that the average tropical storm contributed 7,000 to 11,000 excess deaths (Young and Hsiang, 2024). The study also examined 501 historical storms between 1930 and 2015 and estimated a tropical cyclone-related mortality burden of 3.2–5.1% of all deaths in the Atlantic coastal region between 1930 and 2015 (Young and Hsiang, 2024). Evidence shows an increase in the average number of deaths per tropical cyclone in the United States since 2001, due to a combination of storm factors, shifting of population spatial distribution towards coastal areas, and demographic trends. Young and Hsiang (2024) found no evidence of adaptation reducing the deadliness of these storms.
Floods
Floods are associated with human health impacts, as described below. Many non-climate factors contribute to the risks to human health from floods, such as emergency preparedness and response and the location and condition of infrastructure. Changes in heavy precipitation and sea level rise associated with climate change (see Chapter 3) may also contribute to the risk. A recent global analysis of flood fatalities by Jonkman et al. (2024) found that no trend in flood-related mortality has been observed.
Extreme rainfall and flooding have been linked with hospital admissions and adverse health outcomes, including increased risk of injury, infectious diseases, increased morbidity and mortality from cardiovascular disease and other causes (Aggarwal et al., 2025; He et al., 2024; Lynch et al., 2025; Wettstein et al., 2025). Floods can also damage or impede access to critical infrastructure including hospitals, disrupt medical supply chains, and cut people off from care (Wu et al., 2024). For example, a Veterans Affairs hospital closed for six months after Hurricane Sandy, and the study found that the “temporary period of decreased access to health care services was associated with increased rates of uncontrolled hypertension, but not with increased rates of uncontrolled diabetes or hyperlipidemia, more than 1 year after the Manhattan VA facility reopened” (Baum et al., 2019, p. 2 of 13). More than 700 hazardous waste sites are located in high-risk flood zones, increasing concerns about exposure to toxic chemicals (GAO, 2019). Beyond physical illness and death, exposure to floods can contribute to adverse pregnancy outcomes and leave lasting mental health impacts, especially for children (Wu et al., 2024).
5.5 CLIMATE-SENSITIVE DISEASES
EPA (2009a) noted that many human diseases were sensitive to weather and the USGCRP (2016) report on climate and health subsequently stated with high confidence levels that climate change is harming human health by increasing morbidity and mortality. As discussed above, exposure to increased heat and worsened air quality contributes to negative health outcomes. The effects of changing climate on the distribution of vector-borne disease effects are briefly described in this section. The effects of exposure to changing temperatures, weather events, increased PM2.5, and allergens on a variety of chronic and noncommunicable diseases are also briefly discussed.
Vector-Borne Diseases
Climate suitability for various climate-sensitive pathogens and disease vectors has increased since EPA (2009a). Ticks can carry many diseases, including alpha-gal syndrome, Lyme disease, Babesiosis, Rocky Mountain spotted fever, and others. An increase in the geographic distribution of tick-borne diseases, anticipated in the 2009 report, has been observed and attributed to climate warming. Persistently warming temperatures may not only expand their geographic range but also extend their active season (USGCRP, 2016). One example is the lone star tick, which carries alpha-gal syndrome (inducing meat allergy) and has dramatically increased its U.S. distribution due to warming (Molaei et al., 2019). Similarly, Lyme disease—another tick-borne disease described in the 2009 EPA report—has expanded its range and activity due to climate warming with expansion in the northern United States and decreased southern activity, recognizing also that multiple factors interact to drive disease transmission (Couper et al., 2021; Kugeler et al., 2015; USGCRP, 2016). Lyme disease case report maps from the Centers for Disease Control and Prevention show range expansion from 1995 to 20232 with Lyme disease cases increasing from approximately 23,000 in 2002 to more than 62,000 in 2022 (CDC, 2004; Kugeler et al., 2024). Couper et al. (2021) examined Lyme disease incidence and found that the clearest climate–Lyme disease signal was observed in the Northeast United States, where rising annual temperatures were associated with higher incidence and a high-emissions scenario projected that there could be an estimated 23,619 ± 21,607 additional cases by 2050; however, this projection had uncertainty and no significant increases in disease incidence were projected for other regions. Couper et al. (2021) also found no significant increases or decreases in Lyme disease across any region under a
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2 See https://www.cdc.gov/lyme/data-research/facts-stats/lyme-disease-case-map.html (accessed September 8, 2025).
moderate-emissions scenario, underscoring the multifactor and regionally variable relationship between climate change and Lyme disease dynamics.
Dengue, a mosquito-borne viral disease, has increased in geographic range since EPA (2009a), now appearing in non-travelers in Texas, Florida, Arizona, California, and Hawaii.3 This shift coincides with increased mosquito vector activity due to climate warming (Ebi and Nealon, 2016). With rising temperatures, the aggressive Asian tiger mosquito (Aedes albopictus) has expanded in the United States, raising concerns about potential outbreaks of diseases from Chikungunya and Zika viruses (Rochlin et al., 2013). Mordecai et al. (2019) has described how temperature affects both mosquito biting behavior and transmission of pathogens. West Nile disease, which is also mosquito-borne, has also been seen in the United States since the 2009 report, related to changes in precipitation and heat (Hahn et al., 2015).
Fungal Diseases
Fungal infection is expanding across the United States, particularly in western states (Salazar-Hamm and Torres-Cruz, 2024). Soil dust contains a variety of microorganisms, including bacteria and fungi. Recent studies show association between airborne soil dust or dust storms and Valley fever (Howard, Sayes et al., 2024; Tong et al., 2017). Valley fever, or coccidioidomycosis, is a fungal infection resulting from breathing Coccidioides fungal spores; it can cause fever, cough, fatigue, shortness of breath, and other symptoms; its incidence has increased from approximately 2,000 cases in 1998 to 21,000 cases in 20234 and its area of endemicity has expanded to include 12 states: Arizona, California, Colorado, Idaho, Kansas, Nebraska, Nevada, New Mexico, Oklahoma, Texas, Utah, and Washington (Gorris et al., 2019). In addition, changing weather patterns can affect Coccidioides growth and dispersal, as the fungus grows in the soil after heavy rainfall and disperses into the area in subsequent hot and dry conditions (Head et al., 2022).
Evidence also points to changes in geographical extent of histoplasmosis, the most frequent fungal respiratory infection in the United States The causative agent, Histoplasma capsulatum, is a dimorphic soil-based fungus endemic to the U.S. Midwest, Latin America, Africa, South Asia, and the Caribbean. Approximately 60–90% of people living in areas surrounding the Ohio and Mississippi river valleys have been exposed to Histoplasma.5 Symptoms include fever, chills, headache, muscle aches, fatigue, and cough.6 Histoplasmosis affects people who are immunosuppressed more severely. While histoplasmosis is far less studied than Valley fever, the area of endemicity in the United States is spreading northwest to Minnesota, Wisconsin, Michigan, Montana, and Nebraska (Hepler et al., 2022; Maiga et al., 2018).
Antimicrobial Resistance
EPA (2009a) did not address antimicrobial resistance as a climate-sensitive issue. Antimicrobial resistance causes significant global morbidity, with projections estimating 8.2 million annual deaths associated with antimicrobial resistance by 2050 (Naghavi et al., 2024). Several studies show that higher temperatures accelerate bacterial growth, mutation rates, and gene transfer, increasing resistance risk to antibiotics (McGough et al., 2020; Van Eldjik et al., 2024). Heat waves and droughts concentrate antibiotics and resistant bacteria in water systems, enhancing opportunities for developing resistance, while extreme rainfall spreads antibiotic-resistant pathogens from sewage and farms where antibiotics are used for animal health (MacFadden et al., 2018). Better understanding of this area and its potential intersections with climate change continues to be a topic of exploration.
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3 See https://www.cdc.gov/dengue/outbreaks/2024/index.html.
4 See reported cases of Valley fever at https://www.cdc.gov/valley-fever/php/statistics/index.html (accessed September 8, 2025).
5 See https://www.cdc.gov/histoplasmosis/php/statistics/?CDC_AAref_Val=https://www.cdc.gov/fungal/diseases/histoplasmosis/statistics.html.
6 See https://www.lung.org/lung-health-diseases/lung-disease-lookup/histoplasmosis/symptoms-diagnosis.
Waterborne Diseases
Heavy precipitation and drought are linked to waterborne disease outbreaks. Excessive rainfall mobilizes pathogens into water supplies, while droughts disrupt sanitation. Contrary to the 2009 report’s claim that flood-related infectious disease risks are low in high-income countries (EPA, 2009a), recent studies show heavy precipitation increases gastrointestinal illnesses in the United States (De Roos et al., 2020; Haley et al., 2024). Hurricanes and extreme weather events can also introduce pathogens into water systems via disrupted sanitation infrastructure. For example, Vibrio parahaemolyticus is a waterborne bacterium that causes seafood-associated diarrheal disease in the United States, while Vibrio vulnificus is also found in marine settings and causes serious wound infections and diarrheal illness. Increasing water temperatures and other changes to coastal waters caused by climate are predicted to enhance Vibrio replication with resultant increased infection from contaminated shellfish or wound exposure to contaminated water (Hayden et al., 2023; Schets et al., 2025; USGCRP, 2016). Extension northward along the East Coast and increases in the numbers of reported non-foodborne cases of Vibrio vulnificus since 1988 have been observed (Archer et al., 2023; Brumfield et al., 2025).
Moreover, as water temperatures rise and more humans use recreational water due to heat, increasing infections with Naegleria fowleri, a thermophilic ameba that causes meningoencephalitis, is possible as well as a northward expansion of this disease (Heilmann et al., 2024; USGCRP, 2016). The potential for exposure to toxins associated with harmful algal blooms is another potential impact on health (see Chapter 6).
Noncommunicable Diseases
Exposure to heat, ozone, particulate matter, and extreme events also impact several noncommunicable and chronic diseases associated with the cardiovascular, renal, and pulmonary systems. The potential impacts extend to psychological and mental health and nutrition, areas not addressed in EPA (2009a).
Cardiovascular health: Cardiovascular diseases are the world’s leading cause of disability and death. In 2022, 941,652 U.S. deaths were attributable to cardiovascular disease for all ages (Martin et al., 2025). Pollution has had a major impact on cardiovascular morbidity and mortality. Short-term variation in PM2.5 levels (from hours to days) is associated with increased risks of myocardial infarction, stroke, and death from cardiovascular disease (Rajagopalan and Landrigan, 2021). During heat waves, a meta-analysis of 266 papers found the risk of cardiovascular disease-related mortality increased by 11.7% (95% CI [9.3, 14.1%]) with the risk increasing as heat wave intensity increased (Liu et al., 2022). Both hot and cold temperature extremes increase risk; an analysis of cardiovascular-related deaths across 27 countries including the United States found that “hot days (above 97.5th percentile) and cold days (below 2.5th percentile) accounted for 2.2 (95% empirical CI [eCI], 2.1-2.3) and 9.1 (95% eCI, 8.9-9.2) excess deaths for every 1000 cardiovascular deaths, respectively” (Alahmad et al., 2023). There are several mechanisms for how extreme temperature impacts the cardiovascular system. Heat, for example, strains the cardiovascular system (e.g., dehydration, hypotension, tachycardia, electrolyte shifts). People with heart failure, coronary disease, or arrhythmia are at highest risk; certain drugs (e.g., β-blockers, antiplatelets) can impair heat loss or increase heat-myocardial infarction (MI) risk (Chen et al., 2022). The risk has often been found to be higher in susceptible subgroups, including older people, people with preexisting conditions, and those who are socioeconomically disadvantaged (Singh et al., 2024). Heat waves and factors such as ground-level ozone that worsen air quality are associated with higher rates of cardiovascular mortality (Kazi et al., 2024). Exposure to PM2.5 components, emissions from fossil fuels, and chemical combustion by anthropogenic sources (e.g., gas stations) are associated with increased hypertension (Chen et al., 2025; Xu et al., 2022), another risk factor for cardiovascular morbidity. It has been calculated that for every increment of 10 μg m–3 in PM2.5, the risk of myocardial infarction, stroke, or cardiovascular-related death increases 0.1–1%, and air pollution is estimated to contribute to 14% of all stroke-associated deaths (Rajagopalan and Landrigan, 2021; Verhoeven et al., 2021). A meta-analysis of 35 studies showed “increases in particulate matter concentration were associated with heart failure hospitalisation or death (PM2.5 2.12% per 10 μg/m3, 95% Confidence Interval 1.42-2.82; PM10 1.63% per 10 μg/m3, 95% Confidence Interval 1.20-2.07). Strongest associations were seen on the day of exposure, with more persistent effects
for PM2.5. In the USA, we estimate that a mean reduction in PM2.5 of 3.9 μg/m3 would prevent 7,978 heart failure hospitalisations and save a third of a billion U.S. dollars a year” (Shah et al., 2013, p. 1,039). Air pollution has also been associated with an increased risk of atrial fibrillation and ventricular arrhythmias (Peralta et al., 2020).
Renal health: Extreme temperatures can increase the risk of heat stroke and dehydration leading to heat exhaustion, hypotension, and acute kidney disease (Glaser et al., 2016). There has also been a recent increase in chronic kidney disease of unknown etiology in rural communities, particularly in outdoor working farmworkers, in different parts of the world with increased temperatures and decreased rainfall (Glaser et al., 2016). A current hypothesis suggests that heat along with occupational exposures plays a role in this recent epidemic of unexplained chronic kidney disease in MesoAmerica and Sri Lanka (Hansson et al., 2023; Johnson et al., 2019; Wijkström et al., 2013). Studies are now ongoing looking for the cause of unexplained increased renal disease in agricultural workers in the central valley of California, with a hypothesis that extreme heat is an underlying factor (Bragg-Gresham et al., 2020).
Pulmonary health: Climate change is associated with increased risk factors for respiratory health. This linkage was noted in EPA (2009a); however, the extent of impact was not fully explored. Air pollutants and extreme temperatures have been associated with an increased risk of chronic obstructive pulmonary disease (COPD) and asthma exacerbations and higher mortality rates from these respiratory diseases (Almetwally et al., 2020). Various components of air pollution have been associated with worsened lung function, asthma exacerbations, and COPD, including PM2.5, nitrogen dioxide, and tropospheric ozone (Vongelis et al., 2025). Wildfire smoke—particularly PM2.5—is strongly associated with increased emergency department visits and hospitalizations for respiratory illnesses such as asthma (McArdle et al., 2023). Particularly susceptible populations include children, adults over the age of 65, pregnant women, and people in areas where access to climate adaptation and health care may be compromised (Covert et al., 2023).
Psychological and mental health: The climate’s impact on mental health was not addressed in EPA (2009a). Extreme weather events have been associated with increased rates of anxiety, depression, and mental health disorders (Barbani, 2025). Air pollution (PM2.5) has been associated with increased prevalence of mood and psychotic disorders (e.g., schizophrenia) and suicide (Hoare et al., 2019; Kim et al., 2018; Newbury et al., 2019). Heat waves and high temperatures are associated with increased cases of suicide, hospital visits for mental health issues, crime, and violence (Choi et al., 2024; Mahendran et al., 2021; Nori-Sarma et al., 2022; Thompson et al., 2023). Sleep (particularly deep sleep) is also affected by ambient temperature and humidity, which may be impacted by climate warming, and can further exacerbate mental health declines (Li et al., 2025; Okamoto-Mizuno and Mizuno, 2012). Growing evidence also shows that long-term exposure to air pollution may play a role in dementia (Abolhasani et al., 2023; Best Rogowski et al., 2025). Studies have shown increased anxiety among children and youth around climate change. A global study of 10,000 youth and young adults in 10 countries found that 59% reported being extremely worried about climate change, and more than 50% reported negative emotions such as sadness, anxiety, anger, helplessness, and powerlessness (Hickman et al., 2021). Lastly, one should note the many psychological impacts from events that displace people from their homes (Bellizzi et al., 2023).
Nutrition and food safety: Impacts on nutrition were also not addressed in EPA (2009a). There are several pathways by which climate change impacts nutrition and food safety. The rise in temperatures and variability in precipitation amount and intensity have negatively affected agricultural production. (See Chapter 6 for more detailed discussion of food production and agriculture.) Such events strain global food systems (Romanello et al., 2021) and can affect crop frequency (number of production seasons per year) and caloric yields (crop yield in calories produced per acre). Food safety is also threatened by increased warming. The IPCC report (2022a) noted that increasing temperature accelerated the growth of foodborne pathogens like Salmonella and Campylobacter. Warmer conditions extend the survival of pathogens in soil, and water and food supplies. Alterations to marine ecosystems can disrupt marine food supplies in some communities (USGCRP, 2023). Elevated atmospheric CO2 concentration is associated with lower nutritional value in staple crops. Myers et al. tested several crops from
different countries including the United States and found a significant decrease in the concentration of iron and zinc in C3 (cool season) grasses and legumes (Myers et al., 2014). Zinc and iron deficiencies increase the risk of infections, diarrhea, and anemia. These sequelae would be more impactful in low resourced settings globally or in malnourished children in the United States. Although CO2 concentrations can increase plant growth with potential effects on crop yields (see discussion in Chapter 6), the IPCC 2023 Synthesis Report noted that these benefits are often offset by climate related stresses such as heat, drought, and nutrient limitations.
Diseases Caused by Airborne Allergens
As reported in EPA (2009a), multiple factors influence allergen levels and health consequences such as allergies and asthma, including changes to CO2 and climate that affect plant growth, distribution, and allergenicity. Evidence since 2009 continues to indicate that GHGs and associated climate changes affect airborne allergens in ways that can contribute to allergies and asthma, while recognizing that development of such conditions depends not only on environmental exposures but also on individual and genetic factors (Dharmage et al., 2019).
USGCRP (2023) reported that factors such as rising temperatures, changes in precipitation, elevated CO2, and higher ozone levels are affecting pollen, with effects than can include extending pollen seasons, boosting pollen levels, and broadening the geographic range of allergenic plants, while enhancing pollen allergenicity (Agache et al., 2024; Anderegg et al., 2021; Epstein et al., 2025; Lee et al., 2023; Paudel et al., 2021; USGCRP, 2023; WHO, 2018; Zhang and Steiner, 2022). For instance, ragweed pollen season in parts of the United States grew longer by as much as 13 to 27 days between 1995 and 2009 (Ziska et al., 2011). Increases in rainfall and flooding associated with climate change can also foster mold growth and facilitate the introduction of new allergenic species (Epstein et al., 2025). Extreme weather can worsen respiratory risks. Thunderstorms can rupture pollen grains, leading to “thunderstorm asthma,” associated with increases in asthma events (Agache et al., 2024; Beggs, 2024; D’Amato et al., 2016; Mampage et al., 2022), with a recent analysis finding a 24% increased risk (95% CI [13, 36%]) (Makrufardi et al., 2023). Post-disaster studies of hurricanes have found increased mold and endotoxin exposures and mold reactivity, especially among those with asthma (Chew et al., 2006; Rao et al., 2007; Sampath et al., 2023). Wildfire smoke, dust, and sandstorms can alter pollen structure, increasing allergenicity and inflammation and amplifying allergic and respiratory diseases (WHO, 2025). Drier, hotter conditions may intensify airway inflammation, while air pollution increases allergic inflammation and susceptibility to viral infections (Burbank, 2025; Edwards et al., 2025; Wright and Demain, 2024).
Elevated atmospheric CO2 concentration leads to more vigorous growth in many plant species, which often results in increased pollen production. Research on allergenic plants, such as ragweed, has demonstrated that higher CO2 levels cause them to produce more pollen (Choi et al., 2021). These environmental changes have health consequences. Pollen and mold exposure contribute to allergic rhinitis and asthma (Sapkota et al., 2019), with a limited number of studies identifying relationships with hospitalizations and mortality in people with underlying COPD (Idrose et al., 2022). A meta-analysis found that each exposure increase of 10 grass pollen grains per m3 was associated with a 1.88% increase in asthma emergency department visits (95% CI [0.94, 2.82%]) (Erbas et al., 2018), while high grass and ragweed pollen concentrations were associated with chronic respiratory mortality in a Michigan study (Larson et al., 2025). Elevated fungal spores have also been linked to increased asthma medication use, symptom severity, and hospitalizations (D’Amato et al., 2020).
5.6 COMPOUNDING AND CASCADING EFFECTS
Understanding the compounding effects of climate change is important because multiple hazards—such as heat waves, drought, and wildfires—can occur simultaneously or sequentially in a single individual’s life, amplifying both acute and chronic health risks (see Chapter 3, Section 3.7). Exposure to these compound events can increase morbidity and mortality above what would be expected from any individual event alone, due to synergistic stress on physical and mental health as well as health care infrastructure. For example, simultaneous exposure to heat and air pollution such as wildfire smoke can exacerbate respiratory and cardiovascular illness leading to increased
hospitalizations for people with asthma (Anenberg et al., 2020; Chen et al., 2024; Jones-Ngo et al., 2025). At the same time, exposures to smoke and to extreme heat can aggravate psychological stress and other mental health outcomes (Eisenman and Galway, 2022; Nori-Sarma et al., 2022).
Beyond acute impacts, longitudinal studies underscore that repeated exposures to hazards including extreme weather events may have cumulative detrimental effects on long-term health outcomes, including increased risk for chronic diseases, mental health disorders, and early mortality (Leppold et al., 2022). Multiple exposures over a person’s lifetime can heighten the biological wear and tear known as “allostatic load” and overwhelm coping mechanisms. In addition, multiple or consecutive extreme events experienced by a region can overwhelm health care resources. As a result, the intersecting and repeated challenges of climate change over years or decades pose a threat to health that exceeds the effects of isolated exposures.
