3.1 Introduction

  • Neal Fann
    U.S. Environmental Protection Agency
  • Terry M. Brennan
    Camroden Associates Inc.
  • Patrick Dolwick
    U.S. Environmental Protection Agency
  • Janet L. Gamble
    U.S. Environmental Protection Agency
  • Vito Ilacqua
    U.S. Environmental Protection Agency
  • Laura Kolb
    U.S. Environmental Protection Agency
  • Christopher G. Nolte
    U.S. Environmental Protection Agency
  • Tanya L. Spero
    U.S. Environmental Protection Agency
  • Lewis H. Ziska
    U.S. Department of Agriculture

Changes in the climate affect the air we breathe, both indoors and outdoors. Taken together, changes in the climate affect air quality through three pathways—via outdoor air pollution, aeroallergens , and indoor air pollution. The changing climate has modified weather patterns, which in turn have influenced the levels and location of outdoor air pollutants such as ground-level ozone ( O3 ) and fine particulate matter.1,2,3,4 Increasing carbon dioxide ( CO2 ) levels also promote the growth of plants that release airborne allergens (aeroallergens). Finally, these changes to outdoor air quality and aeroallergens also affect indoor air quality as both pollutants and aeroallergens infiltrate homes, schools, and other buildings.

Climate change influences outdoor air pollutant concentrations in many ways (Figure 3.1). The climate influences temperatures, cloudiness, humidity, the frequency and intensity of precipitation, and wind patterns,5 each of which can influence air quality. At the same time, 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). Over longer time scales, human responses to climate change may also affect the amount of energy that humans use, as well as how land is used and where people live. These changes would in turn modify emissions (depending on the fuel source) and thus further influence air quality.6,7 Some air pollutants such as ozone, sulfates, and black carbon also cause changes in climate.8 However, this chapter does not consider the climate effects of air pollutants, remaining focused on the health effects resulting from climate-related changes in air pollution exposure.

Poor air quality, whether outdoors or indoors, can negatively affect the human respiratory and cardiovascular systems. Outdoor ground-level ozone and particle pollution can have a range of adverse effects on human health. Current levels of ground-level ozone have been estimated to be responsible for tens of thousands of hospital and emergency room visits, millions of cases of acute respiratory symptoms and school absences, and thousands of premature deaths each year in the United States.9,10 Fine particle pollution has also been linked to even greater health consequences through harmful cardiovascular and respiratory effects.11

Allergies caused by ragweed

Higher pollen concentrations and longer pollen seasons can increase allergic sensitization and asthma episodes.

A changing climate can also influence the level of aeroallergens such as pollen, which in turn adversely affect human health. Rising levels of CO2 and resulting climate changes alter the production, allergenicity (a measure of how much particular allergens, such as ragweed, affect people), distribution, and seasonal timing of aeroallergens. These changes increase the severity and prevalence of allergic diseases in humans. Higher pollen concentrations and longer pollen seasons can increase allergic sensitization and asthma episodes and thereby limit productivity at work and school.

Finally, climate change may alter the indoor concentrations of pollutants generated outdoors (such as ground-level ozone), particulate matter, and aeroallergens (such as pollen). Changes in the climate may also increase pollutants generated indoors, such as mold and volatile organic compounds. Most of the air people breathe over their lifetimes will be indoors, since people spend the vast majority of their time in indoor environments. Thus, alterations in indoor air pollutant concentrations from climate change have important health implications.


Figure 3.1: Climate Change and Health—Outdoor Air Quality

Figure 3.1: Climate Change and Health—Outdoor Air Quality
This conceptual diagram for an outdoor air quality example illustrates the key pathways by which humans are exposed to health threats from climate drivers, and potential resulting health outcomes (center boxes). These exposure pathways exist within the context of other factors that positively or negatively influence health outcomes (gray side boxes). Key factors that influence vulnerability for individuals are shown in the right box, and include social determinants of health and behavioral choices. Key factors that influence vulnerability at larger scales, such as natural and built environments, governance and management, and institutions, are shown in the left box. All of these influencing factors can affect an individual’s or a community’s vulnerability through changes in exposure, sensitivity , and adaptive capacity and may also be affected by climate change. See Chapter 1: Introduction for more information.

3.2 Climate Impacts on Outdoor Air Pollutants and Health

Changes in the climate affect air pollution levels.8,12,13,14,15,16,17,18,19,20,21,22 Human-caused climate change has the potential to increase ozone levels,1,4 may have already increased ozone pollution in some regions of the United States,3 and has the potential to affect future concentrations of ozone and fine particles ( particulate matter smaller than 2.5 microns in diameter, referred to as PM 2.5).2,7 Climate change and air quality are both affected by, and influence, several factors; these include the levels and types of pollutants emitted, how land is used, the chemistry governing how these pollutants form in the atmosphere, and weather conditions.

Ground-Level Ozone

Ozone levels and subsequent ozone-related health impacts depend on 1) the amount of pollutants emitted that form ozone, and 2) the meteorological conditions that help determine the amount of ozone produced from those emissions. Both of these factors are expected to change in the future. The emissions of pollutants from anthropogenic (of human origin) sources that form ozone (that is, ozone “precursors”) are expected to decrease over the next few decades in the United States.23 However, irrespective of these changes in emissions, climate change will result in meteorological conditions more favorable to forming ozone. Consequently, attaining national air quality standards for ground-level ozone will also be more difficult, as climate changes offset some of the improvements that would otherwise be expected from emissions reductions. This effect is referred to as the “climate penalty.”7,24

Meteorological conditions influencing ozone levels include air temperatures, humidity, cloud cover, precipitation, wind trajectories, and the amount of vertical mixing in the atmosphere.1,2,25,26 Higher temperatures can increase the chemical rates at which ozone is formed and increase ozone precursor emissions from anthropogenic sources and biogenic(vegetative)sources. Lower relative humidity reduces cloud cover and rainfall, promoting the formation of ozone and extending ozone lifetime in the atmosphere. A changing climate will also modify wind patterns across the United States, which will influence local ozone levels. Over much of the country, the worst ozone episodes tend to occur when the local air mass does not change over a period of several days, allowing ozone and ozone precursor emissions to accumulate over time.27,28 Climate change is already increasing the frequency of these types of stagnation events over parts of the United States,3 and further increases are projected.29 Ozone concentrations near the ground are strongly influenced by upward and downward movement of air (“vertical mixing”). For example, high concentrations of ozone near the ground often occur in urban areas when there is downward movement of air associated with high pressure (“subsidence”), reducing the extent to which locally emitted pollutants are diluted in the atmosphere.30 In addition, high concentrations of ozone can occur in some rural areas resulting from downward transport of ozone from the stratosphere or upper troposphere to the ground.31

Aside from the direct meteorological influences, there are also indirect impacts on U.S. ozone levels from other climate-influenced factors. For instance, higher water vapor concentrations due to increased temperatures will increase the natural rate of ozone depletion, particularly in remote areas,32 thus decreasing the baseline level of ozone. Additionally, potential climate-driven increases in nitrogen oxides (NOx) created by lightning or increased exchange of naturally produced ozone in the stratosphere to the troposphere could also affect ozone in those areas of the country most influenced by background ozone concentrations.33 Increased occurrences of wildfires due to climate change can also lead to increased ozone concentrations near the ground.34 

There is natural year-to-year variability in temperature and other meteorological factors that influence ozone levels.7 While global average temperature over 30-year climatic timescales is expected to increase, natural interannual variability will continue to play a significant role in year-to-year changes in temperature.35 Over the next several decades, the influence of climate change on meteorological parameters affecting average levels of ozone is expected to be smaller than the natural interannual variability.36

To address these issues, most assessments of climate impacts on meteorology and associated ozone formation concurrently simulate global and regional chemical transport over multiple years using “coupled” models. This approach can isolate the influence of meteorology in forming ozone from the effect of changes in emissions. The consensus of these model-based assessments is that accelerated rates of photochemical reaction, increased occurrence of stagnation events, and other direct meteorological influences are likely to lead to higher levels of ozone over large portions of the United States.8,14,16,17 At the same time, ozone levels in certain regions are projected to decrease as a result of climate change, likely due to localized increases in cloud cover, precipitation, and/or increased dilution resulting from deeper mixed layers. These climate-driven changes in projected ozone vary by season and location, with climate and air quality models showing the most consistency in ozone increases due to climate change in the northeastern United States.8,37

Generally, ozone levels will likely increase across the United States if ozone precursors are unchanged (see “Research Highlight: Ozone-Related Health Effects”) .4,7,8 This climate penalty for ozone will offset some of the expected health benefits that would otherwise result from the expected ongoing reductions of ozone precursor emissions, and could prompt the need for adaptive measures (for example, additional ozone precursor emissions reductions) to meet national air quality goals.

Air pollution epidemiology studies describe the relationship between a population’s historical exposure to air pollutants and the risk of adverse health outcomes. Populations exposed to ozone air pollution are at greater risk of dying prematurely, being admitted to the hospital for respiratory hospital admissions, being admitted to the emergency department, and suffering from aggravated asthma , among other impacts.38,39,40

Air pollution health impact assessments combine risk estimates from these epidemiology studies with modeled changes in future or historical air quality changes to estimate the number of air-pollution-related premature deaths and illness.41 Future ozone-related human health impacts attributable to climate change are projected to lead to hundreds to thousands of premature deaths, hospital admissions, and cases of acute respiratory illnesses per year in the United States in 2030.14,42,43,44,45,46

Health outcomes that can be attributed to climate change impacts on air pollution are sensitive to a number of factors noted above—including the climate models used to describe meteorological changes (including precipitation and cloud cover), the models simulating air quality levels (including wildfire incidence), the size and distribution of the population exposed, and the health status of that population (which influences their susceptibility to air pollution; see Ch. 1: Introduction).42,47,48,49   Moreover, there is emerging evidence that air pollution can interact with climate-related stressors such as temperature to affect the human physiological response to air pollution.39,42,50,51,52,53,54,55 For example, the risk of dying from exposure to a given level of ozone may increase on warmer days.51

Particulate Matter

Particulate matter (PM) is a complex mixture of solid- or liquid-phase substances in the atmosphere that arise from both natural and human sources. Principal constituents of PM include sulfate, nitrate, ammonium, organic carbon, elemental carbon, sea salt, and dust. These particles (also known as aerosols) can either be directly emitted or can be formed in the atmosphere from gas-phase precursors. PM smaller than 2.5 microns in diameter (PM2.5) is associated with serious chronic and acute health effects, including lung cancer, chronic obstructive pulmonary disease ( COPD ), cardiovascular disease, and asthma development and exacerbation.11 The elderly are particularly sensitive to short-term particle exposure, with a higher risk of hospitalization and death.56,57

As is the case for ozone, atmospheric PM2.5 concentrations depend on emissions and on meteorology. Emissions of sulfur dioxide (SO2), NOx, and black carbon are projected to decline substantially in the United States over the next few decades due to regulatory controls,58,59,60,61 which will lead to reductions in sulfate and nitrate aerosols.

Climate change is expected to alter several meteorological factors that affect PM2.5, including precipitation patterns and humidity, although there is greater consensus regarding the effects of meteorological changes on ozone than on PM2.5.2 Several factors, such as increased humidity, increased stagnation events, and increased biogenic emissions are likely to increase PM2.5 levels, while increases in precipitation, enhanced atmospheric mixing, and other factors could decrease PM2.5 levels.2,8,37,62 Because of the strong influence of changes in precipitation and atmospheric mixing on PM2.5 levels, and because there is more variability in projected changes to those variables, there is no consensus yet on whether meteorological changes will lead to a net increase or decrease in PM2.5 levels in the United States.2,8,17,21,22,62,63

As a result, while it is clear that PM2.5 accounts for most of the health burden of outdoor air pollution in the United States,10 the health effects of climate-induced changes in PM2.5 are poorly quantified. Some studies have found that changes in PM2.5 will be the dominant driver of air quality-related health effects due to climate change,44 while others have suggested a potentially more significant health burden from changes in ozone.50

PM resulting from natural sources (such as plants, wildfires, and dust) is sensitive to daily weather patterns, and those fluctuations can affect the intensity of extreme PM episodes (see also Ch. 4: Extreme Events, Section 4.6).8 Wildfires are a major source of PM, especially in the western United States during summer.64,65,66 Because winds carry PM2.5 and ozone precursor gases, air pollution from wildfires can affect people even far downwind from the fire location.35,67 PM2.5 from wildfires affects human health by increasing the risk of premature death and hospital and emergency department visits.68,69,70

Climate change has already led to an increased frequency of large wildfires, as well as longer durations of individual wildfires and longer wildfire seasons in the western United States.71 Future climate change is projected to increase wildfire risks72,73 and associated emissions, with harmful impacts on health.74 The area burned by wildfires in North America is expected to increase dramatically over the 21st century due to climate change.75,76 By 2050, changes in wildfires in the western United States are projected to result in 40% increases of organic carbon and 20% increases in elemental carbon aerosol concentrations.77 Wildfires may dominate summertime PM2.5 concentrations, offsetting even large reductions in anthropogenic PM2.5 emissions.22

Likewise, dust can be an important constituent of PM, especially in the southwest United States. The severity and spatial extent of drought has been projected to increase as a result of climate change,78 though the impact of increased aridity on airborne dust PM has not been quantified (see Ch. 4. Extreme Events).2

3.3 Climate Impacts on Aeroallergens and Respiratory Diseases

Climate change may alter the production, allergenicity, distribution, and timing of airborne allergens (aeroallergens). These changes contribute to the severity and prevalence of allergic disease in humans. The very young, those with compromised immune systems, and the medically uninsured bear the brunt of asthma and other allergic illnesses. While aeroallergen exposure is not the sole, or even necessarily the most significant factor associated with allergic illnesses, that relationship is part of a complex pathway that links aeroallergen exposure to the prevalence of allergic illnesses, including asthma episodes.81,82 On the other hand, climate change may reduce adverse allergic and asthmatic responses in some areas. For example, as some areas become drier, there is the potential for a shortening of the pollen season due to plant stress.

Aeroallergens and Rates of Allergic Diseases in the United States 

Aeroallergens are substances present in the air that, once inhaled, stimulate an allergic response in sensitized individuals. Aeroallergens include tree, grass, and weed pollen; indoor and outdoor molds; and other allergenic proteins associated with animal dander, dust mites, and cockroaches.83 Ragweed is the aeroallergen that most commonly affects persons in the United States.84

Girl with inhaler

Nearly 6.8 million children in the United States are affected by asthma , making it a major chronic disease of childhood.

Allergic diseases develop in response to complex and multiple interactions among both genetic and non-genetic factors, including a developing immune system, environmental exposures (such as ambient air pollution or weather conditions), and socioeconomic and demographic factors.85,86,87 Aeroallergen exposure contributes to the occurrence of asthma episodes, allergic rhinitis or hay fever, sinusitis, conjunctivitis, urticaria (hives), atopic dermatitis or eczema, and anaphylaxis (a severe, whole-body allergic reaction that can be life-threatening).84,88 Allergic illnesses, including hay fever, affect about one-third of the U.S. population, and more than 34 million Americans have been diagnosed with asthma.81 These diseases have increased in the United States over the past 30 years (see Ch. 1 Introduction). The prevalence of hay fever has increased from 10% of the population in 1970 to 30% in 2000.84 Asthma rates have increased from approximately 8 to 55 cases per 1,000 persons to approximately 55 to 90 cases per 1,000 persons over that same time period;89 however, there is variation in reports of active cases of asthma as a function of geography and demographics.90

Climate Impacts on Aeroallergen Characteristics 

Climate change contributes to changes in allergic illnesses as greater concentrations of CO2 , together with higher temperatures and changes in precipitation, extend the start or duration of the growing season, increase the quantity and allergenicity of pollen, and expand the spatial distribution of pollens.84,91,92,93,94

Historical trends show that climate change has led to changes in the length of the growing season for certain allergenic pollens. For instance, the duration of pollen release for common ragweed (Ambrosia artemisiifolia) has been increasing as a function of latitude in recent decades in the midwestern region of North America (see Figure 3.4). Latitudinal effects on increasing season length were associated primarily with a delay in first frost during the fall season and lengthening of the frost-free period.95 Studies in controlled indoor environments find that increases in temperature and CO2 result in earlier flowering, greater floral numbers, greater pollen production, and increased allergenicity in common ragweed.96,97 In addition, studies using urban areas as proxies for both higher CO2 and higher temperatures demonstrate earlier flowering of pollen species, which may lead to a longer total pollen season.98,99,100


Figure 3.4: Ragweed Pollen Season Lengthens

Figure 3.4: Ragweed Pollen Season Lengthens
Ragweed pollen season length has increased in central North America between 1995 and 2011 by as much as 11 to 27 days in parts of the United States and Canada, in response to rising temperatures. Increases in the length of this allergenic pollen season are correlated with increases in the number of days before the first frost. The largest increases have been observed in northern cities. (Figure source: Melillo et al. 2014. Photo credit: Lewis Ziska, USDA).35

For trees, earlier flowering associated with higher winter and spring temperatures has been observed over a 50-year period for oak.101 Research on loblolly pine (Pinus taeda) also demonstrates that elevated CO2 could induce earlier and greater seasonal pollen production.102 Annual birch (Betula) pollen production and peak values from 2020 to 2100 are projected to be 1.3 to 2.3 times higher, relative to average values for 2000, with the start and peak dates of pollen release advancing by two to four weeks.103

Climate Variability and Effects on Allergic Diseases

Climate change related alterations in local weather patterns, including changes in minimum and maximum temperatures and rainfall, affect the burden of allergic diseases.104,105,106 The role of weather on the initiation or exacerbation of allergic symptoms in sensitive persons is not well understood.86,107 So-called “thunderstorm asthma” results as allergenic particles are dispersed through osmotic rupture, a phenomenon where cell membranes burst. Pollen grains may, after contact with rain, release part of their cellular contents, including allergen-laced fine particles. Increases in the intensity and frequency of heavy rainfall and storminess over the coming decades is likely to be associated with spikes in aeroallergen concentrations and the potential for related increases in the number and severity of allergic illnesses.108,109

Potential non-linear interactions between aeroallergens and ambient air pollutants (including ozone, nitrogen dioxide, sulfur dioxide, and fine particulate matter) may increase health risks for people who are simultaneously exposed.87,88,106,108,110,111,112,113,114 In particular, pre-exposure to air pollution (especially ozone or fine particulate matter) may magnify the effects of aeroallergens, as prior damage to airways may increase the permeability of mucous membranes to the penetration of allergens, although existing evidence suggests greater sensitivity but not necessarily a direct link with ozone exposure.115 A recent report noted remaining uncertainties across the epidemiologic, controlled human exposure, and toxicology studies on this emerging topic.39

3.4 Climate Impacts on Indoor Air Quality and Health An Emerging Issue

Climate change may worsen existing indoor air problems and create new problems by altering outdoor conditions that affect indoor conditions and by creating more favorable conditions for the growth and spread of pests, infectious agents, and disease vectors that can migrate indoors.116 Climate change can also lead to changes in the mixing of outdoor and indoor air. Reduced mixing of outdoor and indoor air limits penetration of outdoor pollutants into the indoors, but also leads to higher concentrations of pollutants generated indoors since their dilution by outdoor air is decreased.

Moldy archway

Dampness and mold in U.S. homes are linked to approximately 4.6 million cases of worsened asthma .

Indoor air contains a complex mixture of chemical and biological pollutants or contaminants. Contaminants that can be found indoors include carbon monoxide ( CO ), fine particles ( PM 2.5), nitrogen dioxide, formaldehyde, radon, mold, and pollen. Indoor air quality varies from building to building and over the course of a day in an individual building.

Public and environmental health professionals have known for decades that poor indoor air quality is associated with adverse respiratory and other health effects.116,117,118,119,120,121 Since most people spend about 90% of their time indoors,122,123,124,125,126 much of their exposures to airborne pollutants (both those influenced by climate change and those driven by other factors) happen indoors.

Outdoor Air Changes Reflected in Indoor Air

Indoor air pollutants may come from indoor sources or may be transported into the building with outdoor air.127,128 Indoor pollutants of outdoor origin may include ozone, dust, pollen, and fine PM (PM2.5). Even if a building has an outdoor air intake, some air will enter the building through other openings, such as open windows or under doors, or through cracks in the buildings, bypassing any filters and bringing outdoor air pollutants inside.129 If there are changes in airborne pollutants of outdoor origin, such as pollen and mold (see Section 3.4) and fine PM from wildfires (see “Climate Impacts on Aeroallergen Characteristics”), there will be changes in indoor exposures to these contaminants. Although indoor fine PM levels from wildfires are typically lower than outdoors (about 50%), because people spend most of their time indoors, most of their exposure to and health effects from wildfire particles (about 80%) will come from particles inhaled indoors.130 Climate-induced changes in indoor-outdoor temperature differences may somewhat reduce the overall intake of outdoor pollutants into buildings for certain regions and seasons (see “Research Highlight: Residential Infiltration and Indoor Air”).131

Most exposures to high levels of ozone occur outdoors; however, indoor exposures, while lower, occur for much longer time periods. Indoors, ozone concentrations are usually about 10% to 50% of outdoor concentrations; however, since people spend most of their time indoors, most of their exposure to ozone is from indoor air.130 Thus, about 45% to 75% of a person’s overall exposure to ozone will occur indoors.132 About half of the health effects resulting from any outdoor increases in ozone (see “Aeroallergens and Rates of Allergic Diseases in the United States”) will be due to indoor ozone exposures.130 The elderly and children are particularly sensitive to short-term ozone exposure; however, they may spend even more time indoors than the general population and consequently their exposure to ozone is at lower levels for longer periods than the general public.133,134 In addition, ozone entering a building reacts with some organic compounds to produce secondary indoor air pollutants. These reactions lower indoor ozone concentrations but introduce new indoor air contaminants, including other respiratory irritants.135

Climate-related increases in droughts and dust storms may result in increases in indoor transmission of dust-borne pathogens, as the dust penetrates the indoor environment. Dust contains particles of biologic origin, including pollen and bacterial and fungal spores. Some of the particles are allergenic.136 Pathogenic fungi and bacteria can be found in dust both indoors and outdoors.137 For example, in the southwestern United States, spores from the fungi Coccidiodes, which can cause valley fever, are found indoors.138 The geographic range where Coccidiodes is commonly found is increasing. Climate changes, including increases in droughts and temperatures, may be contributing to this spread and to a rise in valley fever (see Ch. 4: Extreme Events).

Legionnaires’ disease is primarily contracted from aerosolized water contaminated with Legionella bacteria.139 Legionella bacteria are naturally found outdoors in water and soil; they are also known to contaminate treated water systems in buildings,140 as well as building cooling systems such as swamp coolers or cooling towers.141 Legionella can also be found indoors inside plumbing fixtures such as showerheads, faucets, and humidifiers.142,143 Legionella can cause outbreaks of a pneumonia known as Legionnaire’s disease, which is a potentially fatal infection.144 Exposure can occur indoors when a spray or mist of contaminated water is inhaled, including mist or spray from showers and swamp coolers.145 The spread of Legionella bacteria can be affected by regional environmental factors.116 Legionnaires’ disease is known to follow a seasonal pattern, with more cases in late summer and autumn, potentially due to warmer and damper conditions.146,147 Cases of Legionnaires’ disease are rising in the United States, with an increase of 192% from 2000 to 2009.148,149 If climate change results in sustained higher temperatures and damper conditions in some areas, there could be increases in the spread and transmission of Legionella.


Contaminants Generated Indoors

Although research directly linking indoor dampness and climate change is not available, information on building science, climate change, and outdoor environmental factors that affect indoor air quality can be used to project how climate change may influence indoor environments.130 Climate change could result in increased indoor dampness in at least two ways: 1) if there are more frequent heavy precipitation events and other severe weather events (including high winds, flooding, and winter storms) that result in damage to buildings, allowing water or moisture entry; and 2) if outdoor humidity rises with climate change, indoor humidity and the potential for condensation and dampness will likely rise. Outdoor humidity is usually the largest contributor to indoor dampness on a yearly basis.127 Increased indoor dampness and humidity will in turn increase indoor mold, dust mites, bacteria, and other bio-contamination indoors, as well as increase levels of volatile organic compounds (VOCs) and other chemicals resulting from the off-gassing of damp or wet building materials.116,119,150 Dampness and mold in U.S. homes are linked to approximately 4.6 million cases of worsened asthma and between 8% and 20% of several common respiratory infections, such as acute bronchitis.151,152 If there are climate-induced rises in indoor dampness, there could be increases in adverse health effects related to dampness and mold, such as asthma exacerbation.

Additionally, power outages due to more frequent extreme weather events such as flooding could lead to a number of health effects (see Ch. 4: Extreme Events). Heating, ventilation, and air conditioning (HVAC) systems will not function without power; therefore, many buildings could have difficulty maintaining indoor temperatures or humidity. Loss of ventilation, filtration, air circulation, and humidity control can lead to indoor mold growth and increased levels of indoor contaminants,153 including VOCs such as formaldehyde.119,154,155,156 Power outages are also associated with increases in hospital visits from carbon monoxide (CO) poisoning, primarily due to the incorrect use of backup and portable generators that contaminate indoor air with carbon monoxide.135 Following floods, CO poisoning is also associated with the improper indoor use of wood-burning appliances, and other combustion appliances designed for use outdoors.157 There were at least nine deaths from carbon monoxide poisoning related to power outages from 2000 to 2009.158

Climate factors can influence populations of rodents that produce allergens and can harbor pathogens such as hantaviruses, which can cause Hantavirus Pulmonary Syndrome. Hantaviruses can be spread to people by rodents that infest buildings,159 and limiting indoor exposure is a key strategy to prevent the spread of hantavirus.160 Climate change may increase rodent populations in some areas, including indoors, particularly when droughts are followed by periods of heavy rain (see Ch. 4: Extreme Events) and with increases in temperature and rainfall.161 Also, extreme weather events such as heavy rains and flooding may drive some rodents to relocate indoors.162 Increases in rodent populations may result in increased indoor exposures to rodent allergens and related health effects.159,163,164 In addition, climate factors may also influence the prevalence of hantaviruses in rodents.163,164 This is a complex dynamic, because climate change may influence rodent populations, ranges, and infection rates.

3.5 Populations of Concern

Certain groups of people may be more susceptible to harm from air pollution due to factors including age, access to healthcare, baseline health status, or other characteristics.57 In the contiguous United States, Blacks or African-Americans, women, and the elderly experience the greatest baseline risk from air pollution.165 The young, older adults, asthmatics, and people whose immune systems are compromised are more vulnerable to indoor air pollutants than the general population.166 Lower  socioeconomic status and housing disrepair have been associated with higher indoor allergen exposures, though higher-income populations may be more exposed to certain allergens such as dust mites.167,168

Nearly 6.8 million children in the United States are affected by asthma, making it a major chronic disease of childhood.169 It is also the main cause of school absenteeism and hospital admissions among children.83 In 2008, 9.3% of American children age 2 to 17 years were reported to have asthma.169 The onset of asthma in children has been linked to early allergen exposure and viral infections, which act in concert with genetic susceptibility.170 Children can be particularly susceptible to allergens due to their immature respiratory and immune systems, as well as indoor or outdoor activities that contribute to aeroallergen exposure (see Table 3.1).170,171,172,173

Table 3.1: Percentage of population with active asthma, by year and selected characteristics: United States, 2001 and 2010.

Characteristic Year 2001 % Year 2010 %
Total 7.3 8.4
Male 6.3 7.0
Female 8.3 9.8
White 7.2 7.8
Black 8.4 11.9
Other 7.2 8.1
Hispanic 5.8 7.2
Non-Hispanic 7.6 8.7
Children (0–17) 8.7 9.3
Adults (18 and older) 6.9 8.2
Age Group    
0–4 years 5.7 6.0
5–14 years 9.9 10.7
15–34 years 8.0 8.6
35–64 years 6.7 8.1
65 years and older 6.0 8.1
Northeast 8.3 8.8
Midwest 7.5 8.6
South 7.1 8.3
West 6.7 8.3
Federal Poverty Threshold    
Below 100% 9.9 11.2
100% to < 250% 7.7 8.7
250% to < 450% 6.8 8.2
450% or higher 6.6 7.1

Source: Moorman et al. 2012179

A recent study of children in California found that racial and ethnic minorities are more affected by asthma.174 Among minority children, the prevalence of asthma varies with the highest rates among Blacks and American Indians/Alaska Natives (17%), followed by non-Hispanic or non-Latino Whites (10%), Hispanics (7%), and Asian Americans (7%).

Minority adults and children also bear a disproportionate burden associated with asthma as measured by emergency department visits, lost work and school days, and overall poorer health status (see Table 3.1).174,175 Twice as many Black children had asthma-related emergency department visits and hospitalizations compared with White children. Fewer Black and Hispanic children reported using preventative medication like inhaled corticosteroids (ICS) as compared to White children. Black and Hispanic children also had more poorly controlled asthma symptoms, leading to increased emergency department visits and greater use of rescue medications rather than routine daily use of ICS, regardless of symptom control.173,176

Children living in poverty were 1.75 times more likely to be hospitalized for asthma than their non-poor counterparts. When income is accounted for, no significant difference was observed in the rate of hospital admissions by race or ethnicity. This income effect may be related to access and use of health care and appropriate use of preventive medications such as ICS.177

People with preexisting medical conditions—including hypertension, diabetes, and chronic obstructive pulmonary disorder—are at greater risk for outdoor air pollution-related health effects than the general population.178 Populations with irregular heartbeats (atrial fibrillation) who were exposed to air pollution and high temperatures experience increased risk.165 People who live or work in buildings without air conditioning and other ventilation controls or in buildings that are unable to withstand extreme precipitation or flooding events are at greater risk of adverse health effects. Other health risks are related to exposures to poor indoor air quality from mold and other biological contaminants and chemical pollutants emitted from wet building materials. While the presence of air conditioning has been found to greatly reduce the risk of ozone-related deaths, communities with a higher percentage of unemployment and a greater population of Blacks are at greater risk.56

3.6 Research Needs

In addition to the emerging issues identified above, the authors highlight the following potential areas for additional scientific and research activity on air quality. Understanding of future air quality and the ability to model future health impacts associated with air quality changes—particularly PM 2.5 impacts—will be enhanced by improved modeling and projections of climate-dependent variables like wildfires and land-use patterns, as well as improved modeling of ecosystem responses to climate change. Improved collection of data on aeroallergen concentrations in association with other ecosystem variables will facilitate research and modeling of related health impacts.

Future assessments can benefit from research activities that:

  • enhance understanding of how interactions among climate-related factors, such as temperature or relative humidity, aeroallergens, and air pollution, affect human health, and how to attribute health impacts to changes in these different risk factors;
  • improve the ability to model and project climate change impacts on the formation and fate of air contaminants and quantify the compounded uncertainty in the projections; and
  • identify the impacts of changes in indoor dampness, such as mold, other biological contaminants, volatile organic compounds, and indoor air chemistry on indoor air pollutants and health.


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Very Likely
≥9 in 10
≥2 in 3
As Likely as Not
≈ 1 in 2
≤ 1 in 3
Very Unikely
≤1 in 10

Confidence Level

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

Documenting Uncertainty: This assessment relies on two metrics to communicate the degree of certainty in Key Findings. See Appendix 4: Documenting Uncertainty for more on assessments of likelihood and confidence.

Key Finding 1: Exacerbated Ozone Health Impacts

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, these climate-driven increases in ozone will cause premature deaths, hospital visits, lost school days, and acute respiratory symptoms [Likely, High Confidence].

Key Finding 2: Increased Health Impacts from Wildfires

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].

Key Finding 3: Worsened Allergy and Asthma Conditions

Changes in climate , specifically rising temperatures, altered precipitation patterns, and increasing concentrations of atmospheric carbon dioxide , are expected to contribute to increasing levels of some airborne allergens and associated increases in asthma episodes and other allergic illnesses [High Confidence].