Air pollution is a familiar environmental health hazard. We know what we’re looking at when brown haze settles over a city, exhaust billows across a busy highway, or a plume rises from a smokestack. Some air pollution is not seen, but its pungent smell alerts you.
It is a major threat to global health and prosperity. Air pollution, in all forms, is responsible for more than 6.5 million deaths each year globally, a number that has increased over the past two decades.
What Is Air Pollution?
Air pollution is a mix of hazardous substances from both human-made and natural sources.
Vehicle emissions, fuel oils and natural gas to heat homes, by-products of manufacturing and power generation, particularly coal-fueled power plants, and fumes from chemical production are the primary sources of human-made air pollution.
Nature releases hazardous substances into the air, such as smoke from wildfires, which are often caused by people; ash and gases from volcanic eruptions; and gases, like methane, which are emitted from decomposing organic matter in soils.
Traffic-Related Air Pollution (TRAP), a mixture of gasses and particles, has most of the elements of human-made air pollution: ground-level ozone, various forms of carbon, nitrogen oxides, sulfur oxides, volatile organic compounds, polycyclic aromatic hydrocarbons, and fine particulate matter.
Ozone, an atmospheric gas, is often called smog when at ground level. It is created when pollutants emitted by cars, power plants, industrial boilers, refineries, and other sources chemically react in the presence of sunlight.
Noxious gases, which include carbon dioxide, carbon monoxide, nitrogen oxides (NOx), and sulfur oxides (SOx), are components of motor vehicle emissions and byproducts of industrial processes.
Particulate matter (PM) is composed of chemicals such as sulfates, nitrates, carbon, or mineral dusts. Vehicle and industrial emissions from fossil fuel combustion, cigarette smoke, and burning organic matter, such as wildfires, all contain PM.
A subset of PM, fine particulate matter (PM 2.5) is 30 times thinner than a human hair. It can be inhaled deeply into lung tissue and contribute to serious health problems. PM 2.5 accounts for most health effects due to air pollution in the U.S.
Volatile organic compounds (VOC) vaporize at or near room temperature—hence, the designation volatile. They are called organic because they contain carbon. VOCs are given off by paints, cleaning supplies, pesticides, some furnishings, and even craft materials like glue. Gasoline and natural gas are major sources of VOCs, which are released during combustion.
Polycyclic aromatic hydrocarbons (PAH) are organic compounds containing carbon and hydrogen. Of more than 100 PAHs known to be widespread in the environment, 15 are listed in the Report on Carcinogens. In addition to combustion, many industrial processes, such as iron, steel, and rubber product manufacturing, as well as power generation, also produce PAHs as a by-product. PAHs are also found in particulate matter.
Air Pollution and Climate Change
Air pollution and climate change affect each other through complex interactions in the atmosphere. Air pollution is intricately linked with climate change because both problems come largely from the same sources, such as emissions from burning fossil fuels. Both are threats to people’s health and the environment worldwide. Read more: Health Impacts of Air Quality.
What Is NIEHS Doing?
Over its 50-plus year history, NIEHS has been a leader in air pollution research. The institute continues to fund and conduct research into how air pollution affects health and the population groups who are most affected.
How does air pollution affect our health?
When the National Ambient Air Quality Standards were established in 1970, air pollution was regarded primarily as a threat to respiratory health. In 1993, NIEHS researchers published the landmark Six Cities Study, which established an association between fine particulate matter and mortality.
Air pollution exposure is associated with oxidative stress and inflammation in human cells, which may lay a foundation for chronic diseases and cancer. In 2013, the International Agency for Research on Cancer of the World Health Organization (WHO) classified air pollution as a human carcinogen.
Research on air pollution and health effects continually advances. Public health concern now includes cancer, cardiovascular disease, respiratory diseases, diabetes mellitus, obesity, and reproductive, neurological, and immune system disorders
- A large study of more than 57,000 women found living near major roadways may increase a woman’s risk for breast cancer.
- The NIEHS Sister Study found other airborne toxic substances, especially methylene chloride, which is used in aerosol products and paint removers, are also associated with increased risk of breast cancer.
- Occupational exposure to benzene, an industrial chemical and component of gasoline, can cause leukemia and is associated with non-Hodgkin’s Lymphoma.
- A long-term study, 2000-2016, found an association between lung cancer incidence and increased reliance on coal for energy generation.
- Fine particulate matter can impair blood vessel function and speed up calcification in arteries.
- NIEHS researchers established links between short-term daily exposure by post-menopausal women to nitrogen oxides and increased risk of hemorrhagic stroke.
- For some older Americans, exposure to TRAP can result in lowered levels of high-density lipoprotein, sometimes called good cholesterol, increasing their risk for cardiovascular disease.
- According to a National Toxicology Program (NTP) report, TRAP exposure also increases a pregnant woman’s risk for dangerous changes in blood pressure, known as hypertensive disorders, which are a leading cause of pre-term birth, low birth weight, and maternal and fetal illness and death.
- Air pollution can affect lung development and is implicated in the development of emphysema, asthma, and other respiratory diseases, such as chronic obstructive pulmonary disease (COPD).
- Increases in asthma prevalence and severity are linked to urbanization and outdoor air pollution. Children living in low-income urban areas tend to have more asthma cases than others. Research published in 2023 tied two air pollutants, ozone and PM2.5, to asthma-related changes in children’s airways.
- PM and nitrogen oxide are linked to chronic bronchitis.
- In 2020, a major public health challenge was confluence of the COVID-19 pandemic and wildfires across the western U.S. Building on a well-established connection between air pollution and respiratory-tract infections, a study linked wildfire smoke with additional COVID-19 cases and deaths.
Whom does air pollution affect the most?
Air pollution affects everyone’s health, but certain groups may be harmed more.
Almost 9 out of 10 people who live in urban areas worldwide are affected by air pollution.
The NIEHS-funded Children’s Health Study at the University of Southern California is one of the largest studies of the long-term effects of air pollution on children’s respiratory health. Among its findings:
- Higher air pollution levels increase short-term respiratory infections, which lead to more school absences.
- Children who play several outdoor sports and live in high ozone communities are more likely to develop asthma.
- Children living near busy roads are at increased risk for asthma.
- Children with asthma who were exposed to high levels of air pollutants were more likely to develop bronchitis symptoms.
- Living in communities with higher pollution levels can cause lung damage.
Other studies on women and children
- NIEHS-funded researchers from the University of California, Davis, Environmental Health Sciences Center are conducting the Bio-Specimen and Fire Effects (B-SAFE) Study. This ongoing project seeks to discover if and how recent wildfires and their smoke affected pregnant women and their babies. Begun in 2017, study participants are pregnant women who were living in Northern California when the 2018, 2019, or 2020 wildfires occurred there.
- Breathing PM 2.5, even at relatively low levels, may alter the size of a child's developing brain, which may ultimately increase the risk for cognitive and emotional problems later in adolescence.
- Prenatal exposure to PAHs was associated with brain development effects, slower processing speed, attention-deficit and hyperactivity disorder (ADHD) symptoms, and other neurobehavioral problems in urban youth.
- In New York City, prenatal exposure to air pollution may play a role in childhood ADHD-related behavior problems.
- Prenatal exposure to particulate matter was associated with low birth weight.
- Women exposed to high levels of fine particulate matter during pregnancy, particularly in the third trimester, may have up to twice the risk of having a child with autism.
- Second and third trimester exposure to PM 2.5 might increase the chance of those children having high blood pressure in early life.
- In California’s agricultural San Joaquin Valley, women who were exposed to high levels of carbon monoxide, nitrogen oxide, or nitrogen dioxide during their first 8 weeks of pregnancy were more likely to have a baby with neural tube defects.
- In Marietta, Ohio, home to a ferromanganese refinery, manganese concentrations in blood and hair, a biomarker of air pollution exposure, were associated with lower child IQ scores.
- Alzheimer’s disease and related dementias are a public health challenge for aging populations. NIEHS-funded researchers at the University of Washington identified a link between air pollution and dementias. This well-conducted study adds considerable evidence that ambient air fine particles increase risk of dementias. Conversely, a multi-year study published in 2022 shows improved air quality is associated with lower risk of dementia in older women. The researchers also stated this decline in dementia risk was equivalent to taking nearly 2 1/2 years off the age of the women studied.
- Air pollution was linked to a greater chance of developing several neurological disorders, including Parkinson's disease, Alzheimer's disease, and other dementias. Hospital admissions data from 63 million older adults in the U.S., obtained over 17 years (2000-2016), was analyzed along with estimated PM 2.5 concentrations by zip code to conduct the study.
- In older adults, long-term exposure to TRAP may significantly hasten physical disabilities. The risk is more pronounced among racial minorities and lower-income people.
- Osteoporosis affects women more than men. A large study associated high levels of air pollutants with bone damage, particularly in the lumbar spine, among postmenopausal women. This study expands previous findings linking air pollution and bone damage.
- Nutrients may counter some harmful effects from air pollution. A 2020 study found omega-3 fatty acids, obtained by eating certain fish, may protect against PM 2.5-associated brain shrinkage in older women.
- An NIEHS-funded study found that concentrations of PM 2.5 in rural Washington State were comparable to urban Seattle. In this study, as regional PM 2.5 increased, there were increased asthma symptoms, such as limitation of activities, more wheezing, and more nighttime waking, in rural children.
- NIEHS supported a translational research project, Addressing Air Pollution and Asthma (1MB), that may lead to improved health for children suffering from asthma. They found that certain agricultural practices contribute to poor air quality and asthma among children. The team combined high-efficiency particulate air (HEPA) cleaners and a home-based education program to reduce children’s exposure to pollutants in the home.
- In the rural U.S., large-scale animal feeding operations might compromise regional air quality through emission of pollutants, such as ammonia gas. A study found acute lung function problems in children with asthma in such areas.
Your genes play a role in respiratory health. NIEHS-funded research discovered that people with specific gene variants, which made them more likely to have lung inflammation, had a greater chance of suffering from asthma if they lived close to major roadways.
NIEHS and community involvement
NIEHS supports community participation in the research process and encourages collaborative approaches that build capacity in communities to address environmental health concerns. Community-engaged research and citizen science are two types of collaborative research approaches.
For example, NIEHS helps residents of Imperial County, California track air pollution through a network of 40 community-run monitors. In this county, long-term improvements in air quality were associated with significant lung-function improvement in children.
In another example, NIEHS grant recipients developed community-level tactics and public policies for reducing exposure to TRAP:
- Using high-efficiency particulate air (HEPA) filtration.
- Building land-use buffers and vegetation barriers.
- Improving urban design with gardens, parks, and street-side trees.
- Creating active-travel options, such as bicycling and walking paths.
THE (Trade, Health, Environment) Impact Project brings together researchers and community groups to find solutions for communities affected by trade-related pollution, such as ports and roadways with trucking.
Join an asthma study!
The goal of the Natural History of Asthma with Longitudinal Environmental Sampling (NHALES) study is to help scientists understand how bacteria and other factors in the environment affect people who have moderate to severe asthma.
Who can participate?
- Moderate to severe asthmatics.
- Males and females, aged 18-60.
- Females should not be pregnant or breastfeeding at the start of the study, but may still participate if they become pregnant during the study.
- Nonsmokers who are also not around significant amounts of secondhand smoke.
- No history of chronic obstructive pulmonary disease, emphysema, cystic fibrosis (CF), pulmonary fibrosis, non-CF bronchiectasis, sarcoidosis, unstable angina, or pulmonary hypertension.
- Not allergic to methacholine.
- Able to provide your own transportation to clinic visits on the NIEHS campus in North Carolina.
For more information about this study:
- NHALES: Asthma Study
- Tel 855-MYNIEHS (855-696-4347)
Why improving air quality matters
- Air pollution and birth outcomes are linked as global public health concerns. Researchers analyzed indoor and outdoor air pollution data from all inhabited continents along with key pregnancy outcomes. Their findings indicate efforts to reduce PM2.5 exposure could lead to significant reductions in the number of low-birth weight and pre-term birth infants worldwide. Air pollution reduction would be especially beneficial for children born in low- and middle-income countries.
- Among children in Southern California, decreases in ambient nitrogen dioxide and PM 2.5 were associated with fewer cases of asthma.
- An NIEHS-funded study found that a mixture of several B vitamins may protect DNA from changes attributable to PM 2.5 air pollution.
- Bronchitis symptoms declined as pollution levels dropped in the Los Angeles region.
- Improving air quality may improve cognitive function and reduce dementia risk, according to studies supported in part by NIH and the Alzheimer's Association.
- When fossil-fuel power plants close, nearby air pollution is reduced. A study found the incidence of preterm births went down within 5 kilometers of retired coal and oil-powered plant locations.
Stories from the Environmental Factor (NIEHS newsletter)
- Indoor Wood-burning May Be Linked to Lung Cancer in U.S. Women (September 2023)
- Everyday Air Pollution Can Harm Brain Development in Adolescents (September 2023)
- Wildfire Smoke, Other Air Pollution Can Harm Brain Health, Expert Says (August 2023)
- Burning Plastic Can Affect Air Quality, Public Health (August 2022)
- Interventions Needed to Slow Climate-driven Air Pollution, Researchers Note (March 2022)
- Air Pollution and Forever Chemicals Continue to Pose Health Risks (March 2022)
- Air Pollution Affects Children’s Brain Structure (February 2022)
- Increasing Evidence Links Air Pollution With Breast Cancer (November 2021)
- Fine Particulate Air Pollution Associated With Higher Dementia Risk (September 2021)
- Better Air Quality May Lower Dementia Risk Among Older Women (August 2021)
- Environmentally Persistent Free Radicals May Affect Lung Health (May 2021)
- Air Quality Monitoring Innovations Merit Small Business Recognition (February 2021)
- Climate Change Worsens Air Pollution, Extreme Weather, Expert Says (July 2020)
- Pregnancy Hypertension Risk Increased by Traffic-related Air Pollution (January 2020)
Printable Fact Sheets
- When Wildfires Hit Close to Home is about NIEHS-funded research on the complexity of urban wildfires and how they may affect human health.
- Wildfire Smoke and Children's Health
- Air Pollution Linked to Dementia Cases (September 2023) – In this edition of NIH Research Matters, read about findings from the Health and Retirement Study, funded by the National Institute on Aging, that showed higher air pollution exposure was linked to an increased risk of dementia. After consideration of all sources, fine particulate matter, or PM2.5, from agriculture and wildfires were specifically associated with an increased risk of dementia. Reducing such exposures might help lower the incidence of dementia. The study was published in JAMA Internal Medicine.
- AirNow, a tool developed in partnership by several government agencies, allows you to monitor air quality in real time anywhere in the U.S. Simply enter your zip code as indicated on the website.
- EPA's Air Sensor Toolbox provides information on the operation and use of air-sensor monitoring systems for technology developers, air-quality managers, citizen scientists, and the public.
- NIH Climate Change and Health Initiative – This solutions-focused research initiative aims to reduce the health consequences associated with extreme weather events and evolving climate conditions. NIH has a strong history of creating innovative tools, technologies, and data-driven solutions to address global environmental problems.
- Smoke-ready Toolbox for Wildfires is a compendium of resources from the EPA to help educate you about the risks of smoke exposure and actions that protect your health.
- Wildfire Smoke Collection – The journal Environmental Health Perspectives has published high-impact papers and reviews exploring exposure and resulting health effects related to wildfires.
Related Health Topics
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