Introduction

Air Pollution

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.

When the National Ambient Air Quality Standards were established in 1970, air pollution was regarded primarily as a threat to respiratory health. Over the next decades as air pollution research advanced, public health concern broadened to include cardiovascular disease; diabetes mellitus; obesity; and reproductive, neurological, and immune system disorders.

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.

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), from motor vehicle emissions, may be the most recognizable form of air pollution. It contains 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.

Fact Sheets

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Air Pollution and Your Health

What is NIEHS Doing?

Over its 50-plus year history, NIEHS has been a leader in air pollution research. In 1993, NIEHS researchers published the landmark Six Cities Study, which established an association between fine particulate matter and mortality. 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?

Respiratory Disease

• 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).
• PM and nitrogen oxide are linked to chronic bronchitis.

Cardiovascular Disease

• 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 a cross-section of 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.

Cancer

• 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.

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.

Children

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.

Older adults

• 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.
• PM 2.5 air pollution is also associated with accelerated memory problems and Alzheimer’s-like brain declines, which was seen among women 65 years of age and older.
• 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.

Rural dwellers

• 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.
• 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.

Different genes

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

Breathing dust from mine tailings, created by active and abandoned mining operations, affects lung function. NIEHS grant recipients address such health hazards in disadvantaged communities, such as Native American people in the West, through culturally relevant health communication.

NIEHS also 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.

Community-level tactics can help reduce 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.

Why improving air quality matters

• Among children in Southern California, decreases in ambient nitrogen dioxide and PM 2.5 were associated with fewer cases of asthma.
• Bronchitis symptoms declined as pollution levels dropped in the Los Angeles region.
• 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.
• An NIEHS-funded study found that a mixture of several B vitamins may protect DNA from changes attributable to PM 2.5 air pollution.

Further Reading

Stories from the Environmental Factor (NIEHS Newsletter)

• Climate Change Worsens Air Pollution, Extreme Weather, Expert Says (July 2020)
• Health Disparities in Congressional Spotlight (June 2020)
• COVID-19 Research Grants Get Rapid NIEHS Funding (May 2020)
• Pregnancy Hypertension Risk Increased by Traffic-related Air Pollution (January 2020)
• NIH Researchers Tackle Questions on Air Pollution and Pregnancy (September 2019)
• Nonsmokers Can Get Emphysema from Air Pollution, Study Finds (September 2019)
• India and NIEHS Partner to Advance Air Pollution Research (August 2019)
• Los Angeles Youth Take on Air Monitoring (February 2019)

Podcasts

• Air Pollution and Your Heart is about the role air pollution plays in heart disease.
• Air Quality Monitoring for Citizen Science explores the potential and limitations of air quality monitors and offers strategies for citizen scientists who want to use monitors.
• When Wildfires Hit Close to Home is about NIEHS-funded research on the complexity of urban wildfires and how they may affect human health.

Additional Resources

•  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.
• NIEHS Virtual Forum: Roadway Pollution and Health is a recorded webcast on correlations between roadway air pollution and human health.
• 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.
• 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.
• 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

• Asthma
• Exposure Science
• Gene and Environment Interaction
• Lung Diseases