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. 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.

1. AIR POLLUTION
Introduction
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,

2. 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.
Image courtesy of EPA
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.

3. 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.

4. 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
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 cancerincidence
and increased reliance on coal for energy generation.
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 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.
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).
• 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.

5. 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

6. 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
• 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.

7. • 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.
• 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.
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
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 tacticsand
public policies for reducing exposure to TRAP:
• Using high-efficiency particulate air (HEPA) filtration.

8. • 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.
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 birthswent down within 5 kilometers of retired coal and
oil-powered plant locations.
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)
nhales@mail.nih.gov