Ozone
Ozone (O3) is a gas composed of three oxygen atoms. It is not usually emitted directly into the air,
but at ground-level is created by a chemical reaction between oxides of nitrogen (NOx) and volatile
organic compounds (VOC) in the presence of sunlight. Ozone has the same chemical structure
whether it occurs miles above the earth or at ground-level and can be "good" or "bad," depending on
its location in the atmosphere.
In the earth's lower atmosphere, ground-level ozone is considered "bad." Motor vehicle exhaust and
industrial emissions, gasoline vapors, and chemical solvents as well as natural sources emit NOx and
VOC that help form ozone. Ground-level ozone is the primary constituent of smog. Sunlight and hot
weather cause ground-level ozone to form in harmful concentrations in the air. As a result, it is known
as a summertime air pollutant. Many urban are National Trends in Ozone Levels
Using a nationwide network of monitoring sites, EPA has developed ambient air quality trends for
ozone. Trends are shown here for the 8-hour ozone standards. Under the Clean Air Act, EPA sets and
reviews national air quality standards for ozone. Air quality monitors measure concentrations of ozone
throughout the country. EPA, state, tribal and local agencies use that data to ensure that ozone is at
levels that protect public health and the environment. Nationally, average ozone levels declined in the
1980's, leveled off in the 1990's, and showed a notable decline after 2002. For information on ozone
standards, sources, health effects, and programs to reduce ozone, please see
www.epa.gov/air/ozonepollution/.
as tend to have high levels of "bad" ozone, but even rural areas are also subject to increased ozone
levels because wind carries ozone and pollutants that form it hundreds of miles away from their
original sources.
"Good" ozone occurs naturally in the stratosphere approximately 10 to 30 miles above the earth's
surface and forms a layer that protects life on earth from the sun's harmful rays. Learn more about
how ozone can be beneficial up high in the stratosphere but harmful at ground level.
Basic Information
Ground-level or "bad" ozone is not emitted directly
into the air, but is created by chemical reactions
between oxides of nitrogen (NOx) and volatile
organic compounds (VOC) in the presence of
sunlight. Emissions from industrial facilities and
electric utilities, motor vehicle exhaust, gasoline
vapors, and chemical solvents are some of the major sources of NOx and VOC.
Breathing ozone, a primary component of smog, can trigger a variety of health problems including
chest pain, coughing, throat irritation, and congestion. It can worsen bronchitis, emphysema, and
asthma. Ground-level ozone also can reduce lung function and inflame the linings of the lungs.
Repeated exposure may permanently scar lung tissue.
Ground-level ozone also damages vegetation and ecosystems. In the United States alone, ozone is
responsible for an estimated $500 million in reduced crop production each year.
Under the Clean Air Act, EPA has set protective health-based standards for ozone in the air we
breathe. EPA and others have instituted a variety of multi-faceted programs to meet these healthbased standards. More about EPA ‘s ozone standards and regulatory actions.
Throughout the country, additional programs are being put into place to cut NOx and VOC emissions
from vehicles, industrial facilities, and electric utilities. Programs are also aimed at reducing pollution
by reformulating fuels and consumer/commercial products, such as paints and chemical solvents that
contain VOC. Voluntary and innovative programs also encourage communities to adopt practices, such
as carpooling, to reduce harmful emissions. More about EPA’s innovative programs to reduce air
pollution.
Health
Breathing ozone can trigger a variety of health problems including chest pain, coughing, throat irritation,
and congestion. It can worsen bronchitis, emphysema, and asthma. Ground-level ozone also can reduce
lung function and inflame the linings of the lungs. Repeated exposure may permanently scar lung tissue.
The Clean Air Act requires EPA to set air quality standards to protect both public health and the public
welfare (e.g. crops and vegetation). Ground-level ozone affects both.
Health Effects
People with lung disease, children, older adults, and people who are active can be affected when ozone
levels are unhealthy. Numerous scientific studies have linked ground-level ozone exposure to a variety
of problems, including:
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airway irritation, coughing, and pain when taking a deep breath;
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wheezing and breathing difficulties during exercise or outdoor activities;
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inflammation, which is much like a sunburn on the skin;
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aggravation of asthma and increased susceptibility to respiratory illnesses like pneumonia and
bronchitis; and,
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permanent lung damage with repeated exposures.
Using a nationwide network of monitoring sites, EPA has developed ambient air quality trends for
ozone. Trends are shown here for the 8-hour ozone standards. Under the Clean Air Act, EPA sets and
reviews national air quality standards for ozone. Air quality monitors measure concentrations of ozone
throughout the country. EPA, state, tribal and local agencies use that data to ensure that ozone is at
levels that protect public health and the environment. Nationally, average ozone levels declined in the
1980's, leveled off in the 1990's, and showed a notable decline after 2002. For information on ozone
standards, sources, health effects, and programs to reduce ozone, please see
www.epa.gov/air/ozonepollution/.
Particulate Matter
Particulate matter," also known as particle pollution or PM, is a complex mixture of extremely small
particles and liquid droplets. Particle pollution is made up of a number of components, including acids
(such as nitrates and sulfates), organic chemicals, metals, and soil or dust particles.
The size of particles is directly linked to their potential for causing health problems. EPA is concerned
about particles that are 10 micrometers in diameter or smaller because those are the particles that
generally pass through the throat and nose and enter the lungs. Once inhaled, these particles can affect
the heart and lungs and cause serious health effects. EPA groups particle pollution into two categories:

"Inhalable coarse particles," such as those found near roadways and dusty industries, are larger
than 2.5 micrometers and smaller than 10 micrometers in diameter.
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"Fine particles," such as those found in smoke and haze, are 2.5 micrometers in diameter and
smaller. These particles can be directly emitted from sources such as forest fires, or they can
form when gases emitted from power plants, industries and automobiles react in the air.
Basic Information
Particle pollution (also called particulate matter or PM) is the term for a mixture of solid particles and
liquid droplets found in the air. Some particles, such as dust, dirt, soot, or smoke, are large or dark
enough to be seen with the naked eye. Others are so small, they can only be detected using an electron
microscope.
Particle pollution includes "inhalable coarse particles," with
How Big is Particle Pollution?
diameters larger than 2.5 micrometers and smaller than 10
micrometers and "fine particles," with diameters that are 2.5
micrometers and smaller. How small is 2.5 micrometers? Think
about a single hair from your head. The average human hair is
about 70 micrometers in diameter – making it 30 times larger
than the largest fine particle.
These particles come in many sizes and shapes and can be
made up of hundreds of different chemicals. Some particles,
known as primary particles are emitted directly from a source,
Enlarge this figure
such as construction sites, unpaved roads, fields, smokestacks
or fires. Others form in complicated reactions in the atmosphere of chemicals such as sulfur dioxides
and nitrogen oxides that are emitted from power plants, industries and automobiles. These particles,
known as secondary particles, make up most of the fine particle pollution in the country.
EPA regulates inhalable particles (fine and coarse). Particles larger than 10 micrometers (sand and large
dust) are not regulated by EPA. More about EPA PM Standards and Regulatory Actions.

Health: Particle pollution contains microscopic solids or liquid droplets that are so small that
they can get deep into the lungs and cause serious health problems. The size of particles is
directly linked to their potential for causing health problems. Small particles less than 10
micrometers in diameter pose the greatest problems, because they can get deep into your
lungs, and some may even get into your bloodstream. More information about health.

Visibility: Fine particles (PM2.5) are the major cause of reduced visibility (haze) in parts of the
United States, including many of our treasured national parks and wilderness areas. More
information about visibility.

Reducing particle pollution: EPA’s national and regional rules to reduce emissions of pollutants
that form particle pollution will help state and local governments meet the Agency’s national air
quality standards. More information about reducing particle pollution.
Health
The size of particles is directly linked to their potential for causing health problems. Small particles
less than10 micrometers in diameter pose the greatest problems, because they can get deep into your
lungs, and some may even get into your bloodstream.
Exposure to such particles can affect both your lungs and your heart. Small particles of concern
include "inhalable coarse particles" (such as those found near roadways and dusty industries), which
are larger than 2.5 micrometers and smaller than 10 micrometers in diameter; and "fine particles"
(such as those found in smoke and haze), which are 2.5 micrometers in diameter and smaller.
The Clean Air Act requires EPA to set air quality standards to protect both public health and the public
welfare (e.g. crops and vegetation). Particle pollution affects both.
Health Effects
Particle pollution - especially fine particles - contains microscopic solids or liquid droplets that are so
small that they can get deep into the lungs and cause serious health problems. Numerous scientific
studies have linked particle pollution exposure to a variety of problems, including:
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increased respiratory symptoms, such as irritation of the airways, coughing, or difficulty
breathing, for example;
decreased lung function;
aggravated asthma;
development of chronic bronchitis;
irregular heartbeat;
nonfatal heart attacks; and
premature death in people with heart or lung disease.
People with heart or lung diseases, children and older adults are the most likely to be affected
by particle pollution exposure. However, even if you are healthy, you may experience
temporary symptoms from exposure to elevated levels of particle pollution.
Environmental Effects
Visibility reduction
Fine particles (PM2.5) are the major cause of reduced visibility (haze) in parts of the United States,
including many of our treasured national parks and wilderness areas. For more information about
visibility, visit www.epa.gov/visibility.
Environmental damage
Particles can be carried over long distances by wind and then settle on ground or water. The effects of
this settling include: making lakes and streams acidic; changing the nutrient balance in coastal waters
and large river basins; depleting the nutrients in soil; damaging sensitive forests and farm crops; and
affecting the diversity of ecosystems. More information about the effects of particle pollution and acid
rain.
Aesthetic damage
Particle pollution can stain and damage stone and other materials, including culturally important
objects such as statues and monuments. More information about the effects of particle pollution and
acid rain.
National Trends in Particulate Matter Levels
Using a nationwide network of monitoring sites, EPA has developed ambient air quality trends for
particle pollution, also called Particulate Matter (PM). Trends from 1990-2009 are shown here for
PM2.5 and PM10. Under the Clean Air Act, EPA sets and reviews national air quality standards for PM.
Air quality monitors measure concentrations of PM throughout the country. EPA, state, tribal and local
agencies use that data to ensure that PM in the air is at levels that protect public health and the
environment. Nationally, average PM concentrations have decreased over the years. For information
on PM standards, sources, health effects, and programs to reduce PM, please see
www.epa.gov/air/particlepollution.
Carbon Monoxide
Carbon monoxide (CO) is a colorless, odorless gas emitted from combustion processes. Nationally
and, particularly in urban areas, the majority of CO emissions to ambient air come from mobile
sources. CO can cause harmful health effects by reducing oxygen delivery to the body's organs (like
the heart and brain) and tissues. At extremely high levels, CO can cause death.
EPA first set air quality standards for CO in 1971. For protection of both public health and welfare,
EPA set a 8-hour primary standard at 9 parts per million (ppm) and a 1-hour primary standard at 35
ppm.
In a review of the standards completed in 1985, EPA revoked the secondary standards (for public
welfare) due to a lack of evidence of adverse effects on public welfare at or near ambient
concentrations.
The last review of the CO NAAQS was completed in 1994 and the Agency chose not to revise the
standards at that time.
Health
CO can cause harmful health effects by reducing oxygen delivery to the body's organs (like the heart
and brain) and tissues. At extremely high levels, CO can cause death.
Exposure to CO can reduce the oxygen-carrying capacity of the blood. People with several types of
heart disease already have a reduced capacity for pumping oxygenated blood to the heart, which can
cause them to experience myocardial ischemia (reduced oxygen to the heart), often accompanied by
chest pain (angina), when exercising or under increased stress. For these people, short-term CO
exposure further affects their body’s already compromised ability to respond to the increased oxygen
demands of exercise or exertion.
National Trends in CO Levels
Using a nationwide network of monitoring sites, EPA has developed ambient air quality trends for
carbon monoxide (CO). Trends from 1980-2008 and from 1990-2008 are shown here. Under the Clean
Air Act, EPA sets and reviews national air quality standards for CO. Air quality monitors measure
concentrations of CO throughout the country. EPA, state, tribal and local agencies use that data to
ensure that CO remains at levels that protect public health and the environment. Nationally, average
CO concentrations have decreased substantially over the years. For information on CO standards,
sources, health effects, and programs to reduce CO, please see www.epa.gov/air/urbanair/co.
Nitrogen Dioxide
Nitrogen dioxide (NO2) is one of a group of highly reactive gasses known as "oxides of nitrogen," or
"nitrogen oxides (NOx)." Other nitrogen oxides include nitrous acid and nitric acid. While EPA’s
National Ambient Air Quality Standard covers this entire group of NOx, NO2 is the component of
greatest interest and the indicator for the larger group of nitrogen oxides. NO 2 forms quickly from
emissions from cars, trucks and buses, power plants, and off-road equipment. In addition to
contributing to the formation of ground-level ozone, and fine particle pollution, NO2 is linked with a
number of adverse effects on the respiratory system.
EPA first set standards for NO2 in 1971, setting both a primary standard (to protect health) and a
secondary standard (to protect the public welfare) at 0.053 parts per million (53 ppb), averaged
annually. The Agency has reviewed the standards twice since that time, but chose not to revise the
standards at the conclusion of each review. All areas in the U.S. meet the current (1971) NO2
standards.
Health
Current scientific evidence links short-term NO2 exposures, ranging from 30 minutes to 24 hours, with
adverse respiratory effects including airway inflammation in healthy people and increased respiratory
symptoms in people with asthma.
Also, studies show a connection between breathing elevated short-term NO2 concentrations, and
increased visits to emergency departments and hospital admissions for respiratory issues, especially
asthma.
NO2 concentrations in vehicles and near roadways are appreciably higher than those measured at
monitors in the current network. In fact, in-vehicle concentrations can be 2-3 times higher than
measured at nearby area-wide monitors. Near-roadway (within about 50 meters) concentrations of
NO2 have been measured to be approximately 30 to 100% higher than concentrations away from
roadways.
Individuals who spend time on or near major roadways can experience short-term NO2 exposures
considerably higher than measured by the current network. Approximately 16% of U.S housing units
are located within 300 ft of a major highway, railroad, or airport (approximately 48 million people).
This population likely includes a higher proportion of non-white and economically-disadvantaged
people.
NO2 exposure concentrations near roadways are of particular concern for susceptible individuals,
including people with asthma asthmatics, children, and the elderly
The sum of nitric oxide (NO) and NO2 is commonly called nitrogen oxides or NOx. Other oxides of
nitrogen including nitrous acid and nitric acid are part of the nitrogen oxide family. While EPA’s
National Ambient Air Quality Standard (NAAQS) covers this entire family, NO2 is the component of
greatest interest and the indicator for the larger group of nitrogen oxides.
NOx react with ammonia, moisture, and other compounds to form small particles. These small
particles penetrate deeply into sensitive parts of the lungs and can cause or worsen respiratory
disease, such as emphysema and bronchitis, and can aggravate existing heart disease, leading to
increased hospital admissions and premature death.
Ozone is formed when NOx and volatile organic compounds react in the presence of heat and sunlight.
Children, the elderly, people with lung diseases such as asthma, and people who work or exercise
outside are at risk for adverse effects from ozone. These include reduction in lung function and
increased respiratory symptoms as well as respiratory-related emergency department visits, hospital
admissions, and possibly premature deaths.
Emissions that lead to the formation of NO2 generally also lead to the formation of other NOx.
Emissions control measures leading to reductions in NO2 can generally be expected to reduce
population exposures to all gaseous NOx. This may have the important co-benefit of reducing the
formation of ozone and fine particles both of which pose significant public health threats.
National Trends in Nitrogen Dioxide Levels
Using a nationwide network of monitoring sites, EPA has developed ambient air quality trends for
nitrogen dioxide (NO2). Trends from 1980-2009 and from 1990-2009 are shown here. Under the
Clean Air Act, EPA sets and reviews national air quality standards for NO2. Air quality monitors
measure concentrations of NO2 throughout the country. EPA, state, tribal and local agencies use that
data to ensure that NO2 in the air is at levels that protect public health and the environment.
Nationally, average NO2 concentrations have decreased substantially over the years. For information
on NO2 standards, sources, health effects, and programs to reduce NO2, please see
www.epa.gov/airquality/nitrogenoxides/.
Sulfur Dioxide
Sulfur dioxide (SO2) is one of a group of highly reactive gasses known as “oxides of sulfur.” The
largest sources of SO2 emissions are from fossil fuel combustion at power plants (73%) and other
industrial facilities (20%). Smaller sources of SO2 emissions include industrial processes such as
extracting metal from ore, and the burning of high sulfur containing fuels by locomotives, large ships,
and non-road equipment. SO2 is linked with a number of adverse effects on the respiratory system.
EPA first set standards for SO2 in 1971. EPA set a 24-hour primary standard at 140 ppb and an
annual average standard at 30 ppb (to protect health). EPA also set a 3-hour average secondary
standard at 500 ppb (to protect the public welfare).
The last review of the SO2 NAAQS was completed in 1996 and the Agency chose not to revise the
standards. In the last review, EPA also considered, but did not set, a five minute NAAQS to protect
asthmatics at elevated ventilation rates from bronchoconstriction and respiratory symptoms
associated with 5-10 minute peaks of SO2.
Health
Current scientific evidence links short-term exposures to SO2, ranging from 5 minutes to 24 hours,
with an array of adverse respiratory effects including bronchoconstriction and increased asthma
symptoms. These effects are particularly important for asthmatics at elevated ventilation rates (e.g.,
while exercising or playing.)
Studies also show a connection between short-term exposure and increased visits to emergency
departments and hospital admissions for respiratory illnesses, particularly in at-risk populations
including children, the elderly, and asthmatics.
EPA’s National Ambient Air Quality Standard for SO2 is designed to protect against exposure to the
entire group of sulfur oxides (SOx). SO2 is the component of greatest concern and is used as the
indicator for the larger group of gaseous sulfur oxides (SOx). Other gaseous sulfur oxides (e.g. SO3)
are found in the atmosphere at concentrations much lower than SO2.
Emissions that lead to high concentrations of SO2 generally also lead to the formation of other SOx.
Control measures that reduce SO2 can generally be expected to reduce people’s exposures to all
gaseous SOx. This may have the important co-benefit of reducing the formation of fine sulfate
particles, which pose significant public health threats.
SOx can react with other compounds in the atmosphere to form small particles. These particles
penetrate deeply into sensitive parts of the lungs and can cause or worsen respiratory disease, such
as emphysema and bronchitis, and can aggravate existing heart disease, leading to increased hospital
admissions and premature death. EPA’s NAAQS for particulate matter (PM) are designed to provide
protection against these health effects.
National Trends in Sulfur Dioxide Levels
Using a nationwide network of monitoring sites, EPA has developed ambient air quality trends for
sulfur dioxide (SO2). Trends from 1980-2009 and from 1990-2009 are shown here. Under the Clean
Air Act, EPA sets and reviews national air quality standards for SO2. Air quality monitors measure
concentrations of SO2 throughout the country. EPA, state, tribal and local agencies use that data to
ensure that SO2 in the air is at levels that protect public health and the environment. Nationally,
average SO2 concentrations have decreased substantially over the years. For information on SO2
standards, sources, health effects, and programs to reduce SO2, please see
www.epa.gov/air/sulfurdioxide/.
Lead
Nature and Sources of Lead
In the past, motor vehicles were the major contributor of lead emissions to the air. As a result of
EPA's regulatory efforts to reduce lead in on-road motor vehicle gasoline, air emissions of lead from
the transportation sector, and particularly the automotive sector, have greatly declined over the past
two decades. Major sources of lead emissions to the air today are ore and metals processing and
piston-engine aircraft operating on leaded aviation gasoline. The highest air concentrations of lead
are usually found near lead smelters. Other stationary sources are waste incinerators, utilities, and
lead-acid battery manufacturers.
Health
In addition to exposure to lead in air, other major exposure pathways include ingestion of lead in
drinking water and lead-contaminated food as well as incidental ingestion of lead-contaminated soil
and dust. Lead-based paint remains a major exposure pathway in older homes. Learn more about
lead in paint, dust and soil.
Once taken into the body, lead distributes throughout the body in the blood and is accumulated in the
bones. Depending on the level of exposure, lead can adversely affect the nervous system, kidney
function, immune system, reproductive and developmental systems and the cardiovascular system.
Lead exposure also affects the oxygen carrying capacity of the blood. The lead effects most
commonly encountered in current populations are neurological effects in children and cardiovascular
effects (e.g., high blood pressure and heart disease) in adults. Infants and young children are
especially sensitive to even low levels of lead, which may contribute to behavioral problems, learning
deficits and lowered IQ.
Lead is persistent in the environment and accumulates in soils and sediments through deposition from
air sources, direct discharge of waste streams to water bodies, mining, and erosion. Ecosystems near
point sources of lead demonstrate a wide range of adverse effects including losses in biodiversity,
changes in community composition, decreased growth and reproductive rates in plants and animals,
and neurological effects in vertebrates.
National Trends in Lead Levels
Under the Clean Air Act, EPA sets and reviews national air quality standards for lead. Air quality
monitors measure concentrations of lead throughout the country. EPA, state, tribal and local agencies
use those data to ensure that lead is at levels that protect public health and the environment. EPA has
tracked air quality trends for lead using data from this network of monitors. Trends from 1980-2009
and from 1990-2009 are shown here. Nationally, average lead concentrations decreased dramatically
after EPA's regulations reduced the lead content in on-road motor vehicle gasoline. For information on
lead standards, sources, health effects, and programs to reduce lead, please see
www.epa.gov/air/lead and www.epa.gov/otaq/aviation.htm.
EPA projects that the Clean Air Act Amendments will prevent over 230,000 early deaths in 2020.
Learn more about the Benefits and Costs of the Clean Air Act.
EPA is celebrating the 40th anniversary of the Clean Air Act. Learn more about how this landmark law
has protected America's health and environment.
The Clean Air Act is the law that defines EPA's responsibilities for protecting and improving the
nation's air quality and the stratospheric ozone layer. The last major change in the law, the Clean Air
Act Amendments of 1990, was enacted by Congress in 1990. Legislation passed since then has made
several minor changes.
The Clean Air Act, like other laws enacted by Congress, was incorporated into the United States Code
as Title 42, Chapter 85. The House of Representatives maintains a current version of the U.S. Code,
which includes Clean Air Act changes enacted since 1990.
This site provides links to sections of the U.S. Code containing the amended text of the Clean Air Act.
Section numbers in the U.S. Code are different than the Clean Air Act's section numbers. The table of
contents below gives corresponding section numbers in the Clean Air Act (CAA) and the U.S. Code
(USC). Another difference is that titles in the Clean Air Act correspond to subchapters in the U.S.
Code.
The legal authority for federal programs regarding air pollution control is based on the 1990 Clean Air
Act Amendments (1990 CAAA). These are the latest in a series of amendments made to the Clean Air Act
(CAA). This legislation modified and extended federal legal authority provided by the earlier Clean Air
Acts of 1963 and 1970.
The Air Pollution Control Act of 1955 was the first federal legislation involving air pollution. This Act
provided funds for federal research in air pollution. The Clean Air Act of 1963 was the first federal
legislation regarding air pollution control. It established a federal program within the U.S. Public Health
Service and authorized research into techniques for monitoring and controlling air pollution. In 1967,
the Air Quality Act was enacted in order to expand federal government activities. In accordance with this
law, enforcement proceedings were initiated in areas subject to interstate air pollution transport. As
part of these proceedings, the federal government for the first time conducted extensive ambient
monitoring studies and stationary source inspections.
The Air Quality Act of 1967 also authorized expanded studies of air pollutant emission inventories,
ambient monitoring techniques, and control techniques.
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Clean Air Act of 1970
The enactment of the Clean Air Act of 1970 (1970 CAA) resulted in a major shift in the federal
government's role in air pollution control. This legislation authorized the development of
comprehensive federal and state regulations to limit emissions from both stationary (industrial) sources
and mobile sources. Four major regulatory programs affecting stationary sources were initiated: the
National Ambient Air Quality Standards (NAAQS, pronounced "knacks"), State Implementation Plans
(SIPs), New Source Performance Standards (NSPS), and National Emission Standards for Hazardous Air
Pollutants (NESHAPs). Furthermore, the enforcement authority was substantially expanded. The
adoption of this very important legislation occurred at approximately the same time as the National
Environmental Policy Act that established the U.S. Environmental Protection Agency (EPA). The EPA was
created on May 2, 1971 in order to implement the various requirements included in the Clean Air Act of
1970.
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Clean Air Act Amendments of 1977
Major amendments were added to the Clean Air Act in 1977 (1977 CAAA). The 1977 Amendments
primarily concerned provisions for the Prevention of Significant Deterioration (PSD) of air quality in
areas attaining the NAAQS. The 1977 CAAA also contained requirements pertaining to sources in nonattainment areas for NAAQS. A non-attainment area is a geographic area that does not meet one or
more of the federal air quality standards. Both of these 1977 CAAA established major permit review
requirements to ensure attainment and maintenance of the NAAQS.
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Clean Air Act Amendments of 1990
Another set of major amendments to the Clean Air Act occurred in 1990 (1990 CAAA). The 1990 CAAA
substantially increased the authority and responsibility of the federal government. New regulatory
programs were authorized for control of acid deposition (acid rain) and for the issuance of stationary
source operating permits. The NESHAPs were incorporated into a greatly expanded program for
controlling toxic air pollutants. The provisions for attainment and maintenance of NAAQS were
substantially modified and expanded. Other revisions included provisions regarding stratospheric ozone
protection, increased enforcement authority, and expanded research programs.
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Milestones
Some principal milestones in the evolution of the Clean Air Act are:
The Air Pollution Control Act of 1955
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First federal air pollution legislation
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Funded research for scope and sources of air pollution
Clean Air Act of 1963
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Authorized the development of a national program to address air pollution related
environmental problems
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Authorized research into techniques to minimize air pollution
Air Quality Act of 1967
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Authorized enforcement procedures for air pollution problems involving interstate transport of
pollutants
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Authorized expanded research activities
Clean Air Act 1970
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Authorized the establishment of National Ambient Air Quality Standards
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Established requirements for State Implementation Plans to achieve the National Ambient Air
Quality Standards
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Authorized the establishment of New Source Performance Standards for new and modified
stationary sources
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Authorized the establishment of National Emission Standards for Hazardous Air Pollutants

Increased enforcement authority
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Authorized requirements for control of motor vehicle emissions
1977 Amendments to the Clean Air Act of 1970
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Authorized provisions related to the Prevention of Significant Deterioration
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Authorized provisions relating to areas which are non-attainment with respect to the National
Ambient Air Quality Standards
1990 Amendments to the Clean Air Act of 1970

Authorized programs for Acid Deposition Control
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Authorized a program to control 189 toxic pollutants, including those previously regulated by
the National Emission Standards for Hazardous Air Pollutants

Established permit program requirements

Expanded and modified provisions concerning the attainment of National Ambient Air Quality
Standards
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Expanded and modified enforcement authority

Established a program to phase out the use of chemicals that deplete the ozone layer.