U.S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Outreach and Information Division
Research Triangle Park, NC
February 2014
EPA-456/F-14-002

A Guide to Air Quality andYour Health

Recycled/recyclable. Printed with vegetable oil-based
inks on 100% postconsumer process, chlorine-freerecycled paper.

“It’s a code red day
for ozone.”“Particle pollution levels are
forecast to be unhealthy
for sensitive groups.”“Local air quality is very
unhealthy today.”
You may hear these alerts on radioor TV or read them in the newspaper.
But what do they mean if you:
 Are active outdoors?
 Have children who play outdoors?
 Are an older adult?
 Have heart or lung disease?

This booklet will help you understandhow to find out about air quality inyour area and protect your health.

1

Air Quality Index

Why is air quality
important?
Local air quality affects how
you live and breathe. Like
the weather, it can change
from day to day or evenhour to hour. The U.S.
Environmental Protection
Agency (EPA) and yourlocal air quality agency
have been working to makeinformation about outdoor
air quality as easy to findand understand as weather Air quality directly affects our qualityof life.
forecasts. A key tool in thiseffort is the Air Quality Index, or AQI. EPA and local officials use the AQI to provide simple information about yourlocal air quality, how unhealthy air may affect you, and how
you can protect your health.

What is the AQI?
The AQI is an index for reporting daily air quality. It tells
you how clean or unhealthy your air is, and what associated health effects might be a concern. The AQI focuses on
health effects you may experience within a few hours or days
after breathing unhealthy air. The AQI is calculated for four
major air pollutants regulated by the Clean Air Act: groundlevel ozone, particle pollution, carbon monoxide, and sulfur
dioxide. For each of these pollutants, EPA has establishednational air quality standards to protect public health.
EPA is currently reviewing the national air quality standard
for nitrogen dioxide. If the standard is revised, the AQI
will be revised as well.

How does the AQI work?
Think of the AQI as a yardstick that runs from 0 to 500.
The higher the AQI value, the greater the level of air pollution and the greater the health concern. For example, an
AQI value of 50 represents good air quality with little or nopotential to affect public health, while an AQI value over
300 represents air quality so hazardous that everyone may
experience serious effects.
An AQI value of 100 generally corresponds to the national air
quality standard for the pollutant, which is the level EPA has set
to protect public health. AQI values at or below 100 are generally
thought of as satisfactory. When AQI values are above 100, air
quality is considered to be unhealthy—at first for certain sensitive
groups of people, then for everyone as AQI values increase.

What do the AQI values mean?
The purpose of the AQI is to help you understand what local
air quality means to your health. To make it easier to understand, the AQI is divided into six levels of health concern:

Air Quality Index(AQI) Values

Levels of
Health Concern

ColorsWhen the AQI is in
this range:...air quality
conditions are:...as symbolized
by this color:

0 to 50

Good

Green

51 to 100Moderate

Yellow

101 to 150

Unhealthy for
Sensitive Groups

Orange151 to 200

UnhealthyRed

201 to 300

Very Unhealthy

Purple301 to 500

Hazardous

Maroon2

Air Quality Index
Each category corresponds to a different level of health concern:• Good. The AQI value for your community is between
0 and 50. Air quality is satisfactory and poses little or nohealth risk.
• Moderate. The AQI is between 51 and 100. Air quality
is acceptable; however, pollution in this range may pose amoderate health concern for a very small number of individuals. People who are unusually sensitive to ozone orparticle pollution may experience respiratory symptoms.
• Unhealthy for Sensitive Groups. When AQI values
are between 101 and 150, members of sensitive groups
may experience health effects, but the general public isunlikely to be affected.
• Ozone: People with lung disease, children, older adults,
and people who are active outdoors are consideredsensitive and therefore at greater risk.
• Particle pollution: People with heart or lung disease,older adults,1 and children are considered sensitive andtherefore at greater risk.
• Unhealthy. Everyone may begin to experience health effects
when AQI values are between 151 and 200. Members ofsensitive groups may experience more serious health effects.
• Very Unhealthy. AQI values between 201 and 300
trigger a health alert, meaning everyone may experience
more serious health effects.
• Hazardous. AQI values over 300 trigger health warningsof emergency conditions. The entire population is evenmore likely to be affected by serious health effects.

How is a community’s AQI calculated
and reported?
Each day, monitors record concentrations of the major pollutants at more than a thousand locations across the country.
1

Due to the normal aging process, older adults may experience increased
health risks from exposure to unhealthy air. Studies indicate that somepeople become more sensitive in their mid-60s. However, the risk ofheart attacks, and thus the risk from particle pollution, may begin as
early as the mid-40s for men and mid-50s for women.

Children active outdoors can be sensitive to air pollutants.

These raw measurements are converted into a separate AQI
value for each pollutant (ground-level ozone, particle pollution, carbon monoxide, and sulfur dioxide) using standard
formulas developed by EPA. The highest of these AQI values
is reported as the AQI value for that day.2
In large cities (more than 350,000 people), state and local
agencies are required to report the AQI to the public daily.
Many smaller communities also report the AQI as a publichealth service.
When the AQI is above 100, agencies must also reportwhich groups, such as children or people with asthma orheart disease, may be sensitive to that pollutant. If two or
more pollutants have AQI values above 100 on a given day,agencies must report all the groups that are sensitive to thosepollutants. For example, if a community’s AQI is 130 forozone and 101 for particle pollution, the AQI value for thatday would be announced as 130 for ozone. The announcements would note that particle pollution levels were alsohigh and would alert groups sensitive to ozone or particle
pollution about how to protect their health.
Many cities also provide forecasts for the next day’s AQI.
These forecasts help local residents protect their health byalerting them to plan their strenuous outdoor activities for atime when air quality is better.
2

For more information on how the AQI is calculated, see “Technical
Assistance Document for the Reporting of Daily Air Quality—the Air
Quality Index (AQI)” in the “Publications” section of www.airnow.gov.

3

Air Quality Index
The AQI is a national index, so the values and colors used
to show local air quality and the levels of health concern are
the same everywhere in the United States.

Where can I find the AQI?
Checking local air quality is as easy as checking the weather.
You can find the latest AQI values on the Internet, in yourlocal media, and on many state and local telephone hotlines.
You can also sign up to receive AQI forecasts by e-mail:• AQI on the Internet. EPA and its federal, tribal, state, andlocal partners have developed an AIRNow Web site toprovide the public with easy access to national air quality
information. At www.airnow.gov, you will find daily AQI
forecasts and real-time AQI conditions for over 300 cities
across the United States, with links to more detailed stateand local air quality Web sites. AIRNow’s reports are
displayed as maps you can use to quickly determine if the
air quality is unhealthy near you.

Example of a national AQI map available on the AIRNow Web site.

• AQI via e-mail. Sign up for EnviroFlash(www.enviroflash.info), a free service that will alert
you via e-mail when air quality is forecast to be aconcern in your area.

• AQI in the media. Many local media—television, radio,
and newspapers—and some national media (such as USA
Today, The Weather Channel, and CNN) provide daily
air quality reports, often as part of the weather forecast.
Here’s the type of report you might hear:
Tomorrow will be a code red air quality day for Center
City. The cold winter air, morning traffic, and woodsmoke are expected to cause particle pollution to rise tounhealthy levels. People with heart or lung disease,older adults, and children should avoid prolonged or
heavy physical activities.

What are typical AQI values inmost communities?
In many U.S. communities, AQI values are usually below
100, with higher values occurring just a few times a year.
Larger cities typically have more air pollution than smaller
cities, so their AQI values may exceed 100 more often. AQI
values higher than 200 are infrequent, and AQI values above
300 are extremely rare—they generally occur only during
events such as forest fires. You can compare the air quality of
U.S. cities and find out about quality trends in your area byvisiting “Air Compare” at www.epa.gov/aircompare/.
AQI values can vary from one season to another. In winter,carbon monoxide may be high in some areas because cold
weather makes it difficult for car emission control systems tooperate effectively. Ozone is often higher in warmer months,
because heat and sunlight increase ozone formation. Particle
pollution can be elevated any time of the year.
AQI values also can vary depending on the time of day.
Ozone levels often peak in the afternoon to early evening.
Carbon monoxide may be a problem during morning or
evening rush hours. And particle pollution can be high any
time of day, and is often elevated near busy roadways, especially during morning or evening rush hours.

4Air Quality Index

How can I avoid being exposed tounhealthy air?
You can take simple steps to reduce your exposure tounhealthy air. First, you need to find out whether AQI levels
are a concern in your area. You can do this, as described
previously, by visiting the AIRNow Web site, signing up forEnviroFlash, or checking your local media. If the AQI forozone, particle pollution, carbon monoxide, or sulfur dioxide
is a concern in your area, you can learn what steps to take toprotect your health by checking the charts on the following
pages. Two important terms you will need to understand are:
• Prolonged exertion. This means any outdoor activity thatyou’ll be doing intermittently for several hours and thatmakes you breathe slightly harder than normal. A good
example of this is working in the yard for part of a day.
When air quality is unhealthy, you can protect your healthby reducing how much time you spend on this type ofactivity.
• Heavy exertion. This means intense outdoor activities that cause you to breathe hard. When air quality
is unhealthy, you can protect your health by reducing
how much time you spend on this type of activity, or by
substituting a less intense activity—for example, go for awalk instead of a jog. Be sure to reduce your activity level
if you experience any unusual coughing, chest discomfort, wheezing, breathing difficulty, or unusual fatigue.

ozone
What is ozone?
Ozone is a gas found in the air we breathe. Ozone can begood or bad, depending where it occurs:• Good ozone is present naturally in the Earth’s upper
atmosphere—approximately 6 to 30 miles above theEarth’s surface. This natural ozone shields us from thesun’s harmful ultraviolet rays.
• Bad ozone forms near the ground when pollutants(emitted by sources such as cars, power plants, industrial
boilers, refineries, and chemical plants) react chemicallyin sunlight. Ozone pollution is more likely to form during warmer months. This is when the weather conditions
normally needed to form ground-level ozone—lots ofsun—occur.

Who is most at risk?
Several groups of people are particularly sensitive to ozone,
especially when they are active outdoors. This is because ozone
levels are higher outdoors, and physical activity causes fasterand deeper breathing, drawing more ozone into the body.
• People with lung diseases, such as asthma, chronic
bronchitis, and emphysema, can be particularly sensitive
to ozone. They will generally experience more serious
health effects at lower levels. Ozone can aggravate theirdiseases, leading to increased medication use, doctor andemergency room visits, and hospital admissions.
• Children, including teenagers, are at higher risk from ozone
exposure because they often play outdoors in warmer weather
when ozone levels are higher, they are more likely to haveasthma (which may be aggravated by ozone exposure), andtheir lungs are still developing.

Heavy exertion means an intense activity that causes you to breathe hard.

• Older adults may be more affected by ozone exposure,possibly because they are more likely to have pre-existinglung disease.

5Air Quality Index• Active people of all ages who exercise or work vigorously
outdoors are at increased risk.
• Some healthy people are more sensitive to ozone. They
may experience health effects at lower ozone levels than
the average person even though they have none of therisk factors listed above. There may be a genetic basis forthis increased sensitivity.
In general, as concentrations of ground-level ozone increase,more people begin to experience more serious health effects.
When levels are very high, everyone should be concerned
about ozone exposure.

What are the health effects?
Ozone affects the lungs and respiratory system in many
ways. It can:
• Irritate the respiratory system, causing coughing,throat soreness, airway irritation, chest tightness, or chest
pain when taking a deep breath.
• Reduce lung function, making it more difficult tobreathe as deeply and vigorously as you normally would,
especially when exercising. Breathing may start to feel
uncomfortable, and you may notice that you are taking
more rapid and shallow breaths than normal.

• Inflame and damage the cells that line the lungs.
Within a few days, the damaged cells are replaced andthe old cells are shed—much like the way your skin peels
after sunburn. Studies suggest that if this type of inflammation happens repeatedly, lung tissue may become
permanently scarred and lung function may be permanently reduced .
• Make the lungs more susceptible to infection. Ozone
reduces the lung’s defenses by damaging the cells thatmove particles and bacteria out of the airways and by
reducing the number and effectiveness of white blood
cells in the lungs.
• Aggravate asthma. When ozone levels are unhealthy,
more people with asthma have symptoms that require a
doctor’s attention or the use of medication. Ozone makes
people more sensitive to allergens—the most common
triggers for asthma attacks. Also, asthmatics may be more
severely affected by reduced lung function and airway
inflammation. People with asthma should ask theirdoctor for an asthma action plan and follow it carefully
when ozone levels are unhealthy.
• Aggravate other chronic lung diseases such as
emphysema and bronchitis. As concentrations ofground-level ozone increase, more people with lungdisease visit doctors or emergency rooms and areadmitted to the hospital.
• Cause permanent lung damage. Repeated short-termozone damage to children’s developing lungs may leadto reduced lung function in adulthood. In adults, ozone
exposure may accelerate the natural decline in lung function that occurs with age.

The risk of exposure to unhealthy levels of ground-level ozone isgreatest during warmer months. Children, who often play outdoors
in warmer weather, are at higher risk.

6

Air Quality Index

How can I protect my health at different
AQI values?
AQI Value

Actions to Protect Your Health
From Ozone

Good(0–50)

None

Moderate(51–100*)

Unusually sensitive people should consider
reducing prolonged or heavy outdoor
exertion.

Unhealthy
forSensitive
Groups(101–150)

The following groups should reduce
prolonged or heavy outdoor exertion:• People with lung disease, such as asthma• Children and older adults• People who are active outdoors
The following groups should avoid
prolonged or heavy outdoor exertion:

Unhealthy(151–200)• People with lung disease, such as asthma• Children and older adults• People who are active outdoors
Everyone else should limit prolonged
outdoor exertion.
The following groups should avoid all
outdoor exertion:

Very
Unhealthy(201–300)• People with lung disease, such as asthma• Children and older adults• People who are active outdoors
Everyone else should limit outdoor
exertion.

*An AQI of 100 for ozone corresponds to an ozone level of 0.075 parts per
million (averaged over 8 hours).

Particle pollution
What is particle pollution?
Particle pollution (also known as “particulate matter”) consists of a mixture of solids and liquid droplets. Some particles
are emitted directly; others form when pollutants emitted
by various sources react in the atmosphere. Particle pollution levels can be very unhealthy and even hazardous during
events such as forest fires. Particle levels can be elevatedindoors, especially when outdoor particle levels are high.
Particles come in a wide range of sizes. Those less than 10
micrometers in diameter (smaller than the width of a single
human hair) are so small that they can get into the lungs,
where they can cause serious health problems.
• Fine particles. The smallest particles (those 2.5
micrometers or less in diameter) are called “fine” particles. These particles are so small they can be detected
only with an electron microscope. Major sources of fineparticles include motor vehicles, power plants, residential
wood burning, forest fires, agricultural burning, someindustrial processes, and other combustion processes.
• Coarse particles. Particles between 2.5 and 10 micrometers in diameter are referred to as “coarse.” Sources ofcoarse particles include crushing or grinding operations,and dust stirred up by vehicles traveling on roads.

What are the health effects and who ismost at risk?
Particles smaller than 10 micrometers in diameter can cause
or aggravate a number of health problems and have been
linked with illnesses and deaths from heart or lung disease.
These effects have been associated with both short-term
exposures (usually over 24 hours, but possibly as short as
one hour) and long-term exposures (years).
Sensitive groups for particle pollution include people withheart or lung disease (including heart failure and coronary
artery disease, or asthma and chronic obstructive pulmonary

7

Air Quality Index
disease), older adults (who may have undiagnosed heart orlung disease), and children. The risk of heart attacks, andthus the risk from particle pollution, may begin as early as
the mid-40s for men and mid-50s for women.
• When exposed to particle pollution, people with heartor lung diseases and older adults are more likely to visit
emergency rooms, be admitted to hospitals, or in somecases, even die.
• Exposure to particle pollution may cause people withheart disease to experience chest pain, palpitations,shortness of breath, and fatigue. Particle pollution has
also been associated with cardiac arrhythmias and heart
attacks.
• When exposed to high levels of particle pollution, people
with existing lung disease may not be able to breathe as
deeply or vigorously as they normally would. They may
experience symptoms such as coughing and shortness ofbreath. Healthy people also may experience these effects,
although they are unlikely to experience more serious effects.

diseases, such as asthma and chronic bronchitis, causing
more use of medication and more doctor visits.

How can I protect my health at different
AQI values?
AQI Value

Actions To Protect Your Health
From Particle PollutionGood(0–50)

None

Moderate(51–100*)

Unusually sensitive people should
consider reducing prolonged or heavy
exertion.

Unhealthy
for Sensitive
Groups(101–150)

The following groups should reduce
prolonged or heavy exertion:• People with heart or lung disease• Children and older adults

The following groups should avoid
prolonged or heavy exertion:• Particle pollution also can increase susceptibility to respiratory infections and can aggravate existing respiratory
Unhealthy(151–200)• People with heart or lung disease• Children and older adults
Everyone else should reduce
prolonged or heavy exertion.
The following groups should avoid all
physical activity outdoors:

Very Unhealthy(201–300)• People with heart or lung disease• Children and older adults
Everyone else should avoid prolonged
or heavy exertion.

* For particles up to 2.5 micrometers in diameter: An AQI of 100 corresponds
to 35 micrograms per cubic meter (averaged over 24 hours).
For particles up to 10 micrometers in diameter: An AQI of 100 corresponds
to 150 micrograms per cubic meter (averaged over 24 hours).
Smoke from old, uncertified wood stoves is a major source of particle
pollution in some communities. For information on cleaner-burning woodstoves that are more energy efficient, go to www.epa.gov/woodstoves.

8Air Quality Index

carbon monoxide
What is carbon monoxide?
Carbon monoxide is an odorless, colorless gas. It forms
when the carbon in fuels does not completely burn. Vehicle
exhaust contributes roughly 75 percent of all carbon monoxide emissions nationwide, and up to 95 percent in cities.
Other sources include fuel combustion in industrial processes
and natural sources such as wildfires. Carbon monoxidelevels typically are highest during cold weather, because cold
temperatures make combustion less complete and cause inversions that trap pollutants close to the ground.

What are the health effects and who ismost at risk?
Carbon monoxide enters the bloodstream through the lungs
and binds to hemoglobin, the substance in blood that carries
oxygen to cells. It reduces the amount of oxygen reaching
the body’s organs and tissues.

About half of all carbon monoxide emissions nationwide come from the exhaustof roadway vehicles. Exhaust from all types of vehicles (including marine vessels,
aircraft, locomotives, and mobile equipment) contributes around three-quartersof all carbon monoxide emissions in the United States.

• People with cardiovascular disease, such as coronary arterydisease, are most at risk. They may experience chest pain
and other cardiovascular symptoms if they are exposed tocarbon monoxide, particularly while exercising.
• People with marginal or compromised cardiovascularand respiratory systems (for example, individuals withcongestive heart failure, cerebrovascular disease, anemia,
or chronic obstructive lung disease), and possibly young
infants and fetuses, also may be at greater risk from carbon monoxide pollution.
• In healthy individuals, exposure to higher levels of carbon monoxide can affect mental alertness and vision.

How can I protect my health at different
AQI values?
AQI Value

Actions To Protect Your Health
From Carbon Monoxide

Good(0–50)

None

Moderate(51–100*)

None

Unhealthy
for Sensitive
Groups(101–150)

People with heart disease, suchas angina, should reduce heavy
exertion and avoid sources of carbon
monoxide, such as heavy traffic.

Unhealthy(151–200)

People with heart disease, such as
angina, should reduce moderate
exertion and avoid sources of carbon
monoxide, such as heavy traffic.

Very Unhealthy(201–300)

People with heart disease, such as
angina, should avoid exertion andsources of carbon monoxide, such as
heavy traffic.

* An AQI of 100 for carbon monoxide corresponds to a level of 9 parts per
million (averaged over 8 hours).

9

Air Quality Indexsulfur dioxide
What is sulfur dioxide?
Sulfur dioxide, a colorless, reactive gas, is produced whensulfur-containing fuels such as coal and oil are burned.
Generally, the highest levels of sulfur dioxide are found nearlarge industrial complexes. Major sources include power
plants, refineries, and industrial boilers.

What are the health effects and who ismost at risk?
Sulfur dioxide is an irritant gas that is removed by the nasal
passages. Moderate activity levels that trigger mouth breathing, such as a brisk walk, are needed for sulfur dioxide tocause health effects in most people.
• People with asthma who are physically active outdoors
are most likely to experience the health effects of sulfur
dioxide. The main effect, even with very brief exposure(minutes), is a narrowing of the airways (called bronchoconstriction). This may be accompanied by wheezing, chest tightness, and shortness of breath, which
may require use of medication that opens the airways.
Symptoms increase as sulfur dioxide levels or breathing

rate increases. When exposure to sulfur dioxide ceases,
lung function typically returns to normal within an hour,even without medication.
• At very high levels, sulfur dioxide may cause wheezing,chest tightness, and shortness of breath even in healthypeople who do not have asthma.
• Long-term exposure to sulfur dioxide may cause respiratory symptoms and illness, and aggravate asthma. People
with asthma are the most susceptible to sulfur dioxide.
However, people with other chronic lung diseases or cardiovascular disease, as well as children and older adults,
may also be susceptible to these effects.

How can I protect my health at different
AQI values?
AQI Value

Actions To Protect Your Health
From Sulfur Dioxide

Good(0–50)

None

Moderate(51–100*)

None

Unhealthy for
Sensitive Groups(101–150)

People with asthma should consider
reducing exertion outdoors.

Unhealthy(151–200)

Children, asthmatics, and people
with heart or lung disease should
reduce exertion outdoors.

Very Unhealthy(201–300)

Children, asthmatics, and people
with heart or lung disease should
avoid outdoor exertion. Everyone
else should reduce exertion
outdoors.

* An AQI of 100 for sulfur dioxide corresponds to a level of 75 parts per billion (averaged over one hour).
Children and adults with asthma whoare active outdoors are most vulnerable
to the health effects of sulfur dioxide.

10

Air Quality Index

Where can I get more information?
For information and resources about air quality, visit the
AIRNow Web site at www.airnow.gov. There you can:• Access maps and information on air quality in your area.
Find out how to protect your health and how to reduce
air pollution.
• Sign up for EnviroFlash (www.environflash.info), a freeservice that will alert you via e-mail when air quality inyour area is forecast to be a concern.
• Download the App to see the air quality for your area on
your smartphone or tablet.
• Access brochures, movies, games, and other air quality
educational resources for adults and kids.
• Visit Air Compare (www.epa.gov/aircompare/), where you
can compare the air quality of U.S. cities and find out
about air quality trends in your area.
• Access Web cameras that provide real-time pictures ofvisibility at many locations across the United States.
• Access training and tools. If you are a health careprovider, teacher, or weathercaster, you can use theseresources to help adults and children understand how air
pollution affects their health and how they can protect
their health.

11