A Guide to Air Quality and 

Your Health

Recycled/recyclable. Printed with vegetable oil-based 

inks on 100% postconsumer process, chlorine-free 

recycled paper.

U.S. Environmental Protection Agency

Office of Air Quality Planning and Standards

Outreach and Information Division

Research Triangle Park, NC

August 2009

EPA-456/F-09-002
“Local air quality is very 

unhealthy today.” 

“It’s a code red day 

for ozone.”

“Particle pollution levels are 

forecast to be unhealthy 

for sensitive groups.”

You may hear these alerts on radio 

or 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 understand 

how to find out about air quality in 

your 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 even 

hour to hour. The U.S. 

Environmental Protection 

Agency (EPA) and your 

local air quality agency 

have been working to make 

information about outdoor 

air quality as easy to find 

and understand as weather 

forecasts. A key tool in this 

effort is the Air Quality Index, or AQI. EPA and local officials use the AQI to provide simple information about your 

local 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 established 

national 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. 

Air quality directly affects our quality 

of life.

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 no 

potential 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

Colors

When the AQI is in 

this range:

...air quality 

conditions are:

...as symbolized 

by this color:

0 to 50 Good Green

51 to 100 Moderate Yellow

101 to 150 Unhealthy for 

Sensitive Groups Orange

151 to 200 Unhealthy Red

201 to 300 Very Unhealthy Purple

301 to 500 Hazardous Maroon

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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 no 

health risk. 

•	 Moderate. The AQI is between 51 and 100. Air quality 

is acceptable; however, pollution in this range may pose a 

moderate health concern for a very small number of individuals. People who are unusually sensitive to ozone or 

particle 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 is 

unlikely to be affected. 

•	 Ozone: People with lung disease, children, older adults, 

and people who are active outdoors are considered 

sensitive and therefore at greater risk.

•	 Particle pollution: People with heart or lung disease, 

older adults,1 and children are considered sensitive and 

therefore at greater risk.

•	 Unhealthy. Everyone may begin to experience health effects 

when AQI values are between 151 and 200. Members of 

sensitive 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 warnings 

of emergency conditions. The entire population is even 

more 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 some 

people become more sensitive in their mid-60s. However, the risk of 

heart attacks, and thus the risk from particle pollution, may begin as 

early as the mid-40s for men and mid-50s for women.

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 public 

health service.

When the AQI is above 100, agencies must also report 

which groups, such as children or people with asthma or 

heart 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 those 

pollutants. For example, if a community’s AQI is 130 for 

ozone and 101 for particle pollution, the AQI value for that 

day would be announced as 130 for ozone. The announcements would note that particle pollution levels were also 

high 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 by 

alerting them to plan their strenuous outdoor activities for a 

time when air quality is better.

2 For more information on how the AQI is calculated, see “Guidelines for 

the Reporting of Daily Air Quality—the Air Quality Index (AQI)” in 

the “Publications” section of www.airnow.gov.

Children active outdoors can be sensitive to air pollutants. 

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 your 

local 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, and 

local partners have developed an AIRNow Web site to 

provide 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 state 

and 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. 

•	 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 a 

concern in your area.

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

•	 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: 

What are typical AQI values in 

most 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 by 

visiting “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 to 

operate 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.

Tomorrow will be a code red air quality day for Center 

City. The cold winter air, morning traffic, and wood 

smoke are expected to cause particle pollution to rise to 

unhealthy levels. People with heart or lung disease, 

older adults, and children should avoid prolonged or 

heavy physical activities.

4
Air Quality Index

How can I avoid being exposed to 

unhealthy air? 

You can take simple steps to reduce your exposure to 

unhealthy 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 for 

EnviroFlash, or checking your local media. If the AQI for 

ozone, particle pollution, carbon monoxide, or sulfur dioxide 

is a concern in your area, you can learn what steps to take to 

protect 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 that 

you’ll be doing intermittently for several hours and that 

makes 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 health 

by reducing how much time you spend on this type of 

activity. 

•	 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 a 

walk 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. 

What is ozone? 

Ozone is a gas found in the air we breathe. Ozone can be 

good or bad, depending where it occurs: 

•	 Good ozone is present naturally in the Earth’s upper 

atmosphere—approximately 6 to 30 miles above the 

Earth’s surface. This natural ozone shields us from the 

sun’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 chemically 

in 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 of 

sun—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 faster 

and 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 their 

diseases, leading to increased medication use, doctor and 

emergency room visits, and hospital admissions.

•	 Children 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 have asthma (which may 

be aggravated by ozone exposure), and their lungs are still 

developing.

•	 Older adults may be more affected by ozone exposure, 

possibly because they are more likely to have pre-existing 

lung disease.

ozone 

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

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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 the 

risk factors listed above. There may be a genetic basis for 

this 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 to 

breathe 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. 

The risk of exposure to unhealthy levels of ground-level ozone is 

greatest during warmer months. Children, who often play outdoors 

in warmer weather, are at higher risk.

•	 Inflame and damage the cells that line the lungs.

Within a few days, the damaged cells are replaced and 

the 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 that 

move 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 their 

doctor 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 of 

ground-level ozone increase, more people with lung 

disease visit doctors or emergency rooms and are 

admitted to the hospital. 

•	 Cause permanent lung damage. Repeated short-term 

ozone damage to children’s developing lungs may lead 

to reduced lung function in adulthood. In adults, ozone 

exposure may accelerate the natural decline in lung function that occurs with age.

6
Air Quality Index

How can I protect my health at different 

AQI values?

* An AQI of 100 for ozone corresponds to an ozone level of 0.075 parts per 

million (averaged over 8 hours).

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 

for 

Sensitive 

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

Unhealthy 

(151–200)

The following groups should avoid 

prolonged or heavy outdoor exertion:

•	People with lung disease, such as asthma

•	Children and older adults

•	People who are active outdoors

Everyone else should limit prolonged 

outdoor exertion.

Very 

Unhealthy 

(201–300)

The following groups should avoid all 

outdoor exertion:

•	People with lung disease, such as asthma

•	Children and older adults

•	People who are active outdoors

Everyone else should limit outdoor 

exertion.

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 elevated 

indoors, 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 fine 

particles include motor vehicles, power plants, residential 

wood burning, forest fires, agricultural burning, some 

industrial processes, and other combustion processes. 

•	 Coarse particles. Particles between 2.5 and 10 micrometers in diameter are referred to as “coarse.” Sources of 

coarse particles include crushing or grinding operations, 

and dust stirred up by vehicles traveling on roads. 

What are the health effects and who is 

most 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 with 

heart or lung disease (including heart failure and coronary 

artery disease, or asthma and chronic obstructive pulmonary 

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Air Quality Index

disease), older adults (who may have undiagnosed heart or 

lung disease), and children. The risk of heart attacks, and 

thus 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 heart 

or lung diseases and older adults are more likely to visit 

emergency rooms, be admitted to hospitals, or in some 

cases, even die. 

•	 Exposure to particle pollution may cause people with 

heart 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 of 

breath. Healthy people also may experience these effects, 

although they are unlikely to experience more serious effects. 

•	 Particle pollution also can increase susceptibility to respiratory infections and can aggravate existing respiratory 

Smoke from old, uncertified wood stoves is a major source of particle 

pollution in some communities. For information on cleaner-burning wood 

stoves that are more energy efficient, go to www.epa.gov/woodstoves.

AQI Value Actions To Protect Your Health 

From Particle Pollution

Good 

(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 outdoor exertion:

•	People with heart or lung disease

•	Children and older adults

Everyone else should limit prolonged 

or heavy exertion.

Unhealthy 

(151–200)

The following groups should avoid all

physical outdoors:

•	People with heart or lung disease

•	Children and older adults

Everyone else should avoid prolonged 

or heavy exertion.

Very Unhealthy 

(201–300)

The following groups should remain 

indoors and keep activity levels low:

•	People with heart or lung disease

•	Children and older adults

Everyone else should avoid all 

physical activity outdoors.

* For particles up to 2.5 micrometers in diameter: EPA intends to update the 

AQI rule to reflect the Agency’s September 2006 standards for fine particle 

pollution (PM2.5). In anticipation of this action, AQI forecasts and reports 

on the AIRNow Web site use the new 24-hour fine particle standard—35 

micrograms per cubic meter—as the 100 level of the AQI.

For particles up to 10 micrometers in diameter: An AQI of 100 corresponds 

to 150 micrograms per cubic meter (averaged over 24 hours).

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?

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 monoxide 

levels 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 is 

most 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.

•	 People with cardiovascular disease, such as coronary artery 

disease, are most at risk. They may experience chest pain 

and other cardiovascular symptoms if they are exposed to 

carbon monoxide, particularly while exercising. 

•	 People with marginal or compromised cardiovascular 

and respiratory systems (for example, individuals with 

congestive 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?

About half of all carbon monoxide emissions nationwide come from the exhaust 

of roadway vehicles. Exhaust from all types of vehicles (including marine vessels, 

aircraft, locomotives, and mobile equipment) contributes around three-quarters 

of all carbon monoxide emissions in the United States. 

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, such 

as 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 and 

sources 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 Index

sulfur dioxide

What is sulfur dioxide? 

Sulfur dioxide, a colorless, reactive gas, is produced when 

sulfur-containing fuels such as coal and oil are burned. 

Generally, the highest levels of sulfur dioxide are found near 

large industrial complexes. Major sources include power 

plants, refineries, and industrial boilers.

What are the health effects and who is 

most 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 to 

cause 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 healthy 

people 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?

* An AQI of 100 for sulfur dioxide corresponds to a level of 0.14 parts per 

million (averaged over 24 hours).

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. 

Children and adults with asthma who 

are 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 free 

service that will alert you via e-mail when air quality in 

your area is forecast to be a concern.

•	 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 of 

visibility at many locations across the United States.

•	 Access training and tools. If you are a health care 

provider, teacher, or weathercaster, you can use these 

resources to help adults and children understand how air 

pollution affects their health and how they can protect 

their health.

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