2 Recycled/Recyclable. Printed with 

vegetable oil-based inks on 100% 

postconsumer process, chlorine-free 

recycled paper. 
communities. But what does it mean to you 

...if you are planning outdoor activities that day? 

...if you have children who play outdoors? 

...if you are an older adult? ...if you have asthma? 

This booklet can help you understand what you 

can do to protect yourself from air pollution. 

“Today’s Air Quality 

Index is 105, which is 

unhealthy for sensitive 

groups.” 

air quality, the health concerns for different 

pollution, and how you can protect your he

pollutants reach unhealthy levels. 

What is the AQI? 

The AQI is an index for reporting daily air quality. It tells you 

how clean or polluted your air 

is, and what associated health 

effects might be a concern for 

you. The AQI focuses on health 

effects you may experience 

within a few hours or days after 

breathing polluted air. EPA 

calculates the AQI for five 

major air pollutants regulated 

by the Clean Air Act: groundlevel ozone, particle pollution Air quality d

(also known as particulate our quality o

matter), carbon monoxide, 

sulfur dioxide, and nitrogen dioxide. For each 

pollutants, EPA has established national air q

standards to protect public health. 
g Q

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 categories: 

Air Quality Index Levels of Health Concern Colors 

(AQI) Values 

When the AQI ...air quality conditions are: ...as symbolized 

is in this range: 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 

Each category corresponds to a different level of health 

concern. The six levels of health concern and what they 

mean are: 

■ “Good” The AQI value for your community is between 

0 and 50. Air quality is considered satisfactory, and air 

pollution poses little or no risk. 

■ “Moderate” The AQI for your community is between 

51 and 100. Air quality is acceptable; however, for 

some pollutants there may be a moderate health 

serious health effects. 

■ “Very Unhealthy” AQI values between 201 a

trigger a health alert, meaning everyone ma

more serious health effects. 

■ “Hazardous” AQI values over 300 trigger he

ings of emergency conditions. The entire po

more likely to be affected. 

AQI colors 

A specific color is assigned to each AQI catego

make it easier for you to understand quickly w

pollution is reaching unhealthy levels in your 

For example, the color orange means that con

“unhealthy for sensitive groups,” while red me

conditions may be “unhealthy for everyone,” a

How is a community’s AQI calculated? 

Air quality is measured by monitors that reco

concentrations of the major pollutants each d

than a thousand locations across the country

measurements are then converted into AQI v

standard formulas developed by EPA. An AQ

calculated for each pollutant in an area (grou

ozone, particle pollution, carbon monoxide, 

dioxide, and nitrogen dioxide). The highest A

for the individual pollutants is the AQI value 
Children active outdoors can be sensitive to some air pollutants. 

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 activities for a 

time when air quality is better. 

The AQI is a national index, so the values and colors 

used to show local air quality and the levels of health 

concern will be the same everywhere you go in the 

United States. Look for the AQI to be reported in your 

local newspaper, on television and radio, on the Internet, 

and on many state and local telephone hotlines. 

Tomorrow will be a code red air quality day 

Center City. The cold winter air, morning t

wood smoke are expected to cause particle po

rise to unhealthy levels. People with heart or 

ease, older adults, and children should avoid 

activities. 

■ AQI on the Internet 

EPA’s AIRNow web site (www.epa.gov/airno

contains general information about air pollu

real-time and forecast air quality data. It also 

facts about the health and environmental eff

pollution, steps you can take to protect your 

to reduce pollution, and links to state and lo

pollution agency web sites. 

What are typical AQI values in most com

In many U.S. communities, AQI values are u

100, with values greater than 100 occurring j

times a year. Typically, larger cities have more 

pollution problems, and the AQI in these are

exceed 100 more often than in smaller cities. 

higher than 200 are infrequent, and AQI val

300 are extremely rare. 
g Qypy

health from air pollution. Each chart contains cautionary 

language to help you when air quality levels are unhealthy. 

In general, you can reduce your risk by “reducing prolonged 

or heavy exertion.” Prolonged exertion is an activity that 

occurs over several hours and makes you breathe slightly 

harder than normal. Reducing prolonged exertion could 

mean reducing the time you spend on this type of activity. 

You can also reduce your risk by cutting back on heavy 

exertion—more intense activities that cause you to breathe 

hard. This might mean walking instead of jogging, or jogging for half your usual time. Your breathing rate is a guide 

to how hard you are exerting yourself. If you experience 

any unusual coughing, chest discomfort, wheezing, or 

breathing difficulty, you should reduce your activity level. 

Charts are provided for four pollutants: ozone, particle 

pollution, carbon monoxide, and sulfur dioxide. Another 

common pollutant, nitrogen dioxide, can cause respiratory 

symptoms such as coughing, wheezing, and shortness of 

breath in children and adults who have respiratory diseases, 

such as asthma. The AQI for nitrogen dioxide is not 

included in this booklet because nitrogen dioxide levels 

across the country have been below the national air quality 

standard for the past several years. Nitrogen dioxide levels 

are usually so low that they pose little direct threat to 

human health. Nitrogen dioxide, however, is a concern 

because it plays a significant role in the formation of 

ozone, particle pollution, haze, and acid rain. 

outdoors

people with lung dise

151 - 200 

201 - 300 

301 - 500 

Unhealthy 

Very Unhealthy 

Hazardous 

asthma, should avoid 

heavy exertion outdoo

else, especially child

reduce prolonged or h

Active children and 

asthma, should avoi

exertion. Everyone el

children, should avoid 

heavy exertion o

Everyone should 

physical activity 

*An AQI of 100 for ozone corresponds to an ozone level of 0.08 parts per m

(averaged over 8 hours). 

What is ozone? 

Ozone is a gas composed of three atoms of o

Ozone occurs both in the Earth’s upper atmo

at ground level. Ozone can be good or bad, d

on where it is found: 

■ Good Ozone. Ozone occurs naturally in the 

atmosphere—6 to 30 miles above the Earth

where it forms a protective layer that shield

the sun’s harmful ultraviolet rays. This bene

is gradually being destroyed by manmade c
The risk of exposure to unhealthy levels of ground-level ozone is greatest 

during summer months. 

What are the health effects and who is most at risk? 

Roughly one out of every three people in the United 

States is at a higher risk of experiencing problems from 

ground-level ozone. 

■ One group at high risk is active children because they 

often spend a large part of the summer playing outdoors. 

■ People of all ages who are active outdoors are at 

increased risk because, during physical activity, ozone 

penetrates deeper into the parts of the lungs that are 

more vulnerable to injury. 

become more rapid and shallow than norm

limit a person’s ability to engage in vigorou

■ Ozone can aggravate asthma. When ozone 

are high, more people with asthma have att

require a doctor’s attention or use of medic

reason this happens is that ozone makes pe

sensitive to allergens such as pets, pollen, a

mites, which are common triggers of asthm

■ Ozone can increase susceptibility to respira

infections. 

■ Ozone can inflame and damage the lining 

lungs. Within a few days, the damaged cel

and replaced—much like the skin peels aft

burn. Studies suggest that if this type of in

happens repeatedly over a long time period 

years, a lifetime), lung tissue may become 

scarred, resulting in permanent loss of lung 

and a lower quality of life. 
, 

older adults, and children should 

older adults, and children should 

151 - 200 

201 - 300 

301 - 500 

Unhealthy 

Very Unhealthy 

Hazardous 

avoid prolonged or heavy exertion. 

Everyone else should reduce 

prolonged or heavy exertion. 

People with heart or lung disease, 

avoid all physical activity outdoors. 

Everyone else should avoid 

prolonged or heavy exertion. 

People with heart or lung disease, 

remain indoors and keep activity 

levels low. Everyone else should 

avoid all physical activity outdoors. 

*An AQI of 100 for particles up to 2.5 micrometers in diameter corresponds to a level of 40 

micrograms per cubic meter (averaged over 24 hours). An AQI of 100 for particles up to 10 

micrometers in diameter corresponds to a level of 150 micrograms per cubic meter 

(averaged over 24 hours). 

What is particle pollution? 

Particle pollution (also known as “particulate matter”) 

in the air includes a mixture of solids and liquid droplets. 

Some particles are emitted directly; others are formed in 

the atmosphere when other pollutants react. Particles come 

in a wide range of sizes. Those less than 10 micrometers in 

diameter are so small that they can get into the lungs, 

potentially causing serious health problems. Ten micrometers is smaller than the width of a single human hair. 

or aggravate a number of health problems and 

linked with illnesses and deaths from heart or l

These effects have been associated with both sh

exposures (usually over a 24-hour period, but p

short as one hour) and long-term exposures (y

■ Sensitive groups for particle pollution includ

with heart or lung disease, older adults (who 

undiagnosed heart or lung disease), and chil

■ People with heart or lung diseases—such as 

heart failure, coronary artery disease, asthm

ic obstructive pulmonary disease—and old

more likely to visit emergency rooms, be ad

hospitals, or in some cases, even die. When 

particle pollution, people with heart disease 

rience chest pain, palpitations, shortness of 

fatigue. Particle pollution has also been asso

cardiac arrhythmias and heart attacks. 

■ When exposed to particles, people with exis

disease may not be able to breathe as deeply 

ly as they normally would. They may experi

toms such as coughing and shortness of brea

people also may experience these effects, alth

are unlikely to experience more serious effec

■ Particle pollution also can increase suscepti

respiratory infections and can aggravate exi

tory diseases, such as asthma and chronic b

causing more use of medication and more 
sources of CO, such as 

disease, such as angina, 

sources of CO, such as 

disease, such as angina, 

and sources of CO, such 

201 - 300 

301 - 500 

Very Unhealthy 

Hazardous 

exertion and avoid 

heavy traffic. 

People with heart 

should avoid exertion and 

heavy traffic. 

People with heart 

should avoid exertion 

as heavy traffic. 

Everyone else should 

reduce heavy exertion. 

* An AQI of 100 for carbon monoxide corresponds to a CO level of 9 parts per million 

(averaged over 8 hours). 

What is carbon monoxide? 

Carbon monoxide (CO) is an odorless, colorless gas. 

It forms when the carbon in fuels does not completely 

burn. Vehicle exhaust contributes roughly 60 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. 

■ In healthy individuals, exposure to higher l

carbon monoxide can affect mental alertnes

Vehicle exhaust contributes roughly 60 percent of all ca

emissions nationwide. 
Children, asthmatics, 

lung disease should 

remain indoors. 

201 - 300 

301 - 500 

Very Unhealthy 

Hazardous 

people with heart or lung 

disease should avoid 

outdoor exertion. 

Everyone else should 

reduce exertion outdoors. 

and people with heart or 

Everyone else should 

avoid exertion outdoors. 

* An AQI of 100 for sulfur dioxide corresponds to an SO2 level of 0.14 parts per million 

(averaged over 24 hours). 

What is sulfur dioxide? 

Sulfur dioxide (SO2), a colorless, reactive gas, is produced when sulfur-containing fuels such as coal and 

oil are burned. Major sources include power plants and 

industrial boilers. Generally, the highest levels of sulfur 

dioxide are near large industrial complexes. 

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. 

Children and adults with asthma who are active outdo

vulnerable to the health effects of sulfur dioxide. 

■ Long-term exposure to sulfur dioxide can 

ratory illness, alter the lung’s defense mech

aggravate existing cardiovascular disease. P

cardiovascular disease or chronic lung dise

as children and older adults, may be most 

to these effects. 
Air and Radiation 

EPA-454/K-03-002 

http://www.epa.gov 

August 2003