United States Air and Radiation EPA-452/K-99-001

Environmental Protection Washington, DC 20460 July 1999

Agency http://www.epa.gov/oar/oaqps

1EPA Smog—Who Does It Hurt?

What You Need to Know About

Ozone and Your Health

2Printed on paper containing at least 

30% postconsumer recovered fiber.
Smog—Who Does It Hurt?

What You Need to Know About Ozone and Your Health

Ozone, when it occurs 

at ground level, presents 

a serious air quality problem

in many parts of the United

States. When inhaled—

even at very low levels—

ozone can cause a number 

of respiratory health effects.

In fact, breathing smoggy air can 

be hazardous because smog contains

ozone, a pollutant that can harm our

health when there are elevated levels 

in the air we breathe. This publication

will tell you what kinds of health

effects ozone can cause, when you

should be concerned, and what you

can do to avoid dangerous exposures. 

What is ozone?

Ozone is an odorless, colorless gas

composed of three atoms of oxygen.

Ozone occurs both in the Earth’s 

upper atmosphere and at ground level.

Ozone can be good or bad, depending

on where it is found:

! Good Ozone. Ozone occurs naturally in the Earth’s upper atmosphere—

10 to 30 miles above the Earth’s 

surface—where it forms a protective

layer that shields us from the sun’s

harmful ultraviolet rays. This “good”

ozone is gradually being destroyed by

manmade chemicals. An area where

ozone has been most significantly

depleted—for example, over the 

North or South pole—is sometimes

called a “hole in the ozone.”

! Bad Ozone. In the Earth’s lower

atmosphere, near ground level, ozone 

is formed when pollutants emitted by

cars, power plants, industrial boilers,

refineries, chemical plants, and other

sources react chemically in the presence

of sunlight. 

The booklet Ozone: Good Up High, 

Bad Nearby, which can be found on 

the web at http://www.epa.gov/oar/

oaqps/gooduphigh, contains additional information about both good and

bad ozone.

This publication, Smog—Who Does It

Hurt?, focuses on bad ozone—that is,

ozone that occurs at ground level and

can affect the health of people who

breathe it.

On a hot, smoggy summer day,

have you ever wondered: Is the air safe to breathe? 

Should I be concerned about going outside?

Smog—Who Does It Hurt? 1
Should I be concerned about 

exposure to ground-level ozone?

That depends on who you are and 

how much ozone is in the air. Most

people only have to worry about 

ozone exposure when ground-level

concentrations reach high levels. 

In many U.S. communities, this can 

happen frequently during the summer

months. In general, as ground-level

ozone concentrations increase, more

and more people experience health

effects, the effects become more serious, and more people are admitted to

the hospital for respiratory problems.

When ozone levels are very high, 

everyone should be concerned about

ozone exposure.

Scientists have found that about 

one out of every three people in the

United States is at a higher risk of

experiencing ozone-related health

effects (see box on page 3). If you 

are a member of a “sensitive group,”

you should pay special attention to

ozone levels in your area. This publication describes several tools that the

U.S. Environmental Protection Agency

(EPA), in partnership with State and

local agencies, has developed to inform

the public about local ozone levels.

These tools provide the information

you need to decide whether ozone 

levels on any particular day may be

harmful to you. When ozone concentrations reach unhealthy levels, you 

can take simple precautions (described

on page 6 in “What can I do to avoid

unhealthy exposure to ozone?”) to 

protect your health. 

How might ozone affect 

my health?

Scientists have been studying the

effects of ozone on human health for

many years. So far, they have found

that ozone can cause several types of

short-term health effects in the lungs:

! Ozone can irritate the respiratory system. When this happens,

you might start coughing, feel an irritation in your throat, and/or experience an uncomfortable sensation in

your chest. These symptoms can last

for a few hours after ozone exposure

and may even become painful.

! Ozone can reduce lung function.

When scientists refer to “lung function,” they mean the volume of air that

you draw in when you take a full breath

and the speed at which you are able to

blow it out. Ozone can make it more

difficult for you to breathe as deeply

and vigorously as you normally would.

When this happens, you may notice

that breathing starts to feel uncomfortable. If you are exercising or working

outdoors, you may notice that you are

taking more rapid and shallow breaths

than normal. Reduced lung function

can be a particular problem for outdoor workers, competitive athletes, and

other people who exercise outdoors.

! Ozone can aggravate asthma.

When ozone levels are high, more 

asthmatics have asthma attacks that

require a doctor’s attention or the use

of additional medication. One reason

this happens is that ozone makes people more sensitive to allergens, which

are the most common triggers for 

asthma attacks. (Allergens come from

dust mites, cockroaches, pets, fungus,

and pollen.) Also, asthmatics are more

severely affected by the reduced lung

function and irritation that ozone 

causes in the respiratory system.

Children and adults of all ages who

are active outdoors are at risk from

ozone exposure.

2 Smog—Who Does It Hurt?
! Ozone can inflame and damage

the lining of the lung. Some scientists

have compared ozone’s effect on the

lining of the lung to the effect of sunburn on the skin. Ozone damages the

cells that line the air spaces in the lung.

Within a few days, the damaged cells

are replaced and the old cells are shed—

much in the way that skin peels after 

a sunburn. If this kind of damage

occurs repeatedly, the lung may change

permanently in a way that could cause

long-term health effects and a lower

quality of life. 

! Scientists suspect that ozone may

have other effects on people’s health.

Ozone may aggravate chronic lung 

diseases, such as emphysema and bronchitis. Also, studies in animals suggest

that ozone may reduce the immune

system’s ability to fight off bacterial

infections in the respiratory system. 

Most of these effects are considered 

to be short-term effects because they

eventually cease once the individual 

is no longer exposed to elevated levels 

of ozone. However, scientists are 

concerned that repeated short-term

This photo shows a healthy lung airway (top) and an inflamed lung airway

(bottom). Ozone can inflame the

lung’s lining, and repeated episodes

of inflammation may cause permanent

changes in the lung.

Four groups of people, described below, are particularly sensitive 

to ozone. These groups become sensitive to ozone when they are active

outdoors, because physical activity (such as jogging or outdoor work)

causes people to breathe faster and more deeply. During activity, ozone

penetrates deeper into the parts of the lungs that are more vulnerable to

injury. Sensitive groups include:

! Children. Active children are the group at highest risk from ozone exposure. Such children often spend a large part of their summer vacation

outdoors, engaged in vigorous activities either in their neighborhood or at

summer camp. Children are also more likely to have asthma or other 

respiratory illnesses. Asthma is the most common chronic disease for children and may be aggravated by ozone exposure.

! Adults who are active outdoors. Healthy adults of all ages who exercise or work vigorously outdoors are considered a “sensitive group”

because they have a higher level of exposure to ozone than people who

are less active outdoors.

! People with respiratory diseases, such as asthma. There is no evidence that ozone causes asthma or other chronic respiratory disease, but

these diseases do make the lungs more vulnerable to the effects of ozone.

Thus, individuals with these conditions will generally experience the

effects of ozone earlier and at lower levels than less sensitive individuals.

! People with unusual susceptibility to ozone. Scientists don’t yet

know why, but some healthy people are simply more sensitive to ozone

than others. These individuals may experience more health effects from

ozone exposure than the average person.

Scientists have studied other groups to find out whether they are at

increased risk from ozone. So far there is little evidence to suggest that

either the elderly or people with heart disease have heightened sensitivity

to ozone. However, like other adults, elderly people will be at higher risk

from ozone exposure if they suffer from respiratory disease, are active

outdoors, or are unusually susceptible to ozone as described above.

Who is most at risk from ozone?

Smog—Who Does It Hurt? 3
damage from ozone exposure may 

permanently injure the lung. For 

example, repeated ozone impacts on

the developing lungs of children may

lead to reduced lung function as adults.

Also, ozone exposure may speed up the

decline in lung function that occurs as

a natural result of the aging process.

Research is underway to help us better

understand the possible long-term

effects of ozone exposure.

How can I tell if I am being 

affected by ozone?

Often, people exposed to ozone experience recognizable symptoms, including

coughing, irritation in the airways,

rapid or shallow breathing, and discomfort when breathing or general 

discomfort in the chest. People with

asthma may experience asthma attacks.

When ozone levels are higher than 

normal, any of these symptoms may

indicate that you should minimize the

time spent outdoors, or at least reduce

your activity level, to protect your

health until ozone levels decline.

Ozone damage also can occur without

any noticeable signs. Sometimes there

are no symptoms, or sometimes they

are too subtle to notice. People who 

live in areas where ozone levels are frequently high may find that their initial

symptoms of ozone exposure go away

over time—particularly when exposure

to high ozone levels continues for several days. This does not mean that they

have developed resistance to ozone. 

In fact, scientists have found that ozone

continues to cause lung damage even

when the symptoms have disappeared.

The best way to protect your health 

is to find out when ozone levels are 

elevated in your area and take simple 

precautions to minimize exposure even

when you don’t feel obvious symptoms.

How can I find out about 

ozone levels in my area?

EPA and State and local air agencies

have developed a number of tools 

to provide people with information 

on local ozone levels, their potential

health effects, and suggested activities

for reducing ozone exposure.

Air Quality Index. EPA has developed the Air Quality Index, or AQI, 

(formerly known as the Pollutant

Standards Index) for reporting the 

levels of ozone and other common air

pollutants. The index makes it easier

for the public to understand the health

significance of air pollution levels. 

Air quality is measured by a nationwide monitoring system that records

concentrations of ozone and several

other air pollutants at more than a

thousand locations across the country.

EPA has gathered a great deal of information about the health effects of ozone.

This information comes from a number of sources, including animal research, studies that compare health statistics and ozone levels within communities, and controlled testing of human volunteers to determine how ozone affects lung function.

In these studies, volunteers are exposed to ozone in specially designed chambers

where their responses can be carefully measured. Volunteers are prescreened in

medical examinations to determine their health status, and they are never exposed

to ozone levels that exceed those found in major cities on a very smoggy day. 

Though our understanding of ozone’s effects has increased substantially in recent

years, many important questions still remain to be investigated. For example, does

repeated short-term exposure to high levels of ozone cause permanent lung damage? Does repeated exposure during childhood to high levels of ozone cause

reduced lung function in adults? Scientists are continuing to study these and other

questions to gain a better understanding of ozone’s effects.

How do scientists know about the health effects of ozone?

A volunteer in an ozone

research study breathes into

a spirometer—a device that

measures lung function.

4 Smog—Who Does It Hurt?
EPA “translates” the pollutant concentrations to the standard AQI index,

which ranges from 0 to 500. The higher the AQI value for a pollutant, the

greater the danger. An AQI value of

100 usually corresponds to the national

ambient air quality standard (NAAQS)

for the pollutant. These standards are

established by EPA under the Clean

Air Act to protect public health and

the environment. 

The AQI scale has been divided into

distinct categories, each corresponding

to a different level of health concern. 

In the table below, the AQI ranges are

shown in the middle column and the

associated air quality descriptors are

shown in the right column. The left column shows the ozone concentrations,

measured in parts per million (ppm),

that correspond to each category.

Though the AQI scale extends to 500,

levels above 300 rarely occur in the

United States. This publication and

most other references to the AQI do

not list health effects and cautionary

statements for levels above 300. If

ozone levels above 300 should ever

occur, everyone should avoid physical

exertion outdoors.

When pollutant levels are high, states

are required to report the AQI in large

metropolitan areas (populations over

350,000) of the United States. You

may see the AQI for ozone reported 

in your newspaper, or your local television or radio weathercasters may use

the AQI to provide information about

ozone in your area. Here’s

the type of report you

might hear:

The Air Quality Index

today was 160. Air quality was unhealthy due to

ozone. Hot, sunny weather

and stagnant air caused

ozone in Center City to

rise to unhealthy levels.

AQI Colors. To make it

easier for the public to

quickly understand the 

air quality in their communities, EPA has

assigned a specific color 

to each AQI category. 

You will see these colors

when the AQI is reported

in a color format—such as

in a color-print newspaper, on television broadcasts, or on your State or

local air pollution agency’s web site.

This color scheme can help you quickly determine whether air pollutants are

reaching unhealthy levels in your area.

For example, the color orange means

that conditions are “unhealthy for 

sensitive groups,” the color red means

that conditions are “unhealthy” for

everyone, and so on.

Ozone Maps. In many areas of the

country, measurements of ozone concentrations are converted into color

contours of the AQI categories (green,

yellow, orange, red, and purple, shown

below) and displayed on a map (see

example above) to show ozone levels 

Ozone Concentration (ppm) Air Quality Index Values Air Quality Descriptor

(8-hour average, unless noted)

0.0 to 0.064 0 to 50 Good

0.065 to 0.084 51 to 100 Moderate

0.085 to 0.104 101 to 150 Unhealthy for Sensitive Groups

0.105 to 0.124 151 to 200 Unhealthy

0.125 (8-hr.) to 0.404 (1-hr.) 201 to 300 Very Unhealthy

This map shows ozone levels in the

eastern United States on August 24,

1998. Ozone maps are updated several

times daily to show how ozone levels

change throughout the day.

Smog—Who Does It Hurt? 5
in the local area. The map is updated

throughout the day and shows how

ozone builds during hot summer days.

In some areas, ozone maps are used 

to show a forecast of ozone levels for 

the next day. Once you understand 

the color scheme, you can use the maps 

to quickly determine whether ozone

concentrations are reaching unhealthy

levels in your area. Ozone maps appear

on some televised weather broadcasts

and are also available from EPA’s web

site at http://www.epa.gov/airnow.

What can I do to avoid unhealthy

exposure to ozone?

You can take a number of steps. The

chart on page 7 tells you what types 

of health effects may occur at specific

ozone concentrations and what you can

do to avoid them. If you are a parent,

keep in mind that your children are

likely to be at higher risk, particularly 

if they are active outdoors. You may

therefore want to pay special attention

to the guidance for sensitive groups. 

In general, when ozone levels are 

elevated, your chances of being 

affected by ozone increase the longer

you are active outdoors and the more

strenuous the activity you engage in.

Scientific studies show that:

! At ozone levels above 0.12 ppm,

heavy outdoor exertion for short periods of time (1 to 3 hours) can increase

your risk of experiencing respiratory

symptoms and reduced lung function. 

! At ozone levels between 0.08 and

0.12 ppm, even moderate outdoor

exertion for longer periods of time 

(4 to 8 hours) can increase your risk 

of experiencing ozone-related effects.

EPA recommends limiting outdoor

activities as ozone levels rise to

unhealthy levels. You can limit the

In general, when ozone 

levels are elevated, your

chances of being affected 

by ozone increase the longer

you are active outdoors and

the more strenuous the 

activity you engage in. 

Exercise and outdoor activities can play an important role in maintaining

good health. Physical exertion helps build up strength in the heart and

lungs. But exerting yourself outdoors can actually increase your chances

of experiencing health effects when ozone concentrations are at unhealthy

levels. Why is this true? Think of it this way: Exertion generally causes you

to breathe harder and faster. When this happens, more ozone is taken into

your lungs, and ozone reaches tissues that are susceptible to injury.

Research has shown that respiratory effects are observed at lower ozone

concentrations if either the level or duration of exertion is increased. This

is why EPA recommends decreasing the level or duration of exertion to

avoid ozone health effects.

Examples of typical daily activities that involve moderate exertion include

climbing stairs, light jogging, easy cycling, playing tennis or baseball, and

stacking firewood. Outdoor occupational activities such as simple construction work, pushing a wheelbarrow with a load, using a sledgehammer,

or digging in your garden, would also involve moderate exertion. Activities

that involve heavy exertion include vigorous running or cycling, playing

basketball or soccer, chopping wood, and heavy manual labor. Because fitness levels vary widely among individuals, what is moderate exertion for

one person may be heavy exertion for another. No matter how fit you are,

cutting back on the level or duration of exertion when ozone levels are high

will help protect you from ozone’s harmful effects.

What does exertion have to do with 

ozone-related health effects?

6 Smog—Who Does It Hurt?
Health Effects and Protective Actions for Specific Ozone Ranges

1Members of sensitive groups include children who are active outdoors; adults involved in moderate or strenuous outdoor

activities; individuals with respiratory disease, such as asthma; and individuals with unusual susceptibility to ozone. 

Health Effects and Protective Actions

What are the possible health effects? 

! No health effects are expected.

What are the possible health effects?

! Unusually sensitive individuals may experience respiratory effects from prolonged exposure to

ozone during outdoor exertion.

What can I do to protect my health?

! When ozone levels are in the “moderate” range, consider limiting prolonged outdoor exertion if you

are unusually sensitive to ozone. 

What are the possible health effects?

! If you are a member of a sensitive group,1 you may experience respiratory symptoms (such as coughing or

pain when taking a deep breath) and reduced lung function, which can cause some breathing discomfort.

What can I do to protect my health?

! If you are a member of a sensitive group,1 limit prolonged outdoor exertion. In general, you can 

protect your health by reducing how long or how strenuously you exert yourself outdoors and by

planning outdoor activities when ozone levels are lower (usually in the early morning or evening). 

! You can check with your State air agency to find out about current or predicted ozone levels in your

location. This information on ozone levels is available on the Internet at http://www.epa.gov/airnow.

What are the possible health effects?

! If you are a member of a sensitive group,1 you have a higher chance of experiencing respiratory

symptoms (such as aggravated cough or pain when taking a deep breath), and reduced lung function, which can cause some breathing difficulty. 

! At this level, anyone could experience respiratory effects.

What can I do to protect my health?

! If you are a member of a sensitive group,1 avoid prolonged outdoor exertion. Everyone else—

especially children—should limit prolonged outdoor exertion.

! Plan outdoor activities when ozone levels are lower (usually in the early morning or evening).

! You can check with your State air agency to find out about current or predicted ozone levels in your

location. This information on ozone levels is available on the Internet at http://www.epa.gov/airnow.

What are the possible health effects?

! Members of sensitive groups1 will likely experience increasingly severe respiratory symptoms and

impaired breathing.

! Many healthy people in the general population engaged in moderate exertion will experience some

kind of effect. According to EPA estimates, approximately:

! Half will experience moderately reduced lung function.

! One-fifth will experience severely reduced lung function.

! 10 to 15 percent will experience moderate to severe respiratory symptoms (such as aggravated

cough and pain when taking a deep breath).

! People with asthma or other respiratory conditions will be more severely affected, leading some to

increase medication usage and seek medical attention at an emergency room or clinic.

What can I do to protect my health?

! If you are a member of a sensitive group,1 avoid outdoor activity altogether. Everyone else—especially

children—should limit outdoor exertion and avoid heavy exertion altogether.

! Check with your State air agency to find out about current or predicted ozone levels in your location. This information on ozone levels is available on the Internet at http://www.epa.gov/airnow.

Ozone Level

Good

Moderate

Unhealthy 

for Sensitive

Groups

Unhealthy 

Very

Unhealthy 

Smog—Who Does It Hurt? 7
amount of time you are active outdoors

or your activity level. For example, 

if you’re involved in an activity that

requires heavy exertion, such as running or heavy manual labor (see box 

on page 6), you can reduce the time

you spend on this activity or substitute

another activity that requires less exertion (e.g., go for a walk rather than a

jog). In addition, you can plan outdoor

activities when ozone levels are lower,

usually in the early morning or evening.

What can I do to reduce 

ozone levels?

Ground-level ozone is created when

certain pollutants, known as “ozone

precursors,” react in heat and sunlight

to form ozone. Cars and other vehicles

are the largest source of ozone precursors. Other important sources include

industrial facilities, power plants, gasoline-powered mowers, and evaporation

of cleaners, paints, and other chemicals.

We can all help reduce ozone levels by

taking the following steps:

! Drive less. For example, instead of

using a car, you may want to walk, 

use mass transit, or ride a bike.

! Carpool.

! Make sure your car is well-tuned.

! Take care not to spill gasoline when

you fill the tank of your car or lawn or

recreation equipment.

! Make sure that you tightly seal the

lids of chemical products—such as 

solvents, garden chemicals, or household cleaners—to keep evaporation to

a minimum.

For more ideas about what you can do,

visit EPA’s web site at http://www.epa.

gov/airnow/consumer.html.

Motor vehicles are a major contributor to smog. 

You can help reduce ozone levels by

walking, biking, carpooling, or using

public transportation as an alternative

to driving.

The best way to protect your

health is to find out when

ozone levels are elevated 

in your area and take simple 

precautions to minimize 

exposure—even when you 

don’t feel obvious symptoms.

8 Smog—Who Does It Hurt?