What You Need to Know About Ozone and
Your Health
On a hot, smoggy summer day, have you ever wondered: Is the
air safe to breathe? Should I be concerned about going outside?
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 a 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/airnow/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.
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Children and adults
of all ages who are active
outdoors are at risk from ozone exposure.
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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 "Who is most at risk
from ozone?"). 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
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.
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This photo shows a healthy lung air
way (top) and an inflamed lung air way (bottom). Ozone
can inflame the lung's lining, and repeated episodes of
inflammation may cause permanent changes in the lung. |
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.
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 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.
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.
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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.
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.
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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 "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.
Ozone
Concentration (ppm)
(8-hour
average, unless noted) |
Air
Quality Index
Values |
Air
Quality
Descriptor |
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 |
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 above) and displayed on a map (see example below) to
show ozone levels 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://epa.gov/airnow.
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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. |
What
can I do to avoid unhealthy exposure to ozone?
You can take a number of steps. The chart below, "Health
Effects and Protective Actions for Specific Ozone Ranges,"
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 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 "What
does exertion have to do with ozone-related health effects?"),
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.
Health
Effects and Protective Actions for Specific Ozone Ranges
Ozone Level |
Health
Effects and Protective Actions
|
Good |
What are the possible
health effects?
- No health effects are expected.
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Moderate |
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.
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Unhealthy for Sensitive
Groups |
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://epa.gov/airnow
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Unhealthy |
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.
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Very
Unhealthy |
What
are the possible health effects?
- Members of sensitive
groups(1)
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 elseespecially
childrenshould 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.
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Members 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.
What does exertion
have to do with ozone-related health effects?
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 can I do to reduce ozone levels?
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Motor vehicles are a major contributor to smog. |
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:
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You can help reduce ozone levels by walking, biking, carpooling, or using public transportation as an alternative. |
- 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: http://www.epa.gov/airnow/index.cfm?static.consumer. |
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