Ozone is a colorless gas found in the air we breathe. Ozone can be beneficial or harmful to people, depending where it occurs. Beneficial ozone is present naturally in the Earth’s upper atmosphere, 10–30 miles above the Earth’s surface. This natural ozone shields us from the sun’s ultraviolet rays. Harmful ozone forms near ground level when air pollutants (from sources such as cars, power plants, and chemical plants) react chemically in the presence of sunlight. Ozone pollution is more likely to form during warmer months.
Ozone is mainly formed by chemical reactions between volatile organic compounds (VOCs) and oxides of nitrogen in the presence of sunlight and higher temperatures. The human-caused sources of VOCs and nitrogen oxides are industrial and automobile emissions. Ozone concentrations can be transported hundreds of miles and affect remote areas of the country.
How can ground-level ozone affect your health?
Ozone can irritate your respiratory system. When this happens, you may cough, feel irritation or soreness in your throat, or experience chest tightness or pain when taking a deep breath. Reduced lung function can make it more difficult for you to breathe as deeply and easily as you normally would. This is especially true when exercising at higher elevations.
Exposure to ozone can also increase the susceptibility of the lungs to infections, allergens, and other air pollutants. Medical studies have shown that health problems caused by ozone may continue long after exposure has ended.
Who is at risk?
When ozone levels are very high, everyone should be concerned about ozone exposure. In general, as concentrations of ground-level ozone increase, both the number of people affected and the seriousness of the health effects increase. Several groups of people are particularly sensitive to ozone, especially when they are active outdoors. Ozone levels are higher outdoors, and physical activity causes faster and deeper breathing, drawing more ozone into the body.
People particularly sensitive to ozone include:
People with lung diseases, such as asthma, chronic bronchitis, and emphysema will generally experience more serious health effects at lower ozone levels.
Children are at higher risk from ozone exposure because they often play outdoors in summer when ozone levels are higher 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.
Active people of all ages who exercise or work outdoors have higher exposure to ozone than people who are less active.
Some healthy people 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.
What are National Park Service ozone health advisories?
The NPS issues ozone health advisories when ozone concentrations exceed, or are forecasted to exceed, the National Ambient Air Quality Standard. The ozone health standard is an 8-hour average concentration of 70 parts per billion (ppb). NPS ozone health advisories are based on the EPA air quality index levels shown below. View ozone health advisories issued for National Park Service areas.
Understanding Ozone Health Advisory Levels
Good (0–54 ppb) No cautionary statement.
Moderate (55–70 ppb) Unusually sensitive people should consider reducing prolonged or heavy exertion outdoors.
Unhealthy for Sensitive Groups (71–85 ppb) Children, older adults, active people, and people with lung disease (such as asthma) should reduce prolonged or heavy exertion outdoors.
Unhealthy (86–105 ppb)
Children, older adults, active people, and people with lung disease (such as asthma) should avoid prolonged or heavy exertion outdoors. Everyone else should reduce prolonged or heavy exertion outdoors.
Very Unhealthy (106+ ppb) Children, older adults, active people, and people with lung disease (such as asthma) should avoid all outdoor exertion. Everyone else should avoid prolonged or heavy exertion outdoors.
How does ground-level ozone affect natural resources?
Some plants are actually sensitive to ozone at levels well under the national health standards. Lichens, mosses, and liverworts often are most sensitive within an ecosystem and can serve as early indicators of air pollution effects. Plants can also be injured by ozone, which damages their leaves or needles and weakens plants’ ability to fight off disease and insect infestations. Learn more »