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Ozone: The Good and The Bad

Ozone: Good Up High, Bad Nearby
Ozone, a highly reactive form of oxygen, can be good and bad. High in the atmosphere, it protects the Earth from harmful solar radiation. Near the ground, ozone is unhealthy to breathe and can damage trees and crops.

In North Carolina, ozone is the most widespread air quality problem and the main component in urban smog. Smog also contains dust, soot and other pollutants that can cause a dirty brown haze. But ozone, which is a colorless and odorless gas, can reach high levels even on sunny, clear days with little haze.

Ozone Formation
Cars, trucks, power plants, factories and other sources emit air pollutants that form ozone as a secondary pollutant. Ground-level ozone forms when nitrogen oxides (NOx) react with volatile organic compounds (VOCs) in the presence of heat and sunlight.

VOCs, or hydrocarbons, come from man-made sources such as cars, service stations, dry cleaners and factories as well as natural sources such as trees and other vegetation. NOx, a byproduct of combustion, comes from power plants, industrial boilers, motor vehicles, lawn-care equipment and other sources that burn fuel.

Efforts to control ozone focus on NOx because most of it comes from man-made sources that are concentrated in urban areas. Reducing VOCs is less effective because pines, oaks and other trees that are so abundant in the South emit large amounts of hydrocarbons.

Ozone levels generally are higher in urban areas, which contain more cars, industry and other emissions sources. However, winds can carry ozone from cities to surrounding rural areas and even to other states. Much of the ozone pollution at high elevations in the mountains of Western North Carolina is transported by winds from other states. In mountain valleys, however, ozone-forming pollution can come from both local and out-of-state sources.

Health and Environmental Concerns
Ozone is unhealthy to breathe, particularly among sensitive groups: children, people with asthma and other respiratory ailments, and anyone who works or exercises vigorously outdoors. Symptoms of ozone exposure can include coughing, throat irritation, chest pain, rapid and shallow breathing, and asthma attacks. Emergency room visits for asthma have increased as much as 36 percent on high ozone days, according to some studies.

High ozone levels can damage leaves on trees and crops, reducing growth rates and crop yields. In 1995, ground-level ozone caused $2.7 billion in crop damage nationwide, according to the U.S. Environmental Protection Agency. Due to its reactive nature, ozone also can prematurely degrade and wear out rubber, paints and other materials.

Ozone is primarily a problem during the summer months, when heat and sunlight are more intense. Ozone levels also vary on a daily basis. In most areas of North Carolina, ozone levels peak in the afternoon, when temperatures are higher, and then drop at night. In the mountains, however, ozone levels can remain high throughout the day and night at altitudes above 4,000 feet.

You can prevent unhealthy ozone exposure by limiting outdoor work and exercise in the afternoons on high ozone days. Ozone levels generally are much lower in the mornings, so limit exposure by working and exercising outdoors before noon. Ozone generally is not a problem indoors because it is filtered out by air conditioners and household furnishings.

Ozone Forecasts
The N.C. Division of Air Quality (DAQ) and the Forsyth County Environmental Affairs Department issue daily ozone forecasts from May through September, when ozone levels generally are higher. These forecasts enable citizens to limit their exposure on high-ozone days and take actions that help reduce ozone-forming pollution. Forecasters use a color-coded system to advise the public about ozone exposure.

Ozone forecasts currently are available in the following metropolitan areas: Asheville, Charlotte, Fayetteville, Hickory, the Triad and the Triangle. Many television news shows and newspapers publicize ozone forecasts in their weather reports. Citizens also can obtain forecasts by:

Ozone Color Code Air Quality
Green Good
Yellow Moderate
Orange Unhealthy for Sensitive Groups
Red Unhealthy
Purple Very Unhealthy

Ozone forecasts are a key part of the N.C. Air Awareness Program, which educates and informs the public about air pollution, its causes and effects, and ways to prevent it. The Air Awareness Program also sponsors:

  • Educational activities for children, including games, videos, publications, and puppet shows.
  • Training workshops for teachers and classroom visits.
  • Air quality coalitions with businesses, local government agencies, and other employers.
  • Exhibits and other public outreach efforts at conferences, festivals and other events.

Frequently Asked Questions About Ozone

What are ozone action days?
Ozone action days occur when ozone levels are Code Orange, Red or Purple. On these days, ozone levels exceed the standard, which is 0.08 parts per million averaged over an 8-hour period. (Parts per million, or ppm, is a unit expressing the concentration of a compound in air or water.) Ozone levels above the standard are considered unhealthy to breathe, particularly among sensitive groups. Most ozone action days occur during the warmer months, from May through September.

Is it safe to go outside and exercise on ozone action days?
People can safely go outside on ozone action days, but they should avoid vigorous outdoor exercise or labor in the afternoons. Even on Code Orange or Red days, ozone levels generally do not reach unhealthy levels until the afternoons when temperatures are the hottest. Morning is the best time for outdoor activities on high ozone days. At high elevations in the mountains, ozone levels can remain high throughout the day and night during bad air events -- so people should limit outdoor activities if they notice signs of adverse ozone reactions, such as coughing and breathing difficulty.

Are some people more sensitive to ozone?
Yes. Groups that are more sensitive to ozone include children, people with asthma and other respiratory ailments, adults who work or exercise vigorously outdoors, and some healthy individuals with unusual susceptibility. Children are most at risk because they breathe more air per pound of body weight, their lungs are still developing, and they are more likely to have asthma and other respiratory illnesses.

What is North Carolina doing to reduce ozone pollution?
North Carolina is taking actions to reduce ozone-forming emissions from cars and trucks, power plants and other industries. In 1999, the N.C. General Assembly enacted legislation that will require cleaner gasoline statewide by 2004 and expand the motor vehicle emissions testing program to nearly half the state by 2006. In 2000, the N.C. Environmental Management Commission began developing new rules to require substantial reductions in NOx emissions from power plants and other large industrial sources.

How can individuals help reduce ozone pollution?
Anything you can do to cut energy consumption can help reduce ozone-forming pollution. Examples include: car-pooling or other efforts to drive less; riding the bus, train or mass transit; riding a bicycle or walking; setting thermostats higher on air conditioners; keeping cars tuned up and tires properly inflated; obeying speed limits; and turning off appliances and lights when not in use.

For More Information
Statewide: N.C. Air Awareness Program, Division of Air Quality, toll-free phone 1-888-RU4NCAIR (1-888-784-6224)
Charlotte Area: Mecklenburg County Department of Environmental Protection , phone 704-336-5500
Triad Area: Forsyth County Environmental Affairs Department , phone 336-727-8064
Asheville Area: Western North Carolina Air Pollution Control Agency , phone 828-255-5655
Health and Ozone: N.C. Division of Public Health, Occupational and Environmental Epidemiology , phone 919-733-3410