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.
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.
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: