Ozone is a gas that is formed in the atmosphere when three atoms of oxygen combine. Naturally occurring ozone is found high in the stratosphere surrounding the earth and in ground-level ambient air. Stratospheric ozone forms high in the atmosphere when intense sunlight causes oxygen molecules (O2) to break up and re-form as ozone molecules (O3). Popularly called “good ozone,” it shields people, trees, crops, property, and microorganisms from the harmful effects of the sun’s ultraviolet light. One way that ozone forms low in the atmosphere, at ground level, is when certain substances emitted by trees and other vegetation, soil microorganisms, and lightning react together to form low, background concentrations of ozone. If ground-level ozone were produced only from natural sources of emissions, it would be of no concern. Both animal and plant life tolerate natural background concentrations of ozone. But many contemporary human activities result in emissions of additional chemical compounds, called precursors, that also react in the air to form ozone or “bad ozone” and other harmful gases. These activities include transportation, energy production and some industrial and commercial operations.
Ozone pollution is a key component of smog. It is mainly a daytime problem during the summer months. Strong sunlight and hot weather causes ground level ozone to form in harmful concentrations in the air. Ozone is not emitted directly into the air. Instead it is formed in sunlight, which initiates a series of complex atmospheric chemical reactions. These reactions primarily involve nitrogen oxide (NOx) and volatile organic compound (VOC) emissions, called precursors. At ground level, ozone can harm plants and other materials through a process called oxidation. For these reasons, ozone is called a photochemical oxidant. NOx is produced almost entirely as a by-product of high-temperature combustion.
Common sources of NOx include automobiles, trucks, and marine vessels, construction equipment, and power generation. VOCs include many organic chemicals that vaporize easily, such as those found in gasoline and solvents. They are emitted from many sources, including gasoline stations, motor vehicles, airplanes, petroleum storage tanks and oil refineries. In addition, biogenic, or natural emissions from trees and plants, are a major source of VOCs. The concentration of ozone in the air is determined not only by the amounts of ozone precursor chemicals, but also by weather and climate factors. Intense sunlight, warm temperatures, stagnant high-pressure weather systems, and low wind speeds cause ozone to accumulate in harmful amounts.
Tips to “Do Your Share for Cleaner Air” throughout the Ozone Season
|Color||Index||Cautionary Statements||Air Quality|
|Yellow||51-100||Unusually sensitive people should consider limiting prolonged outdoor exertion.||Moderate|
|Orange||101-150||Active children and adults, and people with respiratory disease, such as asthma, should limit prolonged outdoor exertion.||Unhealthy for Sensitive Groups|
|Red||151-200||Active children and adults and people with respiratory disease, such as asthma, should avoid prolonged outdoor exertion; everyone else, especially children, should limit prolonged outdoor exertion.||Unhealthy|
|Purple||201-300||Active children and adults, and people with respiratory disease, such as asthma, should avoid all outdoor exertion; everyone else, especially children, should limit outdoor exertion.||Very Unhealthy|