Department of
Environmental Quality

Ozone is a gas that forms in the atmosphere when three atoms of oxygen are combined. It is not emitted directly into the air, but is created at ground level by a chemical reaction between oxides of nitrogen, and volatile organic compounds in the presence of sunlight. Ozone has the same chemical structure whether it occurs high above the earth or at ground level and can be "good" or "bad," depending on its location in the atmosphere.

Ozone occurs in two layers of the atmosphere. The layer surrounding the earth's surface is the troposphere. Here, ground-level or "bad" ozone is an air pollutant that damages human health, vegetation, and many common materials. It is a key ingredient of urban smog. The troposphere extends to a level about 10 miles up, where it meets the second layer, the stratosphere. The stratospheric, or "good" ozone layer, extends upward from about 10 to 30 miles and protects life on earth from the sun's harmful ultraviolet rays.

Health Impacts of Exposure

The reactivity of ozone causes health problems because it damages lung tissue, reduces lung function, and sensitizes the lungs to other irritants. Scientific evidence indicates that ambient levels of ozone not only affect people with impaired respiratory systems, such as asthmatics, but healthy adults and children as well. Exposure to ozone for several hours at relatively low concentrations has been found to significantly reduce lung function and induce respiratory inflammation in normal, healthy people during exercise. This decrease in lung function generally is accompanied by symptoms including chest pain, coughing, sneezing, and pulmonary congestion.

Recent health studies have shown ozone to be even more detrimental to health than previously thought. The EPA has issued new, more stringent standards for this criteria pollutant based on these studies.