While ozone in the upper atmosphere protects the earth's surface from ultraviolet rays, ground level ozone negatively affects humans, plants and animals. Ozone (O3) is formed when trace gases chemically react in the presence of sunlight. The two trace gases responsible for this reaction are nitrogen oxides (NOx) and volatile organic compounds (VOCs). Fires are one source of these trace gases. Depending upon meteorological and topographical conditions these gases can be transported downwind and react in the presence of sunlight to form ozone.

Ozone is typically a local and regional scale issue. Short term exposure to ozone can cause coughing, pain upon deep breathing, reduced lung function, and shortness of breath. Long term exposure to ozone is associated with reduced lung function (Galiza and Kinney 1999).

Humans are not the only organisms whose health is affected by ozone. Ozone is the most damaging oxidizing agent for plants, and can lead to tree kills in forests and decreased yield in croplands. In conifers ozone exposure can cause premature needle drop, and increased susceptibility to root rots which will ultimately kill the trees (Sinclair et al. 1987).