Ozone in the lower atmosphere is an air pollutant with harmful effects on the respiratory systems of animals. It will also damage the leaves of sensitive plants. However, ozone near the ground should not be mistaken with the ozone layer in the stratosphere, where it absorbs most of the sun’s ultraviolet light preventing it from reaching the Earth's surface. 
 
Contrary to other air pollutants ozone is not emitted directly into the atmosphere. Man made ozone is formed as a byproduct of a series of chemical reactions of precursor pollutants, such as nitrogen oxides and hydrocarbons. The presence of light is also required for the formation of ozone, hence the term photochemical smog, which describes the situation during which ozone levels can reach harmful concentration levels. These episodes also do only occur during the summer months, when temperature and solar radiation levels are high.

The formation of ozone takes some time and highest concentrations are usually found in rural areas and not in the proximity of its precursors emission sources.