Ozone is a highly reactive gas with a pale blue color and a distinctively pungent smell whose molecules consist of three oxygen atoms. It is present in very low concentrations in the atmosphere, where it is formed from conventional oxygen (whose molecules consist of two oxygen atoms) by the action of ultraviolet light and electrical discharges. Its highest concentration is in the ozone layer of the stratosphere, where it absorbs most of the sun's ultraviolet radiation.
Ozone is also created as a result of human activity. It is produced intentionally for use in many industrial and consumer applications because it is one of the most powerful oxidizing agents. It is also produced in the lower atmosphere by the reaction of sunlight on air containing hydrocarbons and nitrogen oxides that are emitted directly by internal combustion engines and industrial processes. Such ozone reacts directly with some hydrocarbons and, although contributing to their removal from the air, simultaneously creates other compounds that are key components of smog.
The high oxidizing capability of ozone can damage mucous and respiratory tissues in animals. There is also evidence of significant reduction in agricultural yields resulting from increased ground-level ozone and related pollutants, which interfere with photosynthesis and thus stunt the overall growth of some plant species. On the contrary, the ozone in the ozone layer has an extremely beneficial role for both plants and animals.