Ozone is a colorless gas with a pungent odor, and, if inhaled, can damage or kill living cells. It is normally created in the upper atmosphere by the action of sunlight on normal oxygen. Hydrocarbons are released mainly by motor vehicles and evaporation of fuel, paint, and dry cleaning fluids. Nitrogen compounds are made when fuel is burned, such as in power plants and motor vehicles. Additionally, ozone is formed when electrical charges, such as lightning, turn normal oxygen into ozone.
Ozone turns back into oxygen on contact with indoor surfaces such as walls or furniture.
Much indoor ozone comes from outdoor sources. In the upper atmosphere ozone has a beneficial effect by absorbing the harmful ultraviolet rays of sunlight. At the earth's surface, ozone is harmful to crops, forests, building materials, and the health of humans and animals. Warm and sunny days with little wind and no rainfall contribute to ozone formation. When these conditions occur over several days, ozone can continue to build up to unhealthy levels, especially in cities.