One cause of air pollutants is burning fossil fuels, including gasoline. Natural events such as grass and forest fires spread smoke. A mixture of smoke and fog known as smog covers cities like a brown cloud. It can kill people who have problems breathing. Humans have poked holes in the Ozone layer. These allow harmful UV radiation through. Substances called CFCs leak from appliances and probably produce these holes. These substances were once used in aerosol spray cans.