A chemical reaction that releases energy and produces heat, light, and other by-products. For example, in a gasoline engine, when oxygen and gas mix and are ignited by spark, heat, light, carbon monoxide, and water are produced. Substances discharged into the air, usually by human activities such as burning fossil fuels. Buried deposits of decayed plants and animals that over millions of years have been converted to oil, coal, or natural gas by heat, and pressure in the earth's crust. Compounds produced by the incomplete burning of gasoline and the evaporation of such things as industrial solvents and oil paints.
Hydrocarbons contribute to smog and ozone.
A metal once added to some gasoline to improve engine performance. As the engine runs, lead particles escape through the exhaust pipe. Gases produced during higher temperature combustion in motor vehicles and in power plant and industrial furnaces. A harmful form of oxygen which is produced when sunlight stimulates a reaction between nitrogen oxide and hydrocarbons. Near the earth's surface, ozone is a major component of smog.