Incineration is a waste treatment process that involves the combustion of organic substances contained in waste materials. Combustion is a rapid, exothermic reaction between a fuel and oxygen. In incineration applications, the fuel is predominately waste (although fossil fuels may be co-fired) and the oxygen source is air. Incineration and other high-temperature waste treatment systems are described as "thermal treatment". Incineration of waste materials converts the waste into ash, flue gas and heat.