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Diesel is a type of fuel derived from crude oil. Large engines, including those used in many trucks, buses, trains, construction and farm equipment, generators, ships, and in some cars, run on diesel fuel.  Diesel engines convert the chemical energy contained in the fuel into mechanical power. Diesel fuel is injected under pressure into the engine cylinder where it mixes with air and where the combustion occurs. The exhaust gases which are discharged from the engine contain several constituents that are harmful to human health and to the environment.  The exhaust from diesel engines is made up of 2 main parts: gases and soot. Each of these, in turn, is made up of many different substances. The gas portion of diesel exhaust is mostly carbon dioxide, carbon monoxide, nitric oxide, nitrogen dioxide, sulphur oxides, and hydrocarbons, including polycyclic aromatic hydrocarbons (PAHs). The soot (particulate) portion of diesel exhaust is made up of particles such as carbon, organic materials (including PAHs), and traces of metallic compounds. Both the gases and the soot of diesel exhaust contain PAHs.