Mobile sources – Pollutant exhaust gases created by the combustion of fuel. Water and CO2 are not included in this category, but CO, NOx, and hydrocarbons are and are thus subject to legislative control. All three are emitted by gasoline engines, while diesel engines also emit particulates that are regulated.

Stationary sources – The release of sulfur oxides and particulates from power stations that can be influenced by fuel composition. Local authorities control the sulfur content of heavy fuel oils used in such applications.