Unburned hydrocarbons represent another source of air pollution associated with the use of fossil fuels (especially gasoline), even though they are not a result of combustion. Much of the emission of unburned hydrocarbons to the air occurs as a result of evaporation from fuel tanks (remember the smell of gasoline during your last fill-up?) and as a result of leaks or spills. Taken individually, these events seem trivially small. But on any given day millions of vehicles are being refilled with gasoline
