Which of the following causes the least pollution when burnt?

Natural gas burns cleanest of these fuels, giving mainly carbon dioxide and water with little soot or sulphur.

Petrol combustion releases more carbon monoxide, unburnt hydrocarbons and particulates than gas.

Diesel emits heavy particulate matter and nitrogen oxides, a major urban pollutant.

Coal is the dirtiest, releasing large amounts of ash, sulphur dioxide and carbon dioxide.

Gas has the highest hydrogen-to-carbon ratio, so it yields less carbon per unit of energy.

This is why compressed natural gas is promoted for cleaner urban transport.