With all engines, what goes in must come out. If you put in 1kg of fuel and 25kg of air, the end result will be 26kg of exhaust emissions.

All the atoms that enter the engine will also come out, but not necessarily as part of the same compounds they were in before.

The toxic substances contained in exhaust gas as it leaves the engine are known as ‘primary pollutants’, but once they have left the engine some of these may react with each other or with other compounds present in the atmosphere to create what are known as ‘secondary pollutants’.

Examples of secondary pollutants include;

• Ozone
• Acid Rain
• Photochemical smog