Carbon monoxide (CO)

This gas is very dangerous even in low concentrations. It has no smell or taste and is colourless. When inhaled it combines in the body with the red blood cells preventing them from carrying oxygen. If absorbed by the body it can be fatal in a very short time

Nitrogen oxides (NO _x)

Oxides of nitrogen are colourless and odourless when they leave the engine but as soon as they reach the atmosphere and mix with more oxygen, nitrogen oxides are formed. They are reddish brown and have an acrid and pungent smell. These gasses damage the body's respiratory system when inhaled. When combined with water vapour nitric acid can be formed which is very damaging to the windpipe and lungs. Nitrogen oxides are also a contributing factor to acid rain

Hydrocarbons (HC)

A number of different hydrocarbons are emitted from an engine and are part or unburnt fuel. When they mix with the atmosphere they can help to form smog. It is also believed that hydrocarbons may be carcinogenic

Particulate matter (PM)

This heading in the main covers lead and carbon. Lead was traditionally added to petrol to slow its burning rate to reduce detonation. It is detrimental to health and is thought to cause brain damage especially in children. Lead will eventually be phased out as all...