Major Pollutants from Petrol and Diesel Engines

Carbon Monoxide (CO)

Produced when the fuel in the engine does not burn properly. In the UK road traffic produces 91% of all CO emissions.
Problems caused: When inhaled reduces the oxygen carrying capacity of blood and causes headaches, fatigue, stress, respiratory problems and at high levels - death.

Nitrogen Oxides (NOx)

Produced from the burning of fuel in the engine. In the UK road traffic is responsible for 49% of all NOx emissions.
Problems caused: NOx emissions help make 'acid rain'. Also combines with hydrocarbons forming low level ozone pollution and may contribute to lung disease.

Hydrocarbons (HC)

Compounds of hydrogen and carbon are present in petrol and diesel e.g. Benzene. UK petrol contains about 2% benzene. In the UK road traffic is responsible for about 35% of all HC emissions.
Problems caused: HC's react with nitrogen oxides producing a number of pollutants, including ozone. Benzene can cause some forms of cancer.

Smoke (particulate matter or PM)

Made of fine particles of carbon coated with organic chemicals, produced when fuel is only partly burnt in the engine.
Problems caused: Increases in smoke levels associated with increases in death from heart and lung disease and may cause lung cancer.

Ozone

Made when some pollutants from vehicles combine together in the sunlight. This happens some time later and can be much further away from any vehicles, so 'hot spots' of ozone pollution do not usually occur.
Problems caused: Ozone can affect human health and causes plant damage.