Exhaust emissions

What comes out of your car's exhaust?

The main exhaust products and their effects on the environment and our health

European rules affecting exhaust emissions from cars first came into force in 1970 and have been tightened up progressively ever since. This means that cars are cleaner now then they have ever been and are set to get cleaner still.

The limits form part of the European type approval regulations – a collection of safety and environmental tests which a new vehicle must pass before it can be sold in the European Union. Euro standards affect the construction of the car and although this will impact on MoT testing, the tests are not the same.

Euro standards

After the first standard in 1970 the next big change came in 1992 with the introduction of the Euro 1 standard. This heralded the compulsory fitting of catalytic converters to petrol cars to reduce carbon monoxide (CO) emissions.

Euro 2 was introduced in January 1996 and further reduced the acceptable limit of carbon monoxide emissions. It also reduced the combined levels of unburned hydrocarbons and oxides of nitrogen for both petrol and diesel vehicles.

Euro 3 came into force in January 2000 and modified the test by eliminating the 40-second engine warm-up period previously allowed before the beginning of the emission sampling. It further reduced permitted carbon monoxide emissions from petrol and diesel cars well.

Euro 4 (January 2005) and Euro 5 (September 2009) have concentrated on cleaning up emissions from diesel cars, especially reducing particulate matter(PM) and oxides of nitrogen (NOx). Some Euro 4 diesel cars were fitted with particulate filters, but all diesel cars need particulate filters to meet the requirements of Euro 5.

Combustion products

In theory, you should be able to burn a 'hydrocarbon' fuel (petrol, diesel, gas etc) with air in an engine to produce just carbon dioxide (CO₂) and water (H₂O). The rest of the exhaust would be the nitrogen (N₂) that came in with the air.

Unfortunately the fuels we burn comprise hundreds of differently structured hydrocarbons that burn in different ways and at different rates. This means that in practice the exhaust contains some that were partially burned, some that reacted with others and some that reacted with the nitrogen.

What's in the exhaust?

The main exhaust products and their effects on the environment and our health.

Nitrogen (N₂) - no adverse effects
Oxygen (O₂) - no adverse affects
Water (H₂O) - no adverse affects
Carbon Dioxide (CO₂) - non-toxic gas but contributes towards acidification of oceans and one of the most important greenhouse gases. Governments around the world are pursuing policies to reduce CO₂ emissions to combat global warming.
Carbon Monoxide (CO) - results from incomplete combustion of fuel. CO reduces the ability of blood to carry oxygen and can cause headaches, respiratory problems and, at high concentrations, even death.
Oxides of Nitrogen (NOx) - produced in any combustion process, NOx emissions are oxidised in the atmosphere and contribute to acid rain. They also react with hydrocarbons to produce photochemical oxidants, which can harm plants and animals.
Sulphur Dioxide (SO₂) - sulphur occurs naturally in the crude oil from which petrol and diesel are refined. It forms acids on combustion leading to acid rain and engine corrosion. It also contributes to the formation of ozone and of particulate matter. Sulphur can also adversely affect the performance of catalytic converters and is now removed from both petrol and diesel during the refining process.
Hydrocarbons (HC) - HCs are emitted from vehicle exhausts as unburnt fuel and also through evaporation from the fuel tank, from the nozzle when you fill up and also at stages through the fuel supply chain. They react with NOx in sunlight to produce photochemical oxidants (including ozone), which irritate the eyes and throat.
Benzene (C₆H₆) - aturally occurring in small quantities (less than 2%) in petrol and diesel, Benzene is emitted from vehicle exhausts as unburnt fuel and also through evaporation from the fuel system although modern fuel systems are sealed and have carbon canisters to hold the vapours. Benzene is toxic and carcinogenic. Long-term exposure has been linked with leukaemia.
Lead (Pb) - lead accumulates in body systems and is known to interfere with the normal production of red blood cells. Following the introduction of unleaded petrol and withdrawal of leaded petrol lead is essentially eliminated as an exhaust product.
Particulates (PM) - particulate matter is partly burned fuel associated mainly with diesel engines. PM10s are very small particles that can pass deep into the lungs causing respiratory complaints. Modern diesel cars are fitted with Diesel Particulate Filters (DPF) to stop these particles passing into the atmosphere.