Exhaust emissions

What comes out of your car's exhaust?

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

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.

Guide to 'Euro' emissions standards

'Euro' standards

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 effects
  • Water (H2O) - no adverse effects
  • 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 react with hydrocarbons to produce low level ozone which can cause inflammation of the airways, reduced lung function and trigger asthma, and also contribute to the formation of particulate matter.
  • 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 cause breathing problems and increased symptoms in those with asthma.
  • Benzene (C6H6) - naturally 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 and is also formed by the reaction between other pollutants. PM10s and the smaller PM2.5s are particles that can pass deep into the lungs causing respiratory complaints and contributing to the risk of developing cardiovascular diseases. Modern diesel cars are fitted with Diesel Particulate Filters (DPF) to stop these particles passing into the atmosphere.

(updated 01 October 2015)