Exhaust Emissions
The main regulated pollutants from car exhausts
There's no doubt that new cars are cleaner than ever and are still being improved. Stringent emissions standards for new cars were introduced in 1992, since when all new petrol cars have had to be fitted with a catalytic convertor.
Standards have been tightened further since 1992 and will continue to be so in the future. The quantities of pollutants produced are much lower for today's new cars, but they're not zero (yet).
By-products of a Car Engine
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.
The exhaust contains some that were partially burned, some that reacted with others and some that reacted with the nitrogen.
The main exhaust products and their effects on the environment and our health are described in the table below.
| Exhaust Constituent | Nature and Effects |
|---|---|
| Nitrogen (N2) | No adverse effects |
| Oxygen (O2) | No adverse affects |
| Water (H2O) | No adverse affects |
| Carbon Dioxide (CO2) | Non-toxic gas, but contributes to climate change. |
| 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. |
| 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) | 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. 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. |