There's more to motoring and protecting the environment than CO2.
Engines also produce carbon monoxide, particulates, nitrous oxides and hydrocarbons, and all have an impact on the environment, albeit a more localised effect.
They're all a result of the incomplete burning of fuel in the engine, and will continue to be produced despite the fact that modern engines have to become increasingly efficient. All cars are governed by a series of European-wide emissions regulations, and currently they are subject to Euro IV requirements. Howveer, tougher Euro V standard set to be introduced in September 2009.
Breathing in carbon monoxide reduces your blood's ability to carry oxygen. Very high levels of the gas can be fatal, which is why it's always wise to a have a carbon monoxide detector in your home in case your boiler breaks down.
Even the far lower levels produced by cars are thought to pose a health risk, however, especially to those suffering from heart disease. If you were to stand around in a traffic jam for a long time, you might develop headaches and dizziness as a result of exposure.
Particulate matter (PM) is a problem for diesel engines, which produce large amounts of PM10s (so called because the particles are 10 micrometers or less in diameter). This can be seen as smoke coming out of the exhaust.
PM10s are the largest particles that your nose and throat can't effectively filter out, so they end up in your lungs. Here they aggravate existing respiratory diseases and are thought to contribute to higher levels of asthma and heart disease.
Nitric oxide combines with oxygen in the atmosphere to create nitrogen dioxide and various other oxides of nitrogen (NOx).
Like particulates, the gases are thought to be responsible for causing and aggravating respiratory illnesses, but could also increase allergic reactions.
NOx are also linked to the formation of acid rain which damages vegetation and crops.
The fuel you stick in your tank is a hydrocarbon (something with a molecular structure compromised mainly of hydrogen and carbon), so strictly speaking, the emissions we should be talking about here are carbon – a posh word for soot.
Either way, this material contributes to the formation of ground-level ozone which, again, is thought to be responsible for causing and aggravating respiratory conditions.
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