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Levels of pollutants emitted by car exhausts are monitored across urban and rural areas around Britain. We check out the 'hotspots.'

Your car runs by burning petrol or diesel (a complex blend of lots of chemicals collectively called hydrocarbons) to produce harmless carbon dioxide and water. There are, however, some by-products of this process.

Some of these hydrocarbons do not get burned in your engine and pass through the exhaust unchanged. There are two in particular, called benzene and 1:3 butadiene that can be harmful. Not all the fuel burns up completely, so some carbon monoxide (CO) is also produced. These are the main pollutants produced by older, petrol driven cars, with diesels of all kinds burning their fuel much more completely, so producing negligible amounts of them.

Lean Burn

In the 1980s, there was a move towards ‘lean burn’ petrol engine technology - increasing the amount of air with the fuel in the engine so that more of the fuel is completely burned up. This reduces the amount of the first two pollutants but tends to encourage the oxygen and nitrogen in the air to combine to produce nitrogen oxides (NOx).

If your petrol car was registered after 1993, it will be fitted with a catalytic converter which removes 95% of these three pollutants from the exhaust as compared with a similar 1976 model.

Exhausts can also contain sulphur dioxide (SO2) from impurities in the fuel, but only 3% of the total emissions of this substance come from transport, the rest mainly from industry and power generation.

Recently, a health threat has been identified from particulates, or PM10s, which are microscopic soot particles produced in the combustion process. Very little particulate emission (5%) is from petrol engines, though, with much more (19%) coming from diesels, disproportionately from the larger diesels in trucks and buses.

The final element in exhaust pollution is Ozone (O3). This is not emitted directly but made in the air by the action of sunlight on other pollutants to form ‘ground level Ozone,’ which, unlike the ozone layer in the high atmosphere, is regarded as a bad thing if levels are too high. Ozone is actually broken down by nitrogen oxides, so one tends to be lower where the other is higher.

How do we know what the levels of these pollutants are?

Levels of these seven pollutants are monitored across urban and rural areas around the country. The measurements are made public - the best place to see them is on BBC1 Ceefax pp 412-417 where they are updated hourly.

Are these pollutants dangerous?

Most toxic substances are only dangerous when a certain level is exceeded. Your medicine cupboard is full of chemicals which are beneficial if taken as prescribed by your doctor but which will kill you if you swallow the whole bottle.

Many everyday items are poisonous if taken to excess - Vitamin A, for example, is an essential part of your diet, and lack of it is what causes hunger strikers to die.

Four of the pollutants described above (Nitrogen Oxides, Carbon Monoxide, Sulphur Dioxide, and Ozone) are like this - there are accepted levels at which no harmful effects are observed even in sensitive population groups.

For the other three (Benzene, 1:3 butadiene and particulates) there is no way of proving they are safe at any level, so the experts set standards where the risk to health are ‘exceedingly small’ - in other words, they can't actually measure it.

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