* Carbon dioxide, which contributes to global warming.

* Carbon monoxide, a suffocating poison that inhibits the ability of the blood to transport oxygen.

* Nitrogen oxide, which may affect the immune system and which contributes to acid rain.

* Benzene, which attacks the bone marrow’s production of red blood cells. Long-term exposure is linked to leukaemia.

* Polycyclic hydrocarbons, which are thought to be carcinogenic. Causes photochemical smog, irritating the lungs.

* Lead, which interferes with the production and survival of red blood cells. Impairs brain function.

* Suspended solid particles which irritate the respiratory system and a probably contributor to asthma.

Friends of the Earth claims that probably 24,000 people in the UK die prematurely each year as a result of motore vehicle pollution.

Exhaust pollution can be addressed in two ways: 1. Making better used of what we have

* 50 per cent of pollution comes from 20 per cent of cars. In pilot testing schemes, 16 per cent of vehicles break emissions standards. The lessons are regular maintenance, replacement of worn-out cars, enforcement of emissions legislation, for example by roadside checks.

* On-board diagnostics (OBD) which tell the driver when, for example, the catalytic converter is failing.

* Availability, and cheapness, of cleaner fuels, such as ultra-low sulphur diesel, and low sulphur petrol.

* Driver education: smooth driving, gentler acceleration, better use of gears, slower speed can save anything up to 30 per cent of fuel.

2. Cleaner cars

The most practical currently are “hybrids” - small petrol or diesel engine combined with electric motor and batteries. A computer shares out the work according to conditions. The batteries are recharged when the petrol engine is running. Braking energy is also used for recharging.

A hybrid produces 40 per cent less CO2 and does about 60 mpg. But it’s 15 per cent more expensive than its conventional counterpart, and therefore government push will be needed if it’s to be a significant contributor to clean air.

Two models currently on sale are the Honda Insight (pound;17,000) and the Toyotal Prius (pound;16,430). Both are established models (you can get brochures). The Honda Insight is a small coupe. Toyota Prius is more practical as a family car. Both have extremely good fuel economy and very low emissions.

Hydrogen power

Hydrogen is a very clean fuel that can be produced from renewable sources, producing water (H20) as steam from the exhaust. Cars running on hydrogen have a similar performance to equivalent petrol cars. But the fuel is difficult to handle - it has to be stored on board as a liquid at minus 235 degrees centigrade. The world is watching Iceland, which plans to use its geothermal power to produce hydrogen and free its economy entirely from dependence on oil.

Fuel cells

A fuel cell converts its fuel directly into electricity by chemical means. The technology has been around for a long time and was used on the Apollo space vehicles in the Sixties and Seventies. A range of fuels can be used - hydrogen is common, and a number of fleet tests of cars, vans and buses have taken place. The industry, though, is looking to use methanol, mainly because it’s easy to make and store.