Billions of pounds of taxpayers' money is due to be spent on cutting air pollution when not enough is known about what is needed, says an influential committee of MPs.

The Public Accounts Committee says there are major gaps in what is known about the health effects of cancer-causing pollutants.

Main pollutants

Carbon monoxide Ozone Nitrogen dioxide Dust Sulphur dioxide

More work is needed by the government to look at what size of dust particles are the most harmful, say the MPs.

The Department of Environment, Food and Rural Affairs (Defra) should also examine whether there is a safe upper level of concentration of ozone, says their report.

The MPs also press the department to put in place contingency plans to help meets its air quality targets where pollution differed from its forecasts.

Prediction problems

Defra had set up a good system for monitoring air quality across the country, say the MPs on Friday.

But the readings can be affected by unpredictable changes in economic activity and the weather.

Edward Leigh: Information riddled with uncertainty

The committee's report focuses on pollutants like benzene from petrol, which can increase the risk of leukaemia.

Another pollutant, 1,3 butadiene, was studied as it is linked to cancer of the lymphoid system.

Committee chairman Edward Leigh said: "Air quality is much better than it used to be.

"But Defra is proposing further reductions in air pollution that will cost billions, on the basis of evidence and forecasts that are riddled with uncertainty.

Health effects

"Defra must develop a better understanding of the effects of pollution on health to inform such decisions in future."

Scientists in March this year showed that air pollution caused the blood vessels to narrow - even in healthy people.

Some experts said the findings explained why air pollution appears to increase the rate of heart attacks and other cardiovascular problems among people with heart and blood vessel disease.

A government report in 1998 suggested that air pollution from ozone, sulphur dioxide and dust particles brought forward up to 24,000 deaths each year.