Everyone is agreed that we need to reduce air pollution

National Society for Clean Air

Their analysis backed up the original conclusions that people living in cities were more likely to die younger because of pollution.

Their study also highlighted that many medical problems could be attributed to some of the smallest air particles.

The first study the researchers looked at was carried out at the Harvard School of Public Health in Boston in 1993. It compared death rates and pollution levels in six US cities.

It found that the death rates increased in almost direct proportion to the level of pollution.

Death rates and pollution

They suggested that people living in the most polluted city, Steubenville in Ohio,were at greater risk of dying young - by 26% - compared with residents of the cleanest city, which was Portage, Wisconsin.

A larger study, carried out by the American Cancer Society in 1995, tested these findings.

It followed 550,000 adults over seven years and their findings also indicated a strong link between death rates and pollution.

However, critics had accused the authors of both of these studies of failing to take other factors, such as poverty, into account.

As a result, the Health Effects Institute decided to re-analyse the studies' findings by testing for other possible causes.

They looked at factors such as education, ethnicity, income levels and the availability of health care as well as differences in other pollutants, temperature and humidity.

But the re-analysis confirmed the original findings.

Political fallout

"For the most part, the inclusion of these additional factors did not alter the association," said Daniel Krewski of the University of Ottawa.

The Harvard study had suggested that air particles with a diameter of less than 10 micrometeres were a threat to public health.

But this latest study has suggested that tiny particles with a diameter of less than 2.5 micrometres are more dangerous.

These particles can be made up of various elements, including fumes from cars and natural dust particles.

The UK government has committed itself to improving air quality and meeting specific targets by 2005.

But a spokesman for the National Society for Clean Air said much of the pollution was from car exhaust fumes. He said this was causing problems politically.

"Everyone is agreed that we need to reduce air pollution. The problem is that the main source of pollution is from road transport and we are seeing a battle between local and central government over who is responsible to reduce this pollution."

He suggested the real argument related to how pollution was going to be tackled and who is responsible for meeting the targets.