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UP TO a fifth of all lung cancer deaths in cities are caused by tiny particles of pollution, most of them from vehicle exhausts.

That鈥檚 the conclusion of the biggest study into city pollution to date, which tracked half a million Americans for 16 years. It suggests the impact is far greater than feared.

The study is important because it followed individuals, says British expert Roy Harrison of the University of Birmingham, allowing the researchers to separate the effects of smoking and pollution. 鈥淚n the past, we have often just compared urban areas and rural areas. But more people smoke in cities, and it is difficult to compensate for that.鈥�

The research focused on particles less than 2.5 micrometres in diameter, known as PM2.5s. These fine particles are thought to kill by lodging deep in the lungs. The researchers found that the long-term death rate from lung cancer rose by 8 per cent for every 10-microgram increase in the average concentration of PM2.5s per cubic metre. The increased risk is comparable with the risks to long-term passive smokers.

Typical PM2.5 levels in the US are 20 micrograms in Los Angeles and 16 micrograms in New York鈥攖he limit set in 1997 by the Environmental Protection Agency is 15 micrograms. British levels are similar, though one PM2.5 monitor at Marylebone Road in London records an average of 32 micrograms. 鈥淚鈥檇 say London has a special problem because of the high proportion of diesel fumes,鈥� says George Thurston of New York University, co-leader of the study.

The implications are bleakest for developing countries. In heavily polluted cities such as Beijing or Delhi, particulate levels average over 300 micrograms and most of this is probably PM2.5s.