CYCLISTS may be doing themselves more harm than good by pedalling to the office along congested roads, according to pioneering research by the British Heart Foundation.

After just one hour of cycling through traffic, tests showed microscopic particles in diesel fumes caused significant damage to blood vessels, increasing the risk of heart disease.

Those cycling at high speeds in the hope of improving their fitness levels are doing themselves the most damage by breathing in a higher volume of the polluted air.

The current system of locating most cycle paths in bus lanes has the perverse effect of forcing cyclists to inhale the most dangerous air, spewed out by diesel-powered buses and taxis. The number of diesel-engined cars in Britain has also increased dramatically from 1.6m to 5m between 1994 and 2004.

The health warning will dismay the large numbers of commuters who have switched to bicycles to improve their fitness, to avoid high fuel prices or, in London, because they fear another terrorist attack on public transport.

There is no dispute in principle about the health benefits of cycling — it improves the circulation, keeps weight down and boosts overall fitness — yet the new research indicates that they could be outweighed by the polluted conditions of a busy road.

Dr David Newby, British Heart Foundation senior lecturer in cardiology at Edinburgh University, said: “Cycling through congested traffic exposes the cyclist to high levels of air pollution, especially as the exercise of cycling increases breathing and the individual’s exposure. This is bad for the heart.”

In his tests 15 healthy men cycled on exercise bikes in a chamber while being exposed to levels of diluted diesel exhaust comparable to the air they would inhale cycling along a congested city road.

The men cycled for one hour. Six hours after exposure to the fumes, damage was detected to their blood vessels. The blood vessels became less flexible and there was a reduction of a protein that breaks down blood clots in the heart. This damage is associated with the early stages of heart disease.

Diesel exhaust includes nanoparticles of carbon and a range of metals. The particulates are so tiny that experts say it is pointless for cyclists to wear masks, because the mesh cannot be fine enough to block them.

Newby said: “While they are exercising, cyclists breathe two to three times as much air as car drivers. We need to locate cycle lanes away from major roads.” Newby’s research has been submitted to the journal Circulation.

Next month the Committee on the Medical Effects of Air Pollutants, a government advisory body, will publish a report highlighting the risks of heart disease from traffic pollution.

Jon Ayres, chairman of the committee, says that typical urban traffic pollution poses the same risk of heart disease as passive smoking.

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