Shedloads of lycra-clad peddlers nearly careered off the road this week after being told their daily exertion on two wheels could be doing more harm than good. Tiny specks of air pollution belched from diesel-fuelled taxis and buses can damage blood vessels, and, according to the Sunday Times, could outweigh the obvious health benefits of cycling. "The health warning will dismay the large number 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," the report said.

The newspaper issued its warning after research in the laboratory of David Newby, a British Heart Foundation senior lecturer in cardiology at Edinburgh University. In his tests, 15 healthy men cycled on exercise bikes in a chamber while being exposed to levels of diesel pollution similar to those found on a congested city street. After an hour of cycling, the scientists found their blood vessels became less flexible and produced less of a protein that breaks down blood clots in the heart - damage associated with the early stages of heart disease.

All very worrying, particularly that, as the faster a cyclist pedals the more air they breathe in, those who believe they are improving their fitness the most are storing up the biggest trouble for themselves.

"Cycling is a good thing to do," Dr Newby insists. "We don't want to give the message that it isn't." His team has made no direct comparison of the risks versus the benefits, and what the original story failed to mention, he says, is that the situation for car drivers is even worse. Drivers face a bigger risk as car ventilation systems vacuum up pollution at ground level, which cannot then disperse, leading to concentrations inside some three times higher than on the street.

For cyclists there is both good news and bad news. The latter is that no type of expensive mask and filter can block the tiny carbon particles that do the damage. "They're just so small they behave almost like a gas," Dr Newby says.

The positive spin is that the levels of pollution drop off considerably just a few metres from the congested source - so cyclists who stick to less busy roads should have nothing to worry about.