People older than 60 and those with heart and lung problems are advised to exercise in green spaces rather than streetside, according to a new study that shows how air pollution from traffic fumes cancels out the positive benefits from exercise.

Researchers from Imperial College London and Duke University in the US compared volunteers who took brisk walks in London's Hyde Park - which registers less pollution than the rest of the British capital - and along Oxford Street - which frequently exceeds the UN World Health Organization's air quality limits.  The results, as published in the medical journal The Lancet, showed that everyone benefited from a walk in the park, especially in lung capacity.

Hyde Park, London

But those subjected to the smog and particulate matter in the air on Oxford Street barely saw any increase in lung capacity.  Arterial stiffness also did not improve.

Oxford Street, London

"For many people, such as the elderly or those with chronic disease, very often the only exercise they can do is to walk," said Imperial College Professor Kian Fan Chung, the lead author of the study.  "Our research suggests that we might advise older adults to walk in green spaces, away from built-up areas and pollution from traffic," he added.

He's also recommending a second study of younger people who exercise or shop in the city, because the research suggests that the negative impact of smog may well be the same in that group.