City air pollution can cancel out the beneficial effects of exercise in older adults, scientists say.
A London study found that short-term exposure to traffic fumes greatly reduced the positive impact of walking on the heart and lungs.
Previous research has shown that breathing in polluted air, especially fine particles from diesel engines, increases the risk of heart disease and death.
For the new study 119 volunteers over the age of 60 were asked to walk for two hours in a relatively quiet part of Hyde Park and along a busy section of Oxford Street.
All the participants, recruited through London's Royal Brompton Hospital, were either healthy or had a stable lung condition or non-progressing heart disease.
The study showed that all the volunteers benefited from a stroll in the park, with lung capacity improving within the first hour, arteries becoming significantly less stiff, and blood flow increasing and lowering in pressure.
In contrast walking along busy Oxford Street had little impact on arterial stiffness and led to only a small increase in lung capacity.
Professor Fan Chung, from Imperial College London, who led the research reported in The Lancet journal, said: "These findings are important as for many people, such as the elderly or those with chronic disease, very often the only exercise they can do is to walk.
"Our research suggests that we might advise older adults to walk in green spaces, away from built-up areas and pollution from traffic."