The over-60s should stick to green spaces and parks when they go for a walk and avoid the city streets, according to a groundbreaking study that says air pollution from traffic fumes largely wipes out the health benefit from the exercise.

Walking is often recommended for older people, but the study from Imperial College London and Duke University in the USA suggests that the over-60s and those with lung and heart problems should steer clear of urban areas with heavy traffic. The negative effect may well be the same in younger people, say the authors, and it reinforces the urgency of reducing emissions in city streets.

The research compared walking for two hours in Oxford Street with strolling in Hyde Park, which registers some air pollution but far less than in the heart of the capital city’s shopping district.

The researchers recruited 119 people who were either healthy, had stable heart disease or stable COPD (chronic obstructive pulmonary disease) which damages the lungs. They were randomly allotted to walk either in the west end of Oxford Street which is restricted to taxis and buses emitting diesel fumes and which has frequently breached air quality limits set by the World Health Organisation, or a traffic-free area of Hyde Park. Some weeks later they did the other walk. The findings were published in the Lancet medical journal.

Noise and pollution levels were significantly higher on Oxford Street, including increased levels of black carbon, nitrogen dioxide and fine particulate matter. All the participants benefited from a stroll in the park, with lung capacity improving within the first hour, and a significant lasting increase for more than 24 hours in many cases. By comparison, a walk up and down Oxford Street led to only a small increase in lung capacity in participants – far lower than recorded in the park.

Blood flow also increased after exercise, with decreases in blood pressure and an increase in heart rate. Arteries became less stiff in those walking in Hyde Park, with a maximum change from baseline of more than 24% in healthy and COPD volunteers, and more than 19% in heart disease patients.

This effect was drastically reduced when walking along Oxford Street, however, with a maximum change in arterial stiffness of just 4.6% for healthy volunteers, 16% for those with COPD and 8.6% for heart disease.

These findings are important as walking is frequently recommended for older people. “For many people, such as the elderly or those with chronic disease, very often the only exercise they can do is to walk,” said Kian Fan Chung, professor of respiratory medicine at the National Heart and Lung Institute at Imperial and 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.

Chung said traffic pollution might also affect younger people doing their shopping in the heart of a city.

“I think it might well do. The only difference is that young people are much more resilient,” he said. A study in younger people should be done, he said.

Other scientists said the study was important evidence of the harms of air pollution, although they would not advise people to stop walking in the city streets, because the benefits for healthy people were reduced but not completely lost.

Ian Colbeck, professor of environmental science at Essex University, said the paper highlighted the risks to health by walking along polluted roads, for the over-60s with specific pre-existing medical conditions.

“We know from other research that for the vast majority of the population the benefits of any physical activity far outweigh any harm caused by air pollution except for the most extreme air pollution concentrations,” he said. “It’s important to that people continue to exercise. In the UK physical inactivity is the fourth largest cause of disease and mortality and contributes to around 37,000 premature deaths in England every year.”

Prof Stephen Holgate from Southampton University and special adviser to the Royal College of Physicians on air quality, said: “The observation that air pollution encountered on a high street in London removes any health protection produced by exercise outdoors is yet another demonstration that pollution is eroding the health of ordinary people. More than this, it would seem that pollution, in large part related to traffic emissions, has an immediate adverse effect on those with chronic diseases such as COPD and cardiovascular diseases.

“While medication for cardiovascular disease apparently protects against acute pollutant effects, is this the way we should be dealing with the problem? I would say no. This important study mandates action to radically reduce pollution at source to enable our cities and towns to be safe places to live in and move around.”