Pollution wipes out the benefits of walking, a new study suggests. Researchers from Imperial College London found that the toxic air in built-up city centres prevents the positive effects on the lungs and heart which are usually gained from exercise.
Although the experiment was carried out in the over-60s, the scientists say the effects could apply to other groups, and have called for stricter air quality limits, and greater access to green spaces.
To determine the impact of pollution on exercise, researchers asked 119 people to take a two-hour stroll through London's Hyde Park and also Oxford Street, a busy shopping area, the Daily Telegraph reports.
Levels of black carbon, nitrogen dioxide and fine particulate matter in Hyde Park are typically well within healthy boundaries but air in Oxford Street regularly breaches dangerous pollution levels, as defined by the World Health Organisation.
The researchers found that everyone in the study benefited from a stroll in the park, with their lung capacity improving within just one hour, an effect which lasted for 24 hours for many people.
By comparison, a walk along Oxford Street barely registered any improvement at all.
Likewise the increase in blood flow usually associated with exercise was virtually absent in those walking along the busy shopping street. And while the arteries of those walking in the park became 24 per cent less stiff, they improved by just 4.6 per cent for people on Oxford Street.
"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," said senior author Fan Chung, Professor of Respiratory Medicine and Head of Experimental Studies Medicine at National Heart & Lung Institute at Imperial College London.
"Our research suggests that we might advise older adults to walk in green spaces, away from built-up areas and pollution from traffic.
"For people living in the inner city it may be difficult to find areas where they can go and walk, away from pollution. It shows that we can't really tolerate the levels of air pollution that we currently find on our busy streets."
Previous research has found that diesel exhaust fumes, particularly fine particulate matter has been associated with an increased risk of cardiovascular disease and death, and can cause a worsening of diseases of the airways, such as asthma.
It is estimated that pollution is linked to 40,000 early deaths in Britain each year, and the government has pledged to ban the sale of diesel cars by 2040, which are a major contributor to urban toxins.
Responding to the study, Prof Kevin McConway, Emeritus Professor of Applied Statistics, at The Open University, said: The research does convince me, though, that poor air quality does play an important role in the short-term benefit one can get from taking a walk.
"But perhaps that's not all that's going on, and I look forward to reading about future similar studies in other places."
However Prof Ian Colbeck, Professor of Environmental Science, at the University of Essex, said people should not be deterred from exercising, even if they live in the city.
"This paper highlights the risks to health by walking along polluted roads, for the over sixties with specific pre-existing medical conditions.
"However 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.
"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."
The authors say that inner-city stress could account for some of the physiological differences seen between the two settings, with the increased noise and activity of Oxford Street having an effect.
The findings were published in The Lancet.
This article was originally published in the Daily Telegraph and is reproduced with permission.