They also point out that there was no resting control group, so they can't be sure that walking contributed to the changes in lung function and arterial stiffness, although previous studies have shown that walking improves arterial stiffness. The researchers say that stress, such as from the increased noise on Oxford Street, could have had an effect.
"Combined with evidence from other studies, our findings underscore that we can't really tolerate the levels of air pollution that we now find on our busy streets", Chung said.
To do so, they examined 119 individuals over the age of 60, who were either healthy, had stable coronary heart disease or had stable ischemic heart disease.
Patients walked for two hours in two London settings at midday; in a "relatively quiet" part of Hyde Park and along a section of Oxford Street. Some weeks later they did the other walk. According to a new study, published in The Lancet, some people who take walks around city streets could be putting their health at risk.
They were unsurprised to find that noise and pollution levels were significantly higher on Oxford Street than in Hyde Park, including higher levels of black carbon, nitrogen dioxide and fine particulate matter (PM2.5) which has a diameter of less than 2.5 micrometers and can enter the lungs and even the bloodstream.
On the contrary those who walked in the park had a noteworthy health improvement in their lung capacity which endured as long as 24 hours compared to those who walked on a polluted street said the benefits were negligible. By contrast, lung capacity improved only slightly during the Oxford Street walk - and did not last.
Likewise the increase in blood flow usually associated with exercise was virtually absent in those walking along the busy shopping street.
In addition, walking in Hyde Park reduced stiffness in arteries by more than 24 percent among the healthy and COPD volunteers.
Commenting on the results, Fan Chung, Professor of Respiratory Medicine and Head of Experimental Studies Medicine at the National Heart & Lung Institute at Imperial College London and senior author of the study, 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 study suggests that we might advise these people to walk in green spaces, away from built-up areas and pollution from traffic".
"I think it might well do". A study in younger people should be done, he said.
"Our findings suggest that healthy people, as well as those with chronic cardiorespiratory disorders, should minimize walking on streets with high levels of pollution because this curtails or even reverses the cardiorespiratory benefits of exercise", the researchers wrote.
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 need to reduce pollution so that everyone can enjoy the benefits of physical activity in any urban environment", he added.
In the United Kingdom, polluted air contributes to about 40,000 deaths a year, almost a quarter of them in London. People like outdoor exercise.
"Exercise is crucial in reducing the risk of a heart attack or stroke, but it seems unsafe levels of air pollution could be erasing these benefits in older adults".
"While medication for cardiovascular disease apparently protects against acute pollutant effects, is this the way we should be dealing with the problem?"