Dr Penny Woods, chief executive of the British Lung Foundation, said: 'How very sad that our streets are so traffic-polluted that older people trying to keep themselves healthy are in fact doing themselves harm.
"Our hope is that this study will add to the evidence city leaders need to contribute to policies that will encourage preservation of green spaces", said Jim Zhang, professor of global and environmental health at Duke and study's co-author.
"... Researchers in London calculated mothers' exposure to air pollution and traffic noise in various parts of the city from 2006 to 2010".
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. Before and after the walks (which averaged 3.1 miles at each setting), the participants underwent various tests that are created to assess the effects of exercise on heart and lung health.
Simon Gillespie, chief executive at the British Heart Foundation, said: "Air pollution contributes to around 40,000 premature deaths in the United Kingdom every year, and the extent of its damage to our cardiovascular health is becoming clearer all of the time".
In healthy participants the walk in Hyde Park led to a 7.5 per cent improvement in the amount of air they could expel in one breath and improvements in blood flow which persisted up to 26 hours after exercise.
However a walk along Oxford Street led to only a small transient increase in lung capacity, and substantial worsening of arterial stiffness (average 7% increase in pulse wave velocity) associated with greater exposure to black carbon soot and ultrafine particles from diesel exhaust. 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 from Imperial College London and Duke University in the US recruited 119 people for the study who were either healthy, had stable heart disease, or stable chronic obstructive pulmonary disease (COPD) - a type of lung disease. That benefit disappeared - or even reversed itself - after walking on Oxford Street. It decreased it by 19 percent for those with ischemic heart disease. It involved, for example, a small number of participants, all of who were from the London area. The study also had people take only two short walks.
"Exercise is crucial in reducing the risk of a heart attack or stroke, but it seems risky levels of air pollution could be erasing these benefits in older adults". The researchers say that stress, such as from the increased noise on Oxford Street, could have had an effect.
"As long as American automakers drag their feet on fuel standards and other sustainability regulations, they're showing that they care more about their short-term financial interests than they do about long-term benefits for public health, the planet, and even the economy", she said.
"For many people, such as the elderly or those with chronic disease, the only exercise they very often can do is walk", said Kian Fan Chung, the study's senior author and a professor of respiratory medicine at Imperial College London, in a released statement. "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.