These harmful effects were associated with increasing exposure to black carbon soot and ultrafine particles, supporting the view that fossil fuel combustion particles are particularly toxic to individuals with cardiovascular and lung disease. They found no evidence that traffic noise was linked to birth weight but "cannot rule out that an association might be seen in a study area with a wider range of noise exposures".
Physical measurements were taken before and after the walks to show the effects of the exercise on cardiovascular health, including measurements of lung volume exhaled, blood pressure, and the degree to which the blood vessels could expand.
They also call for more green spaces in urban environments.
"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.
Participants were randomly assigned to spend two hours walking along the western end of London's Oxford Street, an area where traffic is restricted to diesel-powered buses and cabs; or through a traffic-free area of the city's Hyde Park.
In healthy participants, walking in Hyde Park led to an improvement in measures of lung capacity (average 7.5% increase in FEV1 at five hours after the start of the walk) and arterial stiffness (5% decrease in pulse wave velocity on average after three hours) that persisted for up to 26 hours.
Their stride through a quiet section of Hyde Park had a positive impact on the heart and lungs, but a similar level of exercise in Oxford Street had a minimal effect on health.
Significant differences in arterial stiffness were also observed.
In the study, researchers recruited 119 volunteers through the Royal Brompton Hospital in London, who were over the age of 60 and were either healthy, had stable Chronic Obstructive Pulmonary Disease (COPD), or had stable heart disease. They asked the volunteers to walk for two hours midday in one of two locations in London. That benefit disappeared - or even reversed itself - after walking on Oxford Street.
The study has several limitations.
"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.
"Combined with evidence from other recent studies, our findings underscore that we can't really tolerate the levels of air pollution that we now find on our busy streets", said Fan Chung, professor of respiratory medicine and head of experimental studies medicine at Imperial College's National Heart and Lung Institute. "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. "But for those living in inner cities, this may be hard to do, and there may be a cost associated with it as they have to travel further away from where they live or work". "That should allow everyone to be able to enjoy the health benefits of physical activity in any urban environment".