It highlights the need for stricter air quality limits and better traffic-control measures in our cities.
Exposure to air pollution may counter the beneficial health effects of exercise in older adults, according to a study published in The Lancet journal.
The findings show that short-term exposure to traffic exhaust on a busy street can cancel out the positive effects a two-hour stroll would otherwise have on older adults heart and lungs.
This is the first study to document these negative effects on healthy people as well as those with pre-existing cardiorespiratory conditions such as chronic obstructive pulmonary disease (COPD) or coronary heart disease.
"This adds to the growing body of evidence showing the negative cardiovascular and respiratory impacts of even a short, two-hour exposure to motor traffic pollution," said Junfeng Zhang, professor at Duke University in the US.
"It highlights the need for stricter air quality limits and better traffic-control measures in our cities," said Zhang.
"Combined with evidence from other recent studies, our findings underscore that we cannot really tolerate the levels of air pollution that we currently find on our busy streets," said Fan Chung, professor at Imperial College in the UK.
Since the research also showed that volunteers who walked for two hours in a large city park experienced significant improvements in lung and vascular functions, "we call for greater access to urban green spaces for people to exercise," Zhang added.
The researchers recruited 119 volunteers over the age of 60 who were either healthy, had stable COPD, or stable ischemic heart disease.
Volunteers walked for two hours midday at one of two London locations: in a relatively quiet part of Hyde Park or along a busy section of Oxford Street, where pollution - including black carbon, nitrogen dioxide and fine particulate matter from diesel exhaust fumes - regularly exceeds air quality limits set by the World Health Organization.
Physical measurements taken before and after the walks revealed the effects activity had on each volunteers cardiopulmonary health, including lung capacity, blood pressure, blood flow and arterial stiffness.
Following a stroll in Hyde Park, volunteers lung capacity improved significantly within the first hour and this improvement lasted for more than 24 hours in many cases.
By comparison, a walk along Oxford Street led to a smaller increase during the first hours and no increase later.
Walking in Hyde Park reduced arterial stiffness by more than 24 per cent in healthy and COPD volunteers and more than 19 per cent in heart disease patients.
Walks along Oxford Street yielded much smaller gains. Healthy volunteers experienced a maximum reduction in arterial stiffness of just 4.6 per cent; COPD patients saw a 16 per cent reduction; and those with heart disease saw an 8.6 per cent reduction.
The researchers also noted that patients with heart disease who took medication to improve their cardiovascular health experienced less severe effects following exposure to the pollution. The medication had a stabilising effect.