Led by Imperial College London along with researchers from Duke University, USA, the new study is the first to show that air pollution can have a negative effect on healthy people, people with a chronic lung condition linked with smoking called Chronic Obstructive Pulmonary Disease (COPD), and those with coronary heart disease -- which affects the supply of blood to the heart.

For the research, the team recruited 119 volunteers over the age of 60 who were healthy, had stable COPD, or had stable heart disease. 

Participants were asked to walk for two hours in two London settings at midday; either in a quiet part of green and leafy Hyde Park or along a busy section of Oxford Street -- which has regularly breached the air quality limits set by the World Health Organization.

The team took physical measurements from the participants before and after the walks and collected data on pollution levels to measure participants' exposure.

New research suggests that the benefits of exercise could be cancelled out by the high levels of pollution found in urban areas. 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.

Walking in Hyde Park was also found to have health benefits for everyone, with lung capacity improving within the first hour and most participants showing a significant lasting increase for more than 24 hours later.

Blood flow and heart rate also increased after a walk in the park, blood pressure decreased, and arteries became less stiff.

However, walking along Oxford Street did not have the same positive effect, with participants showing only a small increase in lung capacity, far lower than what was recorded in the park, and smaller decreases in arterial stiffness.

For those participants with heart disease however, taking their medication was found to have a stabilizing effect and may protect these patients from deteriorating in areas with higher levels of air pollution.

Although the team noted that stress could be a contributing factor, with the increase in noise and the number of people on Oxford Street another reason behind the physiological differences observed, the new findings still add to the growing body of evidence on the dangers of urban air pollution.

The team emphasized that although the study only involved two relatively short walks, repeated exposure to air pollution would also not be beneficial to our respiratory and cardiovascular systems, and although the study included older participants, the findings could also apply to other age groups.

"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, "Our research suggests that we might advise older adults to walk in green spaces, away from built-up areas and pollution from traffic."