Before you get angry, no, this study is not saying that air pollution wipes out the benefits of all exercise. It is instead showing that in very specific circumstances, exposure to traffic pollution may lessen some of the positive effects of a gentle stroll for elderly people.

Even that, however, is not clear.

Published in The Lancet, the study led by Imperial College London and Duke University in North Carolina looked at the effects of short-term exposure to traffic exhaust on busy city streets, such as in London.

In the study, 119 people over the age of 60 were asked to take part. They were either healthy, had stable chronic obstructive pulmonary disease (COPD), or stable ischemic heart disease.

They walked for two hours a day at one of two places in London, a quiet area in Hyde Park devoid of vehicles or along busy Oxford Street, which teems with cars.

On Oxford Street, pollution including black carbon, nitrogen dioxide, and fine particulate matter regularly exceeds air quality limits set by the World Health Organization (WHO).

After a stroll in Hyde Park, it was found that the lung capacity of the volunteers significantly improved in the first hour, lasting for more than 24 hours. However, the walk along Oxford Street led to a much smaller increase, and no increase later on.

In addition, walking in Hyde Park reduced stiffness in arteries by more than 24 percent among the healthy and COPD volunteers. For the heart disease patients, it was more than 19 percent. The improvements from walking along Oxford Street were less but not insignificant, at 4.6 percent for healthy volunteers, 16 percent for COPD volunteers, and 8.6 percent for those with heart disease.

"For many people, such as the elderly or those with chronic disease, the only exercise they very often can do is to walk," co-author Fan Chung from Imperial College London said in a statement. "Our study suggests that we might advise these people to walk in green spaces, away from built-up areas and pollution from traffic."

So this study shows that there is a slight reduction in the benefits of walking on a busy street compared to a quiet park. However, the study certainly does not say you should avoid exercising in a busy city, and there are still positive effects from walking, no matter where you are.

It also isn’t clear that pollution was the cause of the physiological differences. The researchers say that stress, such as from the increased noise on Oxford Street, could have had an effect.

Air pollution is a serious issue, but there are very few cities in the world where it outweighs the benefits of exercise. For the most part, get out and about, and stay healthy.