A London study found that short-term exposure to traffic fumes greatly reduced the positive impact of walking on the heart and lungs.
For the study, 119 volunteers over the age of 60 were asked to walk for two hours in a quiet part of Hyde Park and in a busy section of Oxford Street.
All participants, recruited through the Royal Brompton Hospital in London, were healthy or had a stable lung condition or non-progressive heart disease.
The study showed that all volunteers benefited from a walk in the park, with lung capacity improving in the first hour, arteries become significantly less rigid and blood flow increases.
In contrast, walking on Oxford Street had little impact on arterial stiffness and led to a small increase in lung capacity.
Professor Fan Chung of Imperial College London, who led research funded by the British Heart Foundation, said: "Studies such as this one may support new air quality limits, it shows that we can not really tolerate the levels of air pollution that we currently find in our busy streets. "