The study was funded by the British Heart Foundation, whose chief executive Simon Gillespie said: 'Exercise is crucial in reducing the risk of a heart attack or stroke, but it seems unsafe levels of air pollution could be erasing these benefits in older adults.
A separate study published in The BMJ suggested that pregnant mothers exposed to higher levels of air pollution are more likely to have a baby born at lower birth weights.
"Our findings indicate that in traffic congested streets, like London's Oxford Street, the health benefits of walking do not always outweigh the risk from traffic pollution", said senior author Professor Fan Chung, from the National Heart & Lung Institute at Imperial College London.
London Mayor Sadiq Khan has pledged to make reducing air pollution one of his top priorities and announced a global network to tackle the issue while on a trip to Delhi this week.
The research was carried out in collaboration with U.S. colleagues from Duke University.
Half walked on Oxford Street and the other half through a quiet section of nearby Hyde Park.
The study tested 119 people aged over sixty, 40 of whom were healthy, 40 with a medically-stable lung condition, COPD, and 39 with stable ischaemic heart disease.
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.
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.
They found that overall, those who lived within an average 1 kilometre of a major road were at higher risk for high blood pressure and peripheral artery disease (PAD), a narrowing of the peripheral arteries to the legs, stomach, arms, and head, which increases the risk of coronary artery disease, heart attack and stroke.
Analysis revealed that participants benefitted from walking in the park, with lung capacity improving within the first hour and a significant lasting increase for more than 24 hours in many cases.
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.
The researchers found levels of pollution - including fine particulate matter, black carbon and nitrogen dioxide - were significantly higher on Oxford Street compared to Hyde Park.
Air pollution may be harmful to babies even before they are born, a new study has found.
'Our research suggests that we might advise older adults to walk in green spaces, away from built-up areas and pollution from traffic'.
Walking is the sort of low impact exercise recommended by the NHS for older people to improve their cardiovascular fitness, but the study found that the impact of air pollution negated this.
The study was funded by the British Heart Foundation.