Do you think walking is healthy?
"Our findings suggest that healthy people, as well as those with chronic cardiorespiratory disorders, should minimize walking on streets with high levels of pollution because this curtails or even reverses the cardiorespiratory benefits of exercise", the researchers wrote. "This will ensure you can experience the full benefits of exercise", said chief executive of the British Heart Foundation (BHF) Simon Gillespie, whose organization funded the study.
To do so, they examined 119 individuals over the age of 60, who were either healthy, had stable coronary heart disease or had stable ischemic heart disease. Some weeks later they did the other walk.
The researchers believe the findings would apply to other age groups.
Air pollution levels were monitored before and during their walk, and each participant's lung capacity and arterial stiffness was measured before and after. They were randomly allotted to walk either in the west end of Oxford Street which is restricted to taxis and buses emitting diesel fumes and which has frequently breached air quality limits set by the World Health Organisation, or a traffic-free area of Hyde Park.
But after walking on Oxford Street there were only minor improvements in lung capacity, while the measures of arterial stiffness actually got worse which the study associated with "greater exposure to black carbon soot and ultrafine particles from diesel exhaust".
"For many people, such as the elderly or those with chronic disease, the only exercise they very often can do is walk", said Kian Fan Chung, the study's senior author and a professor of respiratory medicine at Imperial College London, in a released statement.
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Or maybe your doctor has recommended more exercise, but you live in a city and can't escape the air pollution. These findings need to be confirmed with empirical long-term studies examining tradeoffs over months and years. When exercising it's best to avoid highly-polluted areas, swapping them for green spaces or even back streets where pollution is lower. "We agree that this is good advice for recreational walking for people who can make that choice", he added.
"This paper highlights the risks to health by walking along polluted roads for the over-60s with specific pre-existing medical conditions", added Ian Colbeck, professor of environmental science at the University of Essex.
Blood flow also increased after exercise, with decreases in blood pressure and an increase in heart rate.
The research also showed that medications for heart disease, such as statins, might protect against some of the damage caused by air pollution. People like outdoor exercise.
"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".
The authors add that it is possible that stress could account for some of the physiological differences seen between the two settings, with the increased noise and activity of Oxford Street having an effect.
"However, it is important to remember the role that walking and cycling can play in helping to reduce air pollution and noise by removing motorized transport from the streets".