The health benefits of cycling and walking outweigh the harmful effects of air pollution, a study has suggested.
Even in cities with high air pollution levels, the benefits are greater than the risks, the University of Cambridge research found.
Regular exercise can reduce the risk of diabetes, heart disease, and several cancers, according to researchers.
The Royal Colleges of Physicians has said air pollution contributes to 40,000 UK early deaths a year.
Researchers analysed information from epidemiological studies to create computer simulations to compare the risks and benefits of physical activity and air pollution in different locations.
It found that for an average air pollution concentration in an urban area, the tipping point – when the risks begin to outweigh the benefits – comes after seven hours of cycling or 16 hours of walking a day.
The Cambridge study, which has been published in Preventive Medicine, found that in a small number of highly polluted cities, the risks of air pollution could start to overcome the benefits of physical activity after 30 minutes of cycling every day.
But only 1% of cities in the World Health Organization’s Ambient Air Pollution Database had pollution levels high enough for that to happen.
“Even in Delhi, one of the most polluted cities in the world – with pollution levels 10 times those in London – people would need to cycle over five hours per week before the pollution risks outweigh the health benefits,” said Dr Marko Tainio, the lead author of the study.
“Our model indicates that in London health benefits of active travel always outweigh the risk from pollution.
“We should remember, though, that a small minority of workers in the most polluted cities, such as bike messengers, may be exposed to levels of air pollution high enough to cancel out the health benefits of physical activity.”
The average air pollution level for cities around the globe is 22 micrograms per cubic metre, according to the World Health Organisation (WHO). In London the air pollution level was recorded at 16 micrograms per cubic metre in 2011.
Senior author Dr James Woodcock added: “Whilst this research demonstrates the benefits of physical activity in spite of air quality, it is not an argument for inaction in combating pollution.
“It provides further support for investment in infrastructure to get people out of their cars and onto their feet or their bikes – which can itself reduce pollution levels at the same time as supporting physical activity.”
The research was carried out by experts from the University of Cambridge’s Centre for Diet and Activity Research and Medical Research Council Epidemiology Unit, along with researchers from the University of East Anglia.