The health benefits of active travel outweigh the risks from air pollution, even in cities where air pollution is high, new research suggests.
Researchers said the findings strengthen the case for supporting walking and cycling in polluted cities, in an effort to reduce vehicle emissions.
Regular exercise can reduce the risk of diabetes, heart disease and several cancers.
It is thought that the nation's fitness levels could be improved by encouraging people to engage in active travel.
However there were concerns over the negative effects of air pollution in urban environments on those who did.
A new study, led by researchers from the Centre for Diet and Activity Research (CEDAR) and Medical Research Council Epidemiology Unit at the University of Cambridge, found that the benefits of active travel actually outweighed the risks from pollution.
This is despite the fact that air pollution is one of the leading environmental risk factors for people's health.
Researchers from CEDAR used computer simulations to compare the risks and benefits for different levels of intensity and duration of active travel and of air pollution in various locations around the world.
They used information from international epidemiological studies and meta-analyses.
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Using this data, researchers calculated that air pollution risks do not outweigh the health benefits of active travel in the vast majority of urban areas worldwide.
Only 1% of cities in the World Health Organisation's Ambient Air Pollution Database had pollution levels high enough that the risks could start to overcome the benefits of physical activity after half an hour of cycling every day.
The study, published in Preventive Medicine, is the first to model the risks and benefits of walking and cycling across a range of air pollution concentrations around the world.
"Our model indicates that in London, health benefits of active travel always outweigh the risk from pollution," said lead author Dr Marko Tainio from the MRC Epidemiology Unit at the University of Cambridge.
"Even in Delhi, one of the most polluted cities in the world - with pollution levels ten times those in London - people would need to cycle over five hours per week before the pollution risks outweigh the health benefits.
"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."
Co-author Dr James Woodcock, also from CEDAR, says: "While this research demonstrates the benefits of physical activity in spite of air quality, it is not an argument for inaction in combatting 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 authors warned that their research model does not take into account detailed information on conditions within different localities in individual cities, the impact of short-term episodes of increased air pollution, or information on the background physical activity or disease history of individuals.
For individuals who are highly active in non-transport settings, for example recreational sports, the marginal health benefits from active travel will be smaller. And vice versa for those who are less active than average in other settings.
