Half an hour's cycling a day, even in the most polluted cities, still provides health benefits, according to a new study.
The new evidence strengthens the case for cycling in polluted cities, which in turn could help reduce vehicle emissions.
One way for people to increase their levels of physical activity is through 'active travel' - including cycling and walking - which then helps reduce the risk of diseases such as diabetes, heart disease, and several cancers.
A collaborative study from the Centre for Diet and Activity Research (CEDAR) and the Medical Research Council Epidemiology Unit at the University of Cambridge concludes that cycling for half-an-hour a day still provides a benefit in 99 per cent of the world's cities.
Britain's air pollution contributes to the early deaths of around 40,000 people annually, and a recent report suggested that one way to reduce the number of cars on the road would be to encourage active travel.
But fears have been raised that people who travel in such environments would inhale more pollution, particularly from diesel engines.
Previous studies had concluded that the health benefits of active travel outweighed the risks, but those studies were done in areas of relatively low pollution.
The new study, published in the journal Preventive Medicine, used computer simulations to test the risks and benefits of active travel in areas of higher pollution, as well as measuring differences in the intensity of active travel.
Researchers calculated that air pollution risks will not negate the health benefits of active travel in the vast majority of urban areas worldwide.
Only one per cent of cities in the World Health Organization's Ambient Air Pollution Database had pollution levels high enough that the risks of air pollution could start to overcome the benefits of physical activity after half-an-hour of cycling every day.
Dr Marko Tainio from the Epidemiology Unit at the University of Cambridge, who led the study, said: "Our model indicates that in London, health benefits of active travel always outweigh the risk from pollution.
"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."
Senior author Dr James Woodcock, also from CEDAR, said: "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 organisation London Air, run by King's College London, monitors the capital's air pollution and offers live update on the air quality using a scale of one to ten.
People are advised to "consider reducing activity, particularly outdoors", when the readings are considered 'high', between seven and nine.
For 'very high' readings, people are warned to "reduce physical exertion, particularly outdoors, especially if you experience symptoms such as cough or sore throat".
But these new findings suggest people should not be overly concerned with cycling in the capital, where the benefits of active travel outweigh the risk to health from pollution.