The health benefits of walking and cycling outweigh the negative effects on health of air pollution, according to a new study.

This new evidence strengthens the case for people getting out of their cars and onto the streets in an effort to improve physical health and reduce vehicle emissions polluting our cities.

The results are even positive in cities with high levels of air pollution - such as Delhi and London - and should be used by policymakers to change how urban infrastructure is planned, say its authors.

The study was published today in the journal Preventive Medicine by scientists in Brazil, Spain, and Switzerland, and the Universities of Cambridge and Edinburgh, and Imperial College London, in the UK.

Regular physical activity reduces the risk of diseases such as diabetes, heart disease, and several cancers. One way for people to increase their levels of physical activity is through ‘active travel’ – for example walking and cycling. However, concern has been raised about the potential risk due to air pollution while walking and cycling in urban environments.

Dr Audrey de Nazelle from the Centre for Environmental Policy at Imperial College London, a co-author of the study, said: "When you cycle in a city you often end up inhaling more air pollution then if you hopped in your car or on a bus or train, and this has sometimes been used as an excuse by politicians not to promote active travel in polluted environments."

In this study, the researchers used computer simulations to assess the risks and benefits for different levels of intensity and duration of active travel, and of air pollution in different locations around the world.

Using this data, they calculated that only one per cent of cities in the World Health Organization’s Ambient Air Pollution Database had pollution levels high enough that, after half an hour of cycling every day, the risks of air pollution could overcome the benefits of physical activity.

Benefits outweigh risks

Dr Marko Tainio from the MRC 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, from CEDAR, a partnership between the Universities of Cambridge and East Anglia, and the Medical Research Council, said: “Whilst 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.”

Dr de Nazelle concluded: "Now we know that in 99 per cent of world cities it is always beneficial to your health to cycle as long as you want. So it is up to our politicians to set the conditions to encourage walking and cycling and help people lead healthier lives."

The authors caution that their 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.

The research was undertaken by the Centre for Diet and Activity Research, a UKCRC Public Health Research Centre of Excellence. The work was also supported by the project Physical Activity through Sustainable Transportation Approaches, funded by the European Union.

Take part in research

Many questions remain about how to effectively promote walking and cycling for travel and what the health implications of such habits might be. The Imperial College London team, led by Dr de Nazelle, is conducting as ananalysis of travel habits in London, as part of a large European study. London dwellers (and residents of six other European cities) can still participate and help make London a better place to live, work and play.

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'Can air pollution negate the health benefits of cycling and walking?' by Marko Tainio, Audrey J. de Nazelle, Thomas Götschi, Sonja Kahlmeier, David Rojas-Rueda, Mark J. Nieuwenhuijsen, Thiago Hérick de Sá, Paul Kelly, and James Woodcock is published in Preventive Medicine.