The study from the Centre for Diet and Activity Research (CEDAR) and Medical Research Council Epidemiology Unit at the University of Cambridge strengthens the case for supporting cycling even in polluted cities, an effort that in turn can help reduce vehicle emissions.

Researchers used computer simulations to compare 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 information from international epidemiological studies and meta-analyses.

Using this data, the researchers calculated that in practical terms, air pollution risks will not negate the health benefits of active travel in the vast majority of urban areas worldwide. Only 1 percent 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, who led the study, said that the 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.

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 study is published in Preventive Medicine.