How long do you have to cycle in London before effects of pollution outweigh benefits of exercise?

A recent study has concluded that the benefits of physical activity almost always outweigh the negative effects of pollution for those cycling in urban areas. Researchers say their findings should encourage people out of their cars and onto their bikes and in so doing reduce pollution levels further.

London’s air pollution, which is caused primarily by traffic and diesel fumes, is responsible for 9,500 premature deaths each year, according to a 2015 study by King’s College, London. Thanks to their proximity to traffic and deeper breathing, a cyclist can expect greater exposure to that threat than most.

According to a 2011 study by researchers from the London School of Medicine, a cycle commuter inhales more than twice the amount of black carbon particles as a pedestrian making a comparable trip. Stopped on your bike at a set of traffic lights, wallowing in the fumes, these kinds of things have probably crossed your mind.

But how dangerous is it? The FT reports that a recent Cambridge University study found that the health benefits of cycling outweigh the risks caused by air pollution in 99 per cent of cities. Is the capital one of them? How long would you have to cycle in London before the effects of pollution started to outweigh the benefits of cycling?

The researchers established tipping points for each of the cities they looked at – the length of time after which the impact of pollution started to outweigh the positives that come with being more active.

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.”

If air pollution were constantly at the city’s record monthly high, it would take nine hours and 50 minutes of cycling a day for harm from pollutants to exceed the overall benefit from physical activity – and anything less than that would reduce your mortality risk relative to not cycling. At average or rush hour levels of pollution, the benefits of cycling in London will always outweigh the negatives.

Tainio said that in Delhi – which has pollution levels ten times as high as in London – people would need to cycle for over five hours per week before the pollution risks outweighed the health benefits.

Senior author Dr James Woodcock, from the Centre for Diet and Activity Research (CEDAR), 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.”