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Could urban exercise be bad for your health?

It’s a conundrum that has preyed on the minds of outdoor athletes for years – does commuting on bike or on foot do more harm than good? Does the combination of noxious gases and soot particulates in vehicle and industrial exhaust fumes undermine the benefits of exercise?

Air pollution is one of the leading environmental risk factors for people’s health. According to a report by the Royal Colleges of Physicians and of Paediatrics and Child Health, air pollution contributes to about 40,000 early deaths a year in the UK, with urban areas worst affected.

A shift from cars, motorbikes and buses to active travel would help to reduce vehicle emissions, but the people who walk or cycle in these environments will inhale more pollution, which could be detrimental to their health. London will be the first place in the UK to introduce an ultra-low emission zone for vehicles from 2019, and a further 15 UK cities are exploring similar schemes of either barring higher emission vehicles from their streets, or levying a heavy daily charge.

Mayor of London, Sadiq Khan said: “With nearly 10,000 people dying early every year in London due to exposure to air pollution, cleaning up London’s toxic air is now an issue of life and death.” According to London Air, the independent monitoring site run by King’s College London, more people are admitted to hospital for lung and heart problems on days when air quality deteriorates, and an increased number of people visit their GP and need to take more medicine. There is also a building body of evidence that indicates long-term exposure to polluted air can
lead to chronic respiratory conditions and even
neurological illnesses.

Dr Peter Steer, chief executive of Great Ormond Street Hospital for Children NHS Foundation Trust (GOSH) said: “Children living in highly polluted areas are four times more likely to have reduced lung function in adulthood, yet improving air quality has been shown to halt and reverse this effect.”

Against this background, fitness enthusiasts must wonder whether an active commute is actually harmful to health, and whether sitting in a car, bus or train might actually lead to a better outcome. After all, gulping lung fulls of dirty air as your body demands more oxygen, doesn’t sound very enticing. But a new study by academics at the University of Cambridge has calculated that active travel is beneficial, even when increased physical activity leads to an increase in the intake of air pollution.

It found that the benefits of physical activity in air with the global average concentration of particulates “by far outweigh risks from air pollution even under the most extreme levels of active travel.” The harm would only exceed the gains after 90 minutes of cycling per day or more than 10 hours of walking, although if the alternative was driving, the threshold for cycling would rise to 3 hours 30 minutes.

Even in Delhi, India, the most polluted city according to the World Health Organisation (WHO), where particulate concentrations average 153 μg/m3, the tipping point between cycling and driving was 30 minutes on a bike per day.

“For the average urban background PM2.5* concentration (22 μg/m3) in the WHO database, the tipping point would only be reached after 7 hours of cycling and 16 hours of walking per day,” said the scientists in the journal, Preventive Medicine. “When we assumed that time spent cycling would replace time driving a car, benefits always exceeded the risks against background air pollution concentration below 80 μg/m3 – a concentration exceeded in only 2% of cities.” Dr Marko Tainio, from the Medical Research Council Epidemiology Unit at the University of Cambridge, who led the study added: “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 and for long enough to cancel out the health benefits of physical activity.”

PM2.5 – refers to particute matter which is the mixture of solid particles and liquid particles in the air.