Benefits of cycling and walking 'outweigh air pollution risk' in cities

The health benefits of cycling and walking outweigh the harm from inhaling air loaded with traffic fumes in all but the world’s most polluted cities, according to a study. An international team of researchers who have modelled the effects say only 1% of cities in the world have such high levels of air pollution that cycling or walking could make a person’s health worse. “The good news is that across the world, in 99% of cities it is safe to cycle up to two hours a day,” said Dr Audrey de Nazelle from the centre for environmental policy at Imperial College London, one of the study’s authors. “That’s because physical inactivity is such a public health issue – it is not that pollution is not detrimental.” In respect to air pollution, London is one of the safer cities in the world in which to cycle and walk, with the researchers finding it safe to do so all day. “Our model indicates that in London health benefits of active travel always outweigh the risk from pollution,” said Dr Marko Tainio from the Cambridge MRC epidemiology unit, who led the study. “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.” Air pollution in major cities including London is of increasing concern, particularly to those who are regularly exposed to it when commuting to work. In February, the Royal College of Physicians published a report looking at exposure to air pollution across a lifetime and concluded that 40,000 people die as a result of it every year. The report linked polluted air to cancer, asthma, stroke and heart disease, diabetes, obesity, and dementia, and calculated that NHS, business and other costs in the UK came to more than £20bn a year. The college called for tougher action against polluters and for local authorities to be given powers to close roads and divert traffic when pollution levels were high. The authors of the study, from the UK, Switzerland, Spain and Brazil, say their findings published in the journal Preventive Medicine are not a reason for complacency. “While this research demonstrates the benefits of physical activity in spite of air quality, it is not an argument for inaction in combatting pollution,” said Dr James Woodcock from the Centre for Diet and Activity Research. “It provides further support for investment in infrastructure to get people out of their cars and on to their feet or their bikes – which can itself reduce pollution levels at the same time as supporting physical activity.” The researchers modelled the effects of cycling and walking at different levels of air pollution and established a tipping point – the length of time after which there was no further health benefit, and a break-even point, when the harm from air pollution began to outweigh the health benefit. For Delhi, the most polluted city on the World Health Organisation’s database, the tipping and break-even points for cycling were 30 and 45 minutes per day respectively, while for walking they were 90 minutes and six hours and 15 minutes respectively. While the researchers looked at the levels of particulates – PM2.5 – in the air and not NO2, which has also been established as harming health, “we did lots of sensitivity analyses and the message would have been the same”, said De Nazelle.