The positive effects of physical exercise such as walking and cycling still outweigh the harms caused by inhaling air that contains traffic fumes – but only to some extent, a new study revealed.

Comparing the levels of air pollution among cities all over the world, researchers in the United Kingdom found that only 1 percent of cities have such terribly high levels of pollution that walking or cycling could be detrimental to a person's health.

In fact, in 99 percent of cities across the globe, it is still safe to bike around for up to two hours a day, says study author Audrey de Nazelle.

"That's because physical inactivity is such a public health issue – it is not that pollution is not detrimental," said de Nazelle.

Earlier this year, the World Health Organization (WHO) reported that smog, or the combination of smoke and fog, has covered most major cities around the world.

Smog-related deaths could reach 6 million annually should air pollution continue to reach dangerously high levels, the WHO said.

Air pollution has been linked to asthma, cancer, heart disease, stroke, diabetes, obesity, and dementia. The Royal College of Physicians published a report that concluded that 40,000 people die as a result of air pollution every year.

Is London Safe?

Mark Tainio, lead author of the new study, says the city of London is one of the safer places to walk and cycle in respect to air pollution.

Tainio says their models suggest that the health benefits of active travel in London outweigh the risk of air pollution.

Even in Delhi, one of the most polluted cities around the globe due to its pollution level of up to ten times than in London, Tainio says people would have to cycle more than five hours every week before the risks overtake the benefits.

However, Tainio warns that a small minority of workers such as bike messengers are exposed to air pollution levels that are high enough to cancel out the health benefits.

The authors of this study also caution against complacency. Although the findings demonstrate the positive effects of physical activity in spite of the quality of air, it is not an argument for inaction when it comes to battling pollution, says Dr. James Woodcock.

Instead, the findings support the idea that people get out of their cars and on to their bikes or feet, which can reduce pollution while promoting physical exercise, adds Woodcock. Their findings will be featured in the journal Preventive Medicine.

Photo : Yukiko Matsuoka | Flickr