How long can you cycle before the harm from pollution exceeds the benefits of exercise?

Life as Laboratory
Life as Laboratory

There is no silver bullet in medicine, but exercise comes close. The benefits of physical activity are so far-reaching that, if you do nothing but get some gym time every day, you will live a healthier and longer life than most people.

So when I decided to move closer to my office in London, the first thing I thought I’d change about my lifestyle was cycling to work. Yet, given all the harm we know air pollution can cause, does cycling actually help, or could it hurt? After all, I’m not breathing in the foul fumes of a truck when I’m sitting inside an air-conditioned train. I’m certainly not breathing them in deeply, as I would while huffing and puffing on my cycle.

Air pollution kills more than 5 million people every year, yet there has been no analysis of the costs versus benefits of city cycling. Until now. With the help of advanced computer simulations and data on the effect of pollution on human health, researchers at the University of Cambridge found that, in almost all the cities of the world, the health benefits of cycling and walking far exceed the harm caused by air pollution.

Researchers also calculated the theoretical “tipping point,” after which additional cycling isn’t more beneficial to health, and what they called the “break-even point”—really more of a breaking point—after which cycling is actually more harmful than beneficial. When combined with WHO data on pollution in major cities, the research shows that even in the world’s most polluted cities (like Delhi), that breaking point doesn’t occur until after at least 60 minutes of cycling a day.

“We are not disputing the fact that air pollution kills,” said James Woodcock, one of the authors of the study. “[But] it would do so even when you are sitting at home. What we find is that being physically active, even in polluted cities, can cut that risk. So it’s a win-win.”

The study’s methodology is limited. Researchers only used one pollution metric—the amount of fine particulate matter (PM 2.5)—as a proxy for negative effects, noting that science isn’t developed enough to feed other factors, such as larger particulate matter (PM 10) and gases (such as nitrogen oxides), into a single a computer model.

“Even when we are able to take those factors into consideration, the overall message wouldn’t change,” Woodcock said. “Maybe the harm caused in some cities would be slightly greater, but not by a lot.”

Researchers also pegged cyclists’ exposure to harmful pollution as two times the average, but many cities have pockets where pollution levels are much higher.

Still, as the first global analysis of physical activity and pollution in cities, the report’s public-health message is clear: Get moving.

City cycling in PM 2.5 (μg/m3) Tipping point (mins/day)
Delhi 122 30
Dhaka 90 45
Karachi 88 45
Tianjin 87 45
Beijing 85 45
Wuhan 80 45
Cairo 76 45
Chengdu 71 60
Lahore 68 60
Mumbai 63 75
Kolkata 61 75
Hangzhou 61 75
Chongqing 61 75
Hyderabad 59 75
Shanghai 52 90
Guangzhou 48 90
Chennai 44 105
Dongguan 43 105
Johannesburg 41 135
Istanbul 33 165
Tehran 32 165
Hong Kong 29 225
Seoul 24 315
Bangkok 24 315
Bogotá 24 315
Taipei 22 315
Mexico city 20 480
Moscow 20 480
São Paulo 19 480
Rio de Janeiro 18 480
Paris 18 480
Manila 17 480
Osaka 17 480
Nagoya 16 855
Tokyo 15 855
London 15 855
Buenos Aires 14 855
Chicago 12 855
Los Angeles 11 960
New York 9 960
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