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DIMINISHING RETURNS

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

Reuters/David Gray
Filtered gasps.
  • Akshat Rathi
By Akshat Rathi

Senior reporter

This article is more than 2 years old.

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 inPM 2.5 (μg/m3)Tipping point (mins/day)
Delhi12230
Dhaka9045
Karachi8845
Tianjin8745
Beijing8545
Wuhan8045
Cairo7645
Chengdu7160
Lahore6860
Mumbai6375
Kolkata6175
Hangzhou6175
Chongqing6175
Hyderabad5975
Shanghai5290
Guangzhou4890
Chennai44105
Dongguan43105
Johannesburg41135
Istanbul33165
Tehran32165
Hong Kong29225
Seoul24315
Bangkok24315
Bogotá24315
Taipei22315
Mexico city20480
Moscow20480
São Paulo19480
Rio de Janeiro18480
Paris18480
Manila17480
Osaka17480
Nagoya16855
Tokyo15855
London15855
Buenos Aires14855
Chicago12855
Los Angeles11960
New York9960

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