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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

Published Last updated 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.

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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