The Sophie’s Choice of Health: Exercise in Smog or Stay Obese

The benefits of biking or walking in polluted areas outweigh the risk of inhaling dirty air.

Cyclists wearing masks ride along a road in heavy smog in Suzhou, China. (Photo: ChinaFotoPress/Getty Images)

May 7, 2016· 2 MIN READ

Culture and education editor Liz Dwyer has written about race, parenting, and social justice for several national publications. She was previously education editor at Good.

I live about seven miles from my job, which, given Los Angeles’ insanely bad traffic, makes for a 45-minute to an hour-long commute—on a good day. Although I often seek respite on the city bus, two big worries keep me from permanently ditching my car for a bike: fear of being struck by a distracted driver and concern about the negative health effects of inhaling all the smoggy air coming out of vehicle tailpipes.

It’s a legitimate concern—a report released in March by the World Health Organization found that as many as 8.2 million people around the world die every year as a result of breathing in dirty air, which contributes to stroke, heart disease, cancer, and chronic respiratory disease. But it turns out that for most city dwellers, the obesity-curbing and cardiovascular-health benefits of biking or walking probably outweigh the risk of breathing in polluted air.

That’s the main finding of a first-of-its-kind study published this week in Preventive Medicine by researchers from the Centre for Diet and Activity Research and the Medical Research Council Epidemiology Unit at the University of Cambridge. The researchers used data for about 1,600 cities from the World Health Organization’s Ambient Air Pollution Database, along with information from the website Compendium of Physical Activities, which standardizes physical activity intensity data. Through computer simulations, they found the tipping point beyond which the health benefits of biking or walking are outweighed by doing so in smoggy conditions.

“In this study we indicate that physical activity benefits outweigh risks of air pollution for active travellers. This means that walking and cycling makes sense despite air pollution,” Marko Tainio, the study’s lead researcher, wrote in an email to TakePart.

Tainio and his colleagues found that in the typical polluted city, such as Los Angeles or London, a person can spend up to seven hours per day biking, or 16 hours walking, before risks of smog exposure start to outweigh the health benefits of physical activity. Only 1 percent of cities—such as New Delhi, the most polluted urban area on the planet—have such terrible smog that cycling more than five hours per week or walking for 30 hours per week might be a cause for concern.

So does that mean people in New Delhi or in notoriously smoggy Beijing should watch the clock and make sure they aren’t cycling more than five hours per week? According to Tainio, people shouldn’t see the five-hour limit as a strict recommendation.

“Given the uncertainties in this kind of calculations, the border between risks and benefits is too uncertain to be taken as [a] guideline,” wrote Tainio. “In highly polluted cities I would advise still to do some amount of cycling, if possible, since it will generate benefits. Also, by replacing car driving with cycling and walking everyone can help to mitigate [the] air pollution problem, and at the same time get physical activity benefits. Win-win situation.”

Still skeptical? Audrey de Nazelle, a coauthor of the study, acknowledged that the findings might create a bit of confusion about the ill effects of air pollution. “We’re not saying that in the end air pollution is OK for you, but just that physical activity is so important that even at high levels you’re still better off cycling/walking—but that doesn’t diminish in any way the fact that the air pollution kills,” she wrote to TakePart. Essentially, you’ll probably be less obese when you die from all the smog you were breathing in.

Because his team “focused on long-term average concentration of air pollution,” Tainio wrote, it didn’t have any specific health recommendations for dealing with one-off smog-alert days—those times when the air is so awful that schools keep kids indoors and cars are booted off roads. If you’re worried about biking during a smog alert, de Nazelle has a practical suggestion: “Instead of going for a bike ride that day people [should] spend the time writing to their politicians to demand immediate action to curb air pollution in their city.”