The Washington PostDemocracy Dies in Darkness

Air pollution is dangerous. But here’s why you should still walk and bike outdoors

A cyclist along Lake Michigan in Chicago in 2013. (Charles Rex Arbogast/AP)

This story has been updated.

We live in a time when, more and more, science is underscoring the devastating health consequences of air pollution. It seems to be responsible for as many as 3 million deaths globally each year, principally due to tiny particulate matter that can get embedded in our lungs. In the United States alone, the economic cost of these health effects could be more than $100 billion per year, research suggests.

Reading all of this, one might justifiably wonder whether a habit of outdoor walking or biking — for instance, to work every day — does more harm than good. Science has also repeatedly underscored the value of physical activity, but the idea that polluted environments could undermine that may complicate the equation.

New research, though, stacks the two up against each other and reaches a literally heartening conclusion: In the vast majority of cities in the world, it’s still better to be engaging in what is sometimes termed “active travel” (walking, biking) outdoors than not to be, regardless of polluted conditions.

“Cycling and walking make sense for almost everyone and almost everywhere,” said Marko Tainio, a researcher with the University of Cambridge who led the research, recently published in the journal Preventive Medicine. The work was conducted in collaboration with scholars from Spain, Brazil, Switzerland and Britain.

The study used a computerized health model to study air pollution levels ranging from 5 to 200 micrograms per cubic meter of fine particulates (often called PM2.5). These are the smallest particles and considered the most dangerous because of their ability to get into the lungs. At the same time, the research contemplated an individual spending anywhere from zero to 16 hours per day walking or cycling outdoors.

Air pollution levels, it’s important to note, vary widely across cities of the world. The global city average for PM2.5 is 22 micrograms per cubic meter for urban areas. But for Delhi, the city with the worst air pollution, according to the World Health Organization, it’s at 153 micrograms per cubic meter for average concentrations. The WHO’s guideline level is 10 micrograms per cubic meter, so some U.S. cities, although nowhere near as polluted as somewhere such as Delhi, nonetheless have levels that the international agency would also consider too high. (Los Angeles’s level is 20, for instance, according to the WHO.)

Looking across the U.S. and considering both particulate pollution and also ozone, a new report from the American Lung Association similarly found that half of Americans “live in counties where they are exposed to unhealthful levels of these pollutants.”

Returning to PM2.5, at levels which are very close to the global average level of fine particulate pollution in urban areas, the research finds that the health benefits of walking and cycling “far outweigh” any pollution risks — and that’s even if you spend many hours outdoors engaging in these activities.

It was only when levels of particulate matter exceeded 100 micrograms per cubic meter of air that things began to get potentially dangerous, to the extent of outweighing the health benefits of outdoor activity. But even here, up to an hour and a half of cycling was beneficial, the study found, as was up to 10 hours of walking. The research thus suggested that if there’s anyone at risk who is engaging in active outdoor activities, it may be bike messengers in particularly polluted cities.

In these super-polluted places, it was also notable that cycling posed more dangers than walking. “Where the risks become bigger is after 45 minutes per day of cycling,” Tainio said. “Walking is more safe in this context.”

It is important to note that the analysis assumes a person is not just walking or cycling for one day but rather doing so regularly over a year and being steadily exposed to average levels of air pollution in a given place over a similarly expansive time period. The consequent health effect examined in the study was not for particular disease but rather overall mortality, or the chance of dying prematurely from all causes.

The study also assumed that walking or cycling was displacing an amount of time spent at home, where people are also exposed to air pollution in the ambient air. However, if cycling was displacing driving, then the benefits were even greater for health, because air pollution tends to be more concentrated near roads where there are lots of vehicles.

The new research, Tainio noted, sets the stage for a positive feedback: If people bike or walk more, even in polluted cities, it gets cars off the road — which in turn makes cities less polluted.

None of this, of course, in any way lessens concerns about just how bad for health air pollution is. “Air pollution is a major issue,” Tainio said. “We just want to say that it’s not a reason not to exercise.”