You might worry that walking, running or cycling on polluted city streets cancels out any fitness benefits, but the evidence suggests you will still be better off – if you take some simple, common-sense steps.

Air pollution might break your heart at an emotional level but also, it seems, at a physiological level. Air pollution has been strongly linked to heart disease in numerous studies, while a study in Lancet Neurology estimated that air pollution is associated with about a third of the global stroke burden.

It’s not clear why air pollution is associated with heart disease, although a study published in Circulation this year provides some clues. It showed that exposure to nitrogen dioxide and fine particulate matter (a type of air pollution commonly emitted by motor vehicles) was linked to an increase in the size of two of the chambers of the heart, the left and right ventricles. This is a marker that often precedes heart failure.

What does this mean for those who like to run, bike or walk in their city? Is it worth inhaling, deeply, all that toxic air?

Yes, probably; most studies looking at the health benefits and risks of exercising in places with high levels of air pollution have concluded the benefits, generally speaking, outweigh the risks. A study by Brazilian researchers published recently in Medicine & Science in Sports & Exercise showed that exercise could even protect against some of the effects of air pollution, at least in mice.

Some of these unfortunate mice were exposed to air pollution (diesel exhaust particles from the tailpipe of a city bus) while running on a treadmill, while some were exposed to the fumes but didn’t run. The researchers found that exercise ameliorated the inflammatory and oxidative stress caused by exposure to air pollution in the bodies of the mice. Mice aren’t people, of course; as the researchers note, we need more human studies to truly understand how and if exercise might protect against air pollution.

“The study recommended people exercise in green areas and away from busy roads.”

The cost-benefit ratio of exercising in the city is complicated. A Danish study done in 2015 followed 52,000 people for an average of 13 years, combining data on their exercise and lifestyle habits with a sophisticated model of air pollution in Denmark. According to the researchers, “exposure to high levels of traffic-related air pollution did not modify [the] beneficial effects of physical activity on mortality”. However, as the lead author noted, the results pertain to Denmark, and may not be true in cities with much higher levels of pollution. She also recommended people exercise in green areas and away from busy roads.

Pollution can affect our hearts, and our lungs. Exercise will benefit both organs – but location matters. A study in the Lancet looked at the rivaling effects of air pollution and exercise on 119 volunteers over 60 years old; a third had ischemic heart disease, a third had chronic obstructive pulmonary disorder (COPD), and a third were healthy. The participants were randomly selected to either take a two-hour walk along Oxford Street (a congested road in the heart of London) or a walk through the quiet, green spaces of Hyde Park.

While all participants derived measurable cardiopulmonary health benefits from walking, the effects were significantly better among those who took their walk in the park rather than down Oxford Street. As the study’s lead author said, “it shows that we can’t really tolerate the levels of air pollution that we currently find on our busy streets”.

There have been many headlines about how exercising in the city is just not worth the risk, but behind every headline is a more complex story. One study by an international team of researchers concluded that only in the most polluted cities in the world would you be exposed to such high levels of pollution that it would cancel out the benefits of exercising in that polluted air.

So, a bike messenger in a particularly polluted city might be vulnerable, but for most people, in most cities in the world, the benefits of exercising still outstripped the harms caused by air pollution. Another study by University of Cambridge researchers found that even in Delhi – the most air-polluted city in the world – people would need to cycle for over five hours a week before the harm from the smog outweighed the health benefits of cycling in the city.

Still, we clearly need more data and better understanding on where and when we’re most exposed to air pollution; it’s generally lowest in the early morning and late evening. We also need more initiatives like the Wellbeing Walk in London, a signposted backstreet walking route between Euston and King’s Cross stations launched in 2015 and which exposes walkers to 50 percent less pollution than the main roads.

We also need to talk more about air pollution, gain a better understanding of its effects on our hearts, lungs and brains, and what can and should be done to reduce and remediate it – for the sake of our own health, and every other creature that shares the planet with us. It’s not, as they say, all about us.

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