Polluted air is bad news for everyone, but particularly for runners and cyclists, since they breathe in huge volumes of air, and gulp it in directly through the mouth (which bypasses the filtering you get from breathing in through the nose). The pollution penetrates deep into the lungs, where it creates inflammation and oxidative stress, and can contribute to conditions like asthma and chronic obstructive pulmonary disease. That's why, during high-pollution summer days, we get warnings about not exercising outdoors.
But researchers in Sao Paulo were skeptical about this advice. Sure, pollution causes inflammation and oxidative stress; but aerobic exercise fights inflammation and oxidative stress. So maybe the net benefits of exercise, even in polluted conditions, outweigh the risks. To find out, they performed a study with mice. First, they collected diesel exhaust particles from the tailpipe of a city bus. Then they divided their mice into four groups:
A control group that did nothing.
An exercise group that trained five days a week for five weeks.
A pollution group that inhaled diesel particles but didn't exercise.
A pollution + exercise group that inhaled diesel particles while exercising.
At the end of the five-week study, they did a whole series of tests to measure how much inflammation, oxidative stress, and lung remodeling was present in the mice. Here's a sample result, showing exhaled nitric oxide (a marker of oxidative and nitric stress):
Pretty stark. The diesel particles (DEP) cause a major spike in nitric oxide -- unless the mice were exercising, in which case levels didn't really change at all. Similar patterns were observed in pretty much all the tests they ran: running protected the mice from pollution damage.
Now, it's important to bear in mind that mice are not humans, so we have to treat the results with caution. If you have the choice between running alongside a river or running alongside a four-lane highway, choose the former every time. But if you're choosing between running on a polluted day or not exercising at all, this study should tilt you in favor of lacing up the shoes. As the researchers conclude:
Taken together, our results indicate that the decreased inflammatory status achieved by chronic aerobic exercise has a protective effect on DEP-exposed mice, decreasing the pro-inflammatory effects of chronic exposure to air pollution.
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