Research links vigorous exercise with improved memory, mood, and mental clarity. But a new study in Medicine & Science in Sports & Exercise reports that running in a polluted urban area could diminish the positive brain boost you would ordinarily get from a workout. Why? Hard exercise makes for faster and deeper breathing, meaning you take in more air—and everything in it. That exposes your body (and brain) to more toxins. Here's how to breathe easier on your runs. (Even on a clear day, breath control while training is crucial to training success. Learn the Key to Breathing on Your Runs for some tips.)

In the City

Vehicles and industrial sources emit harmful particles and gases that can make your eyes and throat burn, your head ache, your chest tighten, and your breathing labored. Long term, it increases your risk of heart and lung disease.

Manage Your Risk: "The positive benefits of running likely outweigh the risks," says Linsey Marr, Ph.D., an environmental engineering professor at Virginia Tech. Still, it's best to skip outdoor runs when the air-quality index is 100-plus. To reduce exposure on other days, run in the morning in parks or near water.

In the 'Burbs

There's no magic air barrier once you cross into the outskirts; ozone levels are often higher in downwind suburbs than in cities, Marr says. Plus, mowers and weed whackers emit pollutants, and chemicals that kill pests and fertilize grass have been linked to health problems like cancer.

Manage Your Risk: Avoid major roadways, especially where traffic is often at a standstill, says Michael Koehle, M.D., Ph.D., who studies exercise and pollution at the University of British Columbia. If you spot lawn care in progress, cross the road.

Plus:Everything You Need to Know About Trail Running Safety

In the Country

Agricultural facilities, wildfires, and volcanoes pump out harmful particles. Dust from unpaved roads can also lodge itself in your lungs, causing respiratory problems.

Manage Your Risk: Large polluters—sprawling factory farms, power plants, burning forests—pose large risks, so steer clear. If you run on dirt roads, stay on the upwind side, and avoid those heavily trafficked by tractors or other monster-size vehicles, Koehle says. Not only do such vehicles emit pollutants, they further pulverize and distribute dust into the air.

At the Gym

Fitness centers often have sophisticated air filters, says Pamela Dalton, Ph.D., of the Monell Chemical Senses Center in Philadelphia. But chemicals used to clean equipment can irritate your airways. The funk of sweaty treadmill neighbors? Annoying, but harmless.

Manage Your Risk: "Spending two hours at the gym, even every day, is unlikely to lead to health issues," Dalton says. Still concerned? Avoid facilities that are stuffy or dirty—signs of poor air filter maintenance. Complain if cleaning products burn your eyes, nose, or lungs.

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