Researchers say the benefits of a bicycle commute could be at least partially negated by when and where you ride, and for how long. Here are some safety tips.

Exercise is a wonderful health elixir.

But bicyclists who commute to work in big cities may be breathing in harmful doses of pollutants, according to studies.

The upshot: Cycling’s health benefits can be reduced by when and where you ride, and for how long.

Longer commutes during rush hour when tailpipes are spewing the most smog can translate into greater health risks, according to the preliminary results of a five-year study done in New York.

Particulates can dig into the lungs and disperse into the bloodstream, where they may lead to issues like heart problems or lung cancer.

Cyclists are also more at risk than people who walk to work because they’re breathing in more heavily as they pedal.

“People who bike regularly are getting the biggest doses of air pollution during their commuting time,” said Steven Chillrud, a geochemist at the Lamont-Doherty Evart Observatory at Columbia University, who is working on the study.

The culprit, he added, is black carbon.

Part of the problem, added experts, is that many bike lanes in cities are close to trucks that spew more carbon than cars.

So, the purpose of the study was to create a pollution map of New York and an app so cyclists can choose less polluted routes.

Bicyclists are getting more than half of their daily air pollution dose in only 6 to 8 percent of their daily commute, Chillrud told Healthline.

More particulates in lungs can have damaging effects — both short-term and long-term.

“Breathing heavy concentrations of pollutants hour after hour and day and after day has a tipping point,” said Dr. David Systrom, an assistant professor of medicine at Harvard Medical School, and director of the advanced cardiopulmonary exercise testing program at Brigham and Women’s Hospital in Boston. “Yet there are no studies following subjects over time.”

Some bicyclists are already concerned about the toxic air they may be breathing.

After he started getting sick more often, bicycle commuter Bernard Housen, a geology professor at Western Washington University, started worrying about the small particulates he felt he was breathing in.

Though the air quality in his home town of Bellingham is quite good, he was riding next to diesel buses spewing air pollutants.

So Housen began studying the pollution along his biking route. His findings were startling.

There was five to six times more particulate matter along the bus routes than on less busy streets.

So Housen changed his bike route by just a block, riding through a leafy residential neighborhood where particulate levels drop off dramatically.

“If you’re commuting by bike for years, that’s a long-term dose,” Housen told Healthline. “That’s cause for concern.”

Still, Housen and several other experts also acknowledge that there are health benefits from commuting by bike.

For Kyle Hatch, a project coordinator at the New York Bicycling Coalition, who lives in upstate New York, the benefits of commuting to work far outweigh the risks.

For one thing, commuting by bike is more relaxing, he said, than exercising for fitness.

And cycling has its health benefits too, he added, including feeling more motivated. Still, he does alter his biking route to avoid heavy traffic and congestion.

Studies done by the American Lung Association also show that air pollution in cities has improved considerably during the past few decades.

Much of the result, say experts like Chillrud, are due to the Clean Air Act, which was passed in 1970 and strengthened in 1990.

According to experts, the key to healthy cycling is doing it smartly.

Systrom advocates using alternative routes as much as possible.

“Choose a commuter route that minimizes pollution exposure,” he told Healthline.

Avoid tunnel entrances and use protected bicycle routes, too. Systrom also suggested riding during times of day when there’s less pollution.

“Afternoons are better,” he said, “and earlier before rush hour commutes.”

To lessen breathing in bad air, Chillrud advises cyclers to note pollution alerts or breathe through their noses. Anti-pollution masks, he noted, aren’t as practical since they usually don’t fit very well.

“But don’t stop exercising,” he added, “just pick your time of day to reduce exposure.”