The Dirty Secret of Outdoor Exercise
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The Dirty Secret of Outdoor Exercise

Jogging behind a fume-spewing bus is the least of your worries. Invisible pollutants hang in the air whether your routine takes you through city streets or far from them, causing you to run slower, breathe harder, and get a second-rate workout. What you should know to stay safe.

Dangers of Exercising Outside

I love to be outside. So it makes sense to exercise outdoors -- and I do, several times a week, despite living in the gritty heart of Baltimore, which has the dubious honor of being part of the 14th most polluted metropolitan area in the country. I try my best to avoid the often hot, humid metropolis, crammed with bumper-to-bumper traffic. Instead I go to a nearby park and power walk my dog on wooded paths or jog around the reservoir. There I can pretend that I'm circling a lagoon on a Maine island, that the sound of the adjacent highway is really the rush of a mountain stream complete with a flock of honking Canada geese, and that the whoosh of passing cars is a gentle ocean breeze. I exercise my imagination and my body at the same time -- a holistic fitness routine that works.

Sort of. Sometimes.

It works until I am choked out of my fantasy by a layer of hazy gray smog stretching to the horizon before me and I find myself hunched over, gasping for one good deep breath of clean, cool air. That's when I have to admit: This pollution is killing my workout.

You don't have to be sweating in the heart of a big city to feel the effects of pollution. A host of research shows that often when you exercise outdoors, your lungs are drawing in gulps of car exhaust, soot, construction dust, diesel, factory fumes, and possibly microscopic bits of ozone and mercury, all of which are slowing you down and harming your health. What's more, college athletes who biked in a polluted environment (the lab-created equivalent of a busy highway) continued to feel the ramifications when riding again three days later, according to a 2008 study published in the Journal of Strength and Conditioning Research.

The distance they could pedal decreased by 5 percent, perhaps because inflammation in their lungs made it harder to breathe for days afterward, explains study author Kenneth Rundell, PhD, director of the Human Physiology Laboratory at Marywood University in Scranton, Pennsylvania.

Beyond the daily irritation, long-term contact with high levels of pollution can increase your chances of incurring heart disease and heart attack, high blood pressure, and asthma. It can even raise your risk for lung cancer as much as regular exposure to secondhand smoke, a study in the Journal of the American Medical Association found. And one study of runners showed that pollution can actually negate the positive effect exercise has on lungs. "Usually when you exercise, your lung function improves," says researcher George Thurston, ScD, a professor of environmental health science at NYU School of Medicine in New York City. "But as ozone levels rose, we saw that benefit shrink to the point where there was no improvement in lung capacity. In short, the benefits of exercising are absolutely diminished by ozone on high-pollution days."

Pollution Everywhere

If you're a swimmer, you're not off the hook: Oceans, lakes, and rivers can contain overflows of untreated sewage. Play golf? Look out for fertilizers and pesticides on the course. But no matter what your sport, the very air you breathe is cause for concern. Ozone pollution (basically smog), particulate matter (microscopic particles from factories and construction sites), sulfur dioxide (a by-product of industrial facilities and some power plants), and carbon monoxide and nitrogen dioxide (gases from vehicle emissions, generators, lawn mowers, and so on) can cause respiratory difficulties. "Each one affects the body differently, but they all can hinder your workout," says Dan Greenbaum, president of the Health Effects Institute, a nonprofit research institute in Boston that examines how pollution affects health. Ozone, for example, can irritate the throat and respiratory tract and inflame the lining of the lungs. Particulate matter and carbon monoxide have been associated with hardening of the arteries.

Unlike golf courses or polluted lakes, air is impossible to avoid. The most recent data show that six in 10 Americans -- 186 million people -- live in places where the air poses a health threat, according to the American Lung Association (ALA). Despite a burgeoning green movement and progress in cutting pollutants, nearly every major metropolitan area is burdened with significant air pollution: Of the 25 cities with the worst ozone pollution, 16 recorded higher ozone levels in the ALA's 2009 report compared with the year before. This plus the steady decrease in gym membership (down approximately 21 percent in 2009, according to the American Council on Exercise) and an increase in people turning to outdoor activities (Road Runners Club of America saw 10 to 30 percent growth in race participation from 2008 to 2009, for instance) has some experts worried about the well-being of outdoor exercisers.

Why Exercisers Are More Vulnerable

Ironically, on high-pollution days it is exercisers who are especially vulnerable, Thurston says. "When we work out, we breathe more air in and draw deeper breaths into the lungs," he explains. Because of this, more pollutants enter the body. Additionally, when we work out, particularly at a high intensity, we tend to breathe through the mouth. "This means you lose your body's first filters, like nasal hairs and the upper respiratory tract, which keep the larger particles from entering the body," says C. Arden Pope III, PhD, an air pollution epidemiologist at Brigham Young University in Provo, Utah. "The combination of breathing more and having the particles penetrate deeper into the lungs can irritate and inflame them." Even a relatively small amount of pollutants inhaled over extended periods during exercise may cause lung damage. Joggers who ran for 30 minutes alongside a busy New York City road had blood toxin levels similar to those found in regular smokers, a study published in The Physician and Sportsmedicine found.

"For the healthy jogger or biker, unless the pollution level is really high, she probably won't even notice it," Pope says. "But prolonged exposure, day after day, year after year, will ultimately take its toll." These days most people feel the consequences when pollution climbs to 151 or more on the Air Quality Index, a scale devised by the Environmental Protection Agency to gauge air quality on any given day. "At that level, pollutants begin to irritate your throat, get into your lungs, and cause inflammation and shortness of breath," Greenbaum says.

Even the most experienced exerciser struggles on those days. Brenda St. Hilaire, 41, a Rhode Island nutritionist and lifelong exerciser, used to spend her lunch hour jogging through Providence. She recently changed to power walking because the filth coughed up by buses and the thousands of diesel trucks that cut through the city on nearby I-95 was simply too much for her lungs to handle. "On ozone-alert days, I almost felt asthmatic when I ran or biked," Brenda says. "I had a hard time with deep breaths and could feel it coming on later in the evening, too. Even when I'm walking, I still feel asthmatic because I'm breathing the poor air, but at least I'm not placing the demands of running on my body."

Instead of altering their outdoor routines, others, like 24-year-old Ashley Thomure, respond to truck fumes by holding their breath. "I know I'm supposed to inhale deeply when I run, but the exhaust from vehicles is unbearable," the nursing student in Saint Charles, Missouri, says. Unfortunately, her breath-holding habit is doing more damage than good: Muscles need that oxygen to function.

The Asthma Connection

Adding to the problem, athletes may be more prone to asthma than others. For example, 12 percent of high school athletes in an Annals of Allergies, Asthma & Immunology study had asthma, and one in six U.S. Olympic athletes at the 1998 Winter Games suffered. By comparison, asthma affects just one in 15 in the general population. One possible reason: Asthma can be induced by strenuous exercise.

"Asthma is an inflammatory disease, so the airways are chronically irritated," says Norman Edelman, MD, chief medical officer for the American Lung Association. "Even when asthma is under control, ozone and particle pollution can exacerbate inflammation and bring on an attack." Worse, pollutants like ozone narrow airways, magnifying the effects of any other allergens, and an asthmatic's pollution threshold is lower than that of the average adult.

Hidden Dangers

More threats await on the emerald grass of the golf course (pesticides and fertilizers) and the bucolic woodland trails you bike and hike (insecticides). Lakes, rivers, streams, and bays can be spiked with agricultural runoff from fertilizer and pesticide residues or industrial waste and sewage overflows. "Swimming in these conditions can cause gastrointestinal problems, diarrhea, skin rashes, ear and eye infections, even hepatitis," says Michelle Mehta, a representative for the Natural Resources Defense Council, a New York City-based nonprofit environmental action group.

In the United States the number of beach advisories and closures issued in 2008 because of pollution topped 20,000 for the fourth consecutive year. Even people who live near some of the nation's most pristine waterways are worried. "I'm concerned about potential sewage overflow after a rainstorm," says Kristan Drzewiecki, 39, a management consultant who regularly swims in Casco Bay, just offshore from her home on Peaks Island, Maine. Two summers ago the beach just across the bay from where she swims was closed for nine days as a result of high contamination levels. It hasn't stopped Kristan from getting her swim workout, but it's changed her attitude from enthusiastic to anxious. "I try to keep my mouth closed when I swim now," she says, acknowledging that this alone is probably not enough.

Of course, "none of this means you should stop exercising outdoors," Dr. Edelman says. "You simply need to be aware of pollution and do all you can to avoid it -- and not contribute to it yourself." For example, having to change her midday run to a walk inspired Brenda St. Hilaire to volunteer with an environmental group. "I joined Clean Water Action, signing petitions and donating," she says, referring to the 1.2 million-member grassroots organization that attacks the basic causes of water and air pollution. She hopes to be able to run outside again someday. Until then, "at least I feel I'm doing something to help solve the pollution problem."

Protect Yourself

Follow these tips to limit your exposure to pollution, courtesy of the Environmental Protection Agency and others.

If golf is your game...

Call for the spraying schedule before you hit the links; avoid the course for 24 hours after a dusting of fertilizers or pesticides. During your game, wear gloves.

If swimming is your sport...

Google "EPA Beacon Beach Advisory" to find out the conditions at your favorite beach. Skip the swim if advisories are posted or if it has rained heavily within the past 24 hours: The likelihood of sewage overflow and storm runoff is higher. (For lakes and lagoons, keep away for up to 72 hours.) Always wear goggles and close your mouth while swimming; shower and rinse your suit immediately afterward.

If jogging, biking, or hiking is more your speed...

Check the Air Quality Index for your area ( or sign up for EPA e-mails at On days when air quality is poor, work out in the early morning or in the evening; ozone levels are generally at their lowest during those times. And when possible, try to run, walk, or bike upwind to minimize pollution exposure.

Originally published in FITNESS magazine, April 2010.