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A cyclist pedals along in traffic on Danforth Ave. near Woodbine in Toronto on Oct. 21, 2013. A study presented at the annual conference of the Canadian Society for Exercise Physiology in Toronto in October suggested an increase of exercising benefits when pushing harder and breathing polluted air more deeply.Peter Power/The Globe and Mail

When University of British Columbia exercise physiology researcher Luisa Giles noticed some tightness and wheeziness in her chest a few years ago, she quickly zeroed in on a possible culprit.

"I'm an avid cycle commuter and really, really embrace physical activity and its importance," she says. But she couldn't help wondering whether her daily rides, an hour each way along Vancouver's busy streets, were doing more harm than good. So she decided to look into it.

The results of her study, presented at the annual conference of the Canadian Society for Exercise Physiology in Toronto last month, offer a surprising twist in an ongoing debate. When it comes to mixing the benefits of exercise with the health risks associated with breathing polluted air, you may actually be better off when you're pushing harder and breathing more deeply.

No one doubts that clean air is better than dirty air. Breathing particulate-laden air triggers a cascade of inflammation and oxidative damage that spreads from the lungs throughout the body.

You can track the daily rise and fall of air quality readings by looking at the number of people admitted to hospital for respiratory problems and other serious issues such as stroke and heart attack.

On the surface, exercise takes that bad situation and makes it worse, because you'll be sucking in more bad air. But trade-offs in the real world are often less clear: exercising in dirty air versus not exercising at all; or commuting by bike versus sitting on a bus travelling along the same traffic-clogged roads.

One question that Giles and her adviser, Dr. Michael Koehle, at UBC's Environmental Physiology Lab tackled was the effect of prior exposure to polluted air before exercising in clean air – a situation that might occur if you respond to an air-quality alert by driving or busing to a gym instead of exercising outdoors.

They found that pre-workout exposure to polluted air raised heart rates during the workout by six or seven beats, showing that the body was still struggling with after-effects of the pollution.

Another important consideration is the fact that exercise itself has powerful anti-inflammatory and antioxidant properties that accumulate over time – something that short-term experiments miss if they simply evaluate the effects of a single bout of exercise in polluted air.

For example, in a study published last year in Medicine & Science in Sports & Exercise, Brazilian researchers exposed mice to diesel exhaust particles for five weeks. Those that didn't exercise showed high levels of lung inflammation and oxidative stress, as expected. But those that exercised five times a week in the diesel fumes were almost completely protected from the negative effects.

Of course, not all exercise has the same effects. In her latest study, Giles had 18 healthy volunteers cycle in an environmental chamber for 30 minutes at a time, at either low or high intensity, while breathing either clean air or air containing levels of diesel exhaust you might experience when cycling along a busy road.

At the lower exercise intensity, diesel fumes increased the amount of energy needed to maintain pace and forced the subjects to breathe more heavily. The total volume of air inhaled per minute increased from 39.9 litres in clean air to 44.5 litres with diesel added. But at the higher exercise intensity, which corresponded to a moderate but sustainable effort, there were no differences in respiratory or metabolic response between the clean air and the dirty air.

Why would pollution cause problems during easy but not hard exercise? The study isn't able to answer this question, but Giles suggests that different patterns of airflow within the lungs may play a role. Heavier breathing may speed the diesel particulates past irritant receptors in the central airways without triggering them.

The findings may be particularly important for people with heart and lung conditions that prevent them from exercising at higher intensities, Giles notes. People with breathing problems also tend to be the most sensitive to pollution, and should consider minimizing outdoor exercise when air conditions are bad.

For healthy people, Giles's research adds to an increasingly complex picture that doesn't lend itself to absolute statements about when you should or shouldn't exercise in polluted air.

The best approach, she emphasizes, is to minimize your exposure to pollution during workouts whenever possible. For example, the air tends to be cleaner early in the morning than later in the day, and on parallel side streets even just a block away from major arteries.

But if you find yourself biking down a busy street and wondering whether you should stop, don't panic. The best option of all, it turns out, may be to pedal even harder.

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