How harmful is it to exercise outside on a polluted day?
There's no doubt that the air pollution in cities is bad for us. And exercise makes it worse, since we breathe in a greater volume of air and bypass the natural filtering of the nasal passages by inhaling through the mouth.
Exercising indoors, where the air tends be better during smoggy periods, is much healthier than slacking off for the summer.
But if you have to head outside anyway - to get to work, for example - the choice is trickier. Depending on when you go and what route you take, you may be better off running or biking to the office than sitting in rush-hour traffic.
The basic problem is that sucking in a mix of gases and particles irritates our airways, which can result in coughing fits and difficulty breathing. While these are the most common and easily diagnosed symptoms, doctors now recognize that they represent just one part of the problem.
"All the blood in the body courses through the lungs to pick up oxygen," says Ken Chapman, director of the Asthma and Airway Centre at Toronto's University Health Network. "So if the lungs get inflamed, those inflammatory signals get carried throughout the body."
As a result, Dr. Chapman says, emergency room doctors deal with higher numbers of serious problems such as strokes and heart attacks on high-pollution days.
The problem is most severe in high-traffic areas. Australian researchers recently asked test subjects to jog back and forth alongside a four-lane highway, and published the results earlier this year in the British Journal of Sports Medicine. Not surprisingly, blood levels of volatile organic compounds, which are commonly found in gasoline, were elevated after just 20 minutes.
But the results may have been different had the subjects been jogging along a riverside bike trail. A 2006 study in the journal Inhalation Toxicology found that combustion-related particulate levels drop exponentially as you move away from a roadway. Even just 200 metres away, levels are four times lower than next to the road. Trees have a further protective effect.
Whether you're better off inside or outside a vehicle seems to depend on the vehicle and location. A Danish study in 2001 measured pollution exposure while driving or biking along identical routes in Copenhagen. The air inside the cars was bad enough that, even taking into account that cyclists were taking longer and breathing more deeply, the drivers were worse off.
On the other hand, an Irish study in 2007 found that the air on buses was worse than the air breathed by cyclists, but that the higher breathing rates led to greater total exposure for cyclists.
Individual differences also matter, Dr. Chapman says. A healthy 20-year-old may be able to tolerate some airway irritation, while a similar level of inflammation could spell trouble for a 55-year-old, sedentary ex-smoker.
Taking these various factors into account, you may find that tweaking when, where and how hard you go will allow you to get outside even when the pollution count is elevated.
"Maybe the balance is that you still bike to work," Dr. Chapman says, "but you lobby for measures that will reduce pollution to make that ride safer."
Alex Hutchinson blogs about research on exercise and athletic performance at SweatScience.com.