We all know that exercise is healthy and desirable, and in the case of outdoor activities, a great way to enjoy fresh air.  However, what if the air where we live is not very fresh?  This not only applies to people who inhabit large cities, but also to areas where traffic is lighter but industrial pollution is rampant.  Certainly, living in downtown Toronto has led me to ask myself this very question.  Like many health-related issues, the answer is not as it may seem. When it comes to balancing the benefit of exercise with the risk of breathing polluted air, it appears that you may actually be better off when pushing yourself harder and breathing more deeply.  Of course, there is no doubt that clean air is better than dirty air, and that breathing particulate-laced air triggers a cascade of inflammation and oxidative damage that spreads from the lungs throughout the body.  With this in mind, one would come to the conclusion that exercise takes a bad situation and makes it worse, on account of sucking in more bad air.  Questions that arise in the real world, however, are 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-congested roads. One study looked at the effects of prior exposure to polluted air before exercising in clean air – for example, responding to an air-quality alert by driving to a gym instead of exercising outdoors – and found that pre-workout exposure to polluted air raised heart rates during the workout by six or seven beats.  This demonstrated that the body was still struggling with the after-effects of exposure.

Another important consideration is the fact the 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.  A study published in the journal Medicine & Science in Sports & Exercise last year exposed mice to diesel exhaust particles for five weeks.  Those that did not exercise showed high levels of lung inflammation and oxidative stress.  However, those that exercised five times per week in the diesel fumes were almost completely protected from the negative effects - regular exercise appeared to offset exposure to dirty air. Luisa Giles, an exercise physiology researcher at University of British Columbia, 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 one might experience when cycling along a busy road.  At the lower intensity, diesel fumes increased the amount of energy needed to maintain pace and forced the subjects to breath more heavily.  However, 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 and dirty air.  Giles was not able to answer this perplexing question in her study, but she suggested 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.  This has implications for people with health problems who are unable to engage in higher intensity exercise, suggesting that these individuals should take extra care not to exercise in dirty air. However, for those who are healthy, fit, and accustomed to intense exercise, outdoor exercise in the hazy days of summer – preferably not along busy roads, though -  may not be as dangerous as once believed. Reference: Hutchinson, Alex. Dirty air keeping you back?  Push forward.  The Globe and Mail, November 4 2013.