The rise of pollution worldwide, combined with the increase in the popularity of running, has raised the question of whether there comes a point at which exercising outdoors is no longer healthful. Last month, the Beijing Marathon took place despite hazardous pollution levels, causing some runners to drop out and others to question whether it was worth it.

A literature review published in the November issue of Sports Medicine provides partial answers to that question. It suggests that exercising in polluted air may undo the positive cognitive effects of exercise, while also making it clear that there may still be other benefits to exercising in polluted air.

It is well established that regular exercise can improve brain health, and there is growing research suggesting that the brain-deprived neurotropic factor (BDNF), a neurotropin, plays a key role in the process. There is also increasing research that exposure to air pollution can have adverse effects on the brain as well as the lungs.

The research team reviewed literature published from November 2009 to November 2013, including three of their own studies, seeking to answer whether these two things cancel each other out.

The intake of air pollution increases considerably when we exercise, because we take deeper and more frequent breaths. The research team cited a study indicating that an athlete running at 70 percent of VO2 max (roughly equivalent to easy running pace) for about three hours inhales the same volume of air as a sedentary person would over the course of two days.

The research team conducted two studies on human subjects. In the first, they had subjects perform two identical cycling tests, but one was in a lab with particles removed from the air. The other was performed along a busy road with moderate pollution. Inside the lab, participants’ BDNF levels rose while performing the test. Along the busy road, they did not.

In a second study, subjects undertook 12-week training programs, with some of them located in a rural environment, and others in an urban environment. The participants in the rural location performed better on a test of executive function (which includes working memory and problem-solving ability) at the end of the training program, while the participants in the urban location did not.

While the findings are troubling, researchers are not saying exercise in an urban setting is not worth it.

“The evidence suggests that regular exercise in highly polluted air might not result in the same neurological benefits that are observed in non-polluted air," they wrote. "However, to the authors’ knowledge there is not enough evidence to suggest that regular exercise in highly polluted air causes more damage to the brain due to air pollution exposure than benefits of physical activity.”

They suggest that people find a “green environment” for their daily exercise, and avoid close proximity to traffic and polluted urban environments. They also suggest avoiding exercising during rush hour, and point out that pollution tends not to be as bad in rainy and windy conditions.

The negative effects of exposure to pollution are well documented, and according to a recent Running Times article, athletes who train in cities have been found to have higher levels of lead in their blood. Exposure to particulate matter can lead to respiratory problems ranging from changes in lung function to premature mortality.

In answering the question of whether exercising in pollution is worth doing, there are many factors to consider, and the answers may be different for different people.

“It should also be emphasized that all possible health effects for the different organ systems and populations should be taken into consideration before any accurate general recommendation can be given concerning exercising in an environment with air pollution,” wrote the researchers.

“The risks for harmful effects on the brain, for example, might be different from those on the cardiovascular system. In addition, the balance between the risks and benefits may also be different in diverse populations, e.g. healthy populations versus populations susceptible to certain diseases.”

Related:

Vigorous Exercise Less Affected by Air Pollution

Running Protects Your Lungs From Pollution

Polluted Runs

Air Pollution Major Cause of Cancer, WHO Says

Pollution Cited as Threat to Chinese Running Boom

Despite Smog, Chinese Flock to Road Races

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