With cooler temps and shortened days upon us, many exercisers are heading indoors for their daily exercise fix. Gyms offer respite from the cold and dark with treadmills, rowing machines, weights, and exercise classes that can help you work up a sweat as well as any outdoor pursuit. But is the air at your gym as healthy as the exercises you're performing?

A new study published this month in the journal Building and the Environment took a look at this very question. For the study, researchers at the University of Lisbon in Portugal and the Technical University of Delft in Holland decided to test the indoor air quality of various gyms in Lisbon. These facilities are similar to those found in the U.S., with a main room for weights or machines as well as smaller studio rooms for aerobics, yoga or other classes.

Researchers got permission from 11 fitness centers to position air-quality monitors in each site’s weight room and within the smaller exercise rooms. The machines were set to measure pollutants for about two hours at the times when the gyms were most crowded — in the late afternoon and evening hours. The monitors were looking for indoor air pollutants such as carbon monoxide, carbon dioxide, ozone, airborne particulates such as dust, and formaldehyde or other chemicals that off-gas from carpeting, cleaning products, furniture or paint. 

The results were unnerving. The monitors showed high levels of airborne dust, formaldehyde and carbon dioxide in concentrations that exceeded most accepted standards for indoor air quality. The levels were especially high during evening exercise classes, when many people were packed into small rooms, stirring up dust and fumes and puffing out large doses of carbon dioxide with every breath. The high concentrations of dust and formaldehyde were the biggest concern, because consistent exposure to these chemicals has been linked to asthma and other respiratory issues.

Poor indoor air quality is a big concern for any building that you frequent regularly. But even more so for a fitness center where you tend to breathe heavily. "When we exercise, we take in more air with each breath and most of that air goes through the mouth, bypassing the natural filtration system” in the nostrils, said Carla Ramos, a graduate student at the University of Lisbon and the lead researcher for the study. “The pollutants go deeper into the lungs compared to resting situations.”

Does this mean you should skip the gym and head back out into the cold — or worse yet, park it on the couch? Not at all. But it might be a good idea to avoid the busiest times at the gym. And if the air at your gym smells stale or chemical-laden, talk to the owner about ways to improve indoor air quality.

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High levels of carbon dioxide, formaldehyde and dust were common in the fitness centers tested for the study.