Every time you gasp for air in a crowded gym, you’re polluting the place. Those deep sputtering breaths after a HIIT session spew carbon dioxide into the confined workout chamber. 
 
 Get enough breathless people doing burpees in the same room and you could create abnormal levels of air pollution in the gym, according to a new study out next month in Building and Environment. 
 
 Researchers from Portugal monitored the air quality in 11 Lisbon gyms and found heightened concentrations of carbon dioxide, carbon monoxide, formaldehyde, and airborne dust. 
 
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 Poor ventilation, small spaces, and breathless exercisers appear to be the culprits. 
 
 From the New York Times: 
 
 “Their findings were disquieting. In general, the gyms showed high levels of airborne dust, formaldehyde and carbon dioxide. The concentrations of these substances generally exceeded most accepted standards for indoor air quality. (No government agency in the United States formally monitors air quality in gyms.) The levels were especially high during evening aerobics classes, when many people were packed into small studios, stirring up dust and fumes and puffing heavily, producing carbon dioxide with every breath.” 
 
 There isn’t a whole lot the average gym-goer can do, as ventilation problems need to be solved by the gym. But you can look for clean, open facilities and go at less crowded times. Or, you know, you could just not breath while you work out. ||||| Next time your yoga teacher tells you to take a "deep, cleansing breath" you may want to rethink that—not the breathing part (please don't stop breathing!) but the cleansing part. Gym air is definitely not clean air, according to a new study. 
 
 Researchers from Portugal and Holland placed air quality monitors in gyms all over Lisbon and looked at the indoor environments. They measured levels of dangerous gases like carbon monoxide, carbon dioxide, and ozone; airborne particulates such as dust; and various chemicals released by carpeting, cleaning products, furniture or paint, including formaldehyde. They collected readings over the course of a month, focusing on the times of day when the gyms were particularly busy like the evening after-work rush. 
 
 What they found will make you want to hold your breath during your next weight set, no matter what your trainer says: high levels of pollutants that exceeded all the safety standards for indoor air quality. Airborne dust was the biggest culprit as researchers said people stirred up dirt and debris as they moved vigorously. Formaldehyde (yes, the same formaldehyde preservative that was in the jar with that pig you dissected in high school and still have nightmares about) was also found in very high amounts. (And your gym isn't the only dirty place you hang out. Check out these 5 Places Mold Grows in Your Home.) 
 
 However, the biggest surprise were the high levels of carbon dioxide (CO2) which were so high precisely because of all that healthy exercising going on. The effect is compounded by the large number of people breathing in confined spaces like aerobics studios. CO2 on its own can cause fatigue, light-headedness, and foggy thinking but working out magnifies the detrimental effects, says Carla Ramos, the lead author of the study. “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. The pollutants go deeper into the lungs compared to resting situations.” 
 
 But no one is telling you to stop exercising and with cold weather here, working out indoors is the only option for many people. Rather, Ramos says this should be a wake-up call for the fitness industry to make sure gyms are properly ventilated and cleaned so that clients have a safe space that will help them achieve their healthy goals. If you're worried about your gym, have a chat with the manager about ways to improve air circulation and cleaning.
– You might want to start your next workout at the gym with a sniff test. A new study suggests that gyms have high levels of air pollutants such as formaldehyde, airborne dust, and carbon dioxide, reports the New York Times. In fact, levels for those three substances generally exceeded accepted standards for air quality. The samples were taken at gyms in Portugal, but the gyms were similar to those commonly found in the US. Researchers found particularly high levels of carbon dioxide during aerobics classes, thanks to all that huffing and puffing. But the need for better ventilation seemed clear in all areas, especially considering the physiology involved with a workout. "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" of the nostrils, says the lead researcher. "The pollutants go deeper into the lungs compared to resting situations." So what to do? The researchers stress that people shouldn't stop going to gyms, notes a blog post at Shape. But they might want to sniff their gym's air, and if it has a stale or chemical odor, ask the manager about ventilation or the most recent air-quality test. Beyond that, exercisers could make a point to go at less-crowded times, suggests Men's Fitness. (Exercise might be bad for your teeth, too.)