Working out is good for you, right? Not if the air in your gym is polluted. As it gets chilly outside, it's all the more tempting to snuggle up on your sofa and save any thoughts of exercise for the good intentions of the New Year. But as many of us don't want to sacrifice our fitness (and good on you if this applies to you), working out at the gym, in the warm, seems like the best option. However, a new study claims this might not be as healthy as you think - because you could be breathing in toxins every time you hit the treadmill.

Researchers from Portugal and Holland took air quality monitors and placed them in gyms all over Lisbon to check out the environment people were exercising in. Levels of gases including carbon monoxide, carbon dioxide and ozone were monitored. In addition to this, airborne dust particles were measured, as well as chemicals from carpeting and cleaning products and paint, including formaldehyde.

Readings were taken over the course of a month, with particular focus on times when the gym was really busy - such as the after-work rush. The result: levels of pollutants deemed much too high for safety standards relating to indoor air quality. The biggest problem was airborne dust and dirt, stirred up by people working out vigorously. Formaldehyde was also found in large quantities.

Do you often work out in smaller aerobics studios with a whole class of keen gym bunnies? Because this is where high levels of CO2 were found. With so many people breathing heavily as they get sweaty, the gas is released in large quantities. The effects: light-headedness, fatigue and clouded thinking.

"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," Carla Ramos, the lead author of the study, explained.

Of course this study isn't saying the gym is bad for you and should be avoided at all costs (sorry!) but more that air quality in them needs drastic improvement. If you're worried, speak to the manager at your fitness centre about changes that could be made. Or, if you want to exercise with peace of mind and save some cash, try out fitness DVDs, which you can use in the comfort of your own home. Then, come summer, embrace the fresh air and get active outside!