Professional athletes often wear training face masks during workouts. Also known as altitude and elevation masks, these products are said to give your exercise regimen a boost.

If you’ve seen someone wearing a training mask, you probably wondered what it is — and why anyone would wear it. These masks, which cover your mouth and nose, cut down on the amount of oxygen you take in while exercising. Using the mask when training forces your heart and lungs to work harder. When you remove the mask for a race or game, your body uses oxygen more efficiently. Fans of altitude masks claim that using them can improve speed, endurance, and strength.

This concept is based on the fact that some athletes, especially distance runners, train at higher altitudes. Altitude training is believed to increase your body’s red blood cell count, allegedly giving you a temporary performance boost when you return to sea level.

The Training Mask also claim exercising with one of these special masks can benefit your body by helping you sleep, giving you more energy, and improving your metabolism. And at the very least, it gives you a show-stopping gym selfie opportunity.

At best, training masks might help you breathe more efficiently, but it’s still unclear if or how that might improve athletic performance. Keep in mind that wearing an altitude mask might cause you to hyperventilate or even faint, due to its restrictive breathing. People who have high blood pressure or other cardiovascular problems should avoid using training masks.