It turns out exercising with a mask on isn’t the worst thing you can do. But a Mashable article points out that a recent study shows that you should always aim to breathe slowly through your nose, even when working out, and that a little bit of extra carbon dioxide actually helps your body absorb more oxygen. This is a well-established rule known as the Bohr effect.
So, wearing a mask might help remind you to breathe through your nose as opposed to your mouth, and provide that tiny bit of carbon dioxide while working out. Another study in November found that there was “no current evidence to support sex-based or age-based differences in the physiological responses to exercise while wearing a facemask.” Still, you should check with your doctor before doing any exercise. (Mashable)