From wikipedia:
Typically masks are worn by people who are very ill (e.g. patients in the midst of cancer treatments) or medical professionals on duty. Neither of these are doing anything more strenuous than standing or walking short distances while wearing the mask. The orders to wear a mask in public amount to an involuntary and unethical experiment, and I would not be surprised if some susceptible people suffer real injury (e.g. heart attacks) as a result.Inhaled air is by volume 78.08% nitrogen, 20.95% oxygen and small amounts of other gasses including argon, carbon dioxide, neon, helium, and hydrogen.[15]
The gas exhaled is 4% to 5% by volume of carbon dioxide, about a 100 fold increase over the inhaled amount. The volume of oxygen is reduced by a small amount, 4% to 5%, compared to the oxygen inhaled.