I'd like to add that with my 15 years experience as a nurse, wearing a mask for any period of time decreases your O2 levels which is needed to send O2 throughout your body to reach your cells for proper cell function. When you are exhaling, it is CO2, the blockage of the mask will have you inhaling CO2 back into your lungs which results a lack of O2 and brings an increase of acidity causing headaches, fatigue and increases your risk of having optimal health. During sugery, we are limited in our movement as we are mostly standing. While walking or excessive mouvement demands increase O2.