they also cause CO2 poisoning because when you breathe out most of the CO2 stays trapped in the space between your mouth/nose and the mask. it takes a while but it makes your heart have to work harder and lungs to extract enough O2, if the mask were efficient enough, the mesh would allow virus particles right in. unless you have a mask like athletes use when training , hooked up to plenty of oxygen, if you do much physical activity for long enough you'll pass out if youndont take the mask off. for very short time periods masks may be nothing more than a security blanket