All I know is whenever I go anywhere (hardly at all), if I need to wear a mask for more than 15 minutes I need to chew some gum. I brush my teeth beforehand, but after a certain amount of time it starts to smell like hot breath inside the mask and I find that gross. I'm not sure if it's the recycling of used air, not enough fresh air movement, or what it is, but wearing a mask effects the moisture content of my mouth. Maybe because it's harder to breathe through my nose with it on. I really have no idea, but gum is a necessity right now.