A rebreather mask is typically used by scuba divers to recycle oxygen.Rebreathers are more expensive than traditional diving masks, but offer a number of advantages.The most common type of rebreather is called the closed-circuit system, which recycles exhaled gas back into the breathing loop.This means that you can stay underwater for longer periods without having to worry about running out of air.Another advantage of these types of systems is that they produce less carbon dioxide so you don't have to worry about getting too much CO2 in your lungs and experiencing hypercapnia (a condition where there's too much dissolved CO2 in the blood).When using this type of system, it's important not to forget to change your mouthpiece regularly because otherwise you'll be consuming bacteria from other people's mouths with each breath.