With contributions by MaddieFay. - Thanks!
If you deprive your brain and body of oxygen, the result is death. Depending on lung capacity and how oxygenated the blood is, there can be PERMANENT harmful effects such as brain damage with only 3 minutes. Those folks who hold their breath while they swim the pool- they worked up to that... A safer version of breath play is to have the sub hold their breath, and but be able to breathe if they need to.
A form of asphyxiation (smothering) featuring the use of a bag placed
over the smothered individual's mouth and nose, face, or, less
commonly, entire body. The most common type of bag used is a simple
plastic bag, though any other material which does not easily allow
oxygen to pass through can be used.
Breath play can KILL you. Never, ever play this way alone, or allow ANYONE to be deprived of oxygen for more than 90 seconds. PERIOD.