I think the time after which brain damage begins is 4mins. That is when the
brain has been deprived of fresh oxygen for more than 4 mins. This will
occur if inhaling a gas w/o O2 or holding one's breath over 4mins (very
rare). Although, at this stage, the damage is still reversible. It's
10mins before irreversible damage begins to occur.
maneesh.shanbhag at sympatico.ca