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The CPAP machine produces a continuous pressure in the airway which keeps the airway open and is useful for COPD
patients and those who are overweight
and snorer. The pressure acts as strut to keep the passage wide open. when you come off the mask the airway suddenly gets a recoil and you feel the congestion, then your body takes over, the muscles tone up the extra saliva and mucus goes down and you feel less congested.
this is just a perception, as such the CPAP will not lead to congestion at all.
I hope the advise would be informative and useful for you.