and why specifically a paper bag?

It doesn't help your breathing...it helps to restore the normal pH in your blood.  When a person hyperventilates, excessive carbon dioxide is breathed off.  Carbon dioxide forms carbonic acid in blood.  The body normally has an acid to alkaline (or base) ratio that must be maintained (I believe it is a 20/1 ratio) in order for cells to function properly.  The excess carbon dioxide breathed off leaves the body in a state of respiratory alkalosis (too alkaline).  By breathing into a bag, you breathe back in the carbon dioxide you just breathed out, which enters the blood and restores the  carbonic acid level to rebalance the normal acid/base ratio. It doesn't have to be a paper bag...any bag will do.

This is used if you are hyperventilating and removing too much carbon dioxide from your lungs.  You will inhale the carbon dioxide you were exhaling.

It gives you less air to breathe and more carbon dioxide for your blood or something like that.

HELLO

   IF YOUR CAR SICK

OR  HIC CUPS.

OTHER WISE NONE OTHER

BEST