I already saw how people were
breathing back in the bags to recycle what came out of their lungs...
Some guests tended to breath slightly out just a very little bit in the bag before drawing deep, too.
It's even in the manual. Don't share bags and use a new one after 2 weeks at least. (I shared with wifey because I fuck her anyway and I used the same bag for 4 weeks at least, so I wasn't sticking to the manual, neither. But group sex is on your own risk. Needle sharing too). But I might just flip.
Edit: think of contaminated material, too. (For those who aren't medical). I buyed contaminated material in Venlo from a coffee shop around 2006. Several times. The volcano blows all the stuff like mold or fillers in the bags (glass dust, sands, pk13/14, plumbum and I don't know what cruel people put on their weeds in illegal, sometimes even legal states)
But if you are a single medical user, why not keep the underwear, ahm, bags, for more than a few months.