Breathing air form you BC..hmmmmm.If ther is one area of dive gear that gets neglected the most, I would say its the inside of the BC...ESPECIALLY if its a rental unit. I had this picture painted for me. A BC gets used in all sorts of diving conditions. depending on your skill level and dive profile you might be constantly adjusting your BC. As your doing this your letting in water ladden with all sorts of bacteria and stuff in it. its likely that the BC will be used in more than one site, so there are different types of bacteria being introduced all the time. sediment water, stagnant water. fresh water mixed with salt water. all sorts of stuff. then you do a rinse of your gear and put it away. if its a rental, it might get a splash before its hung up to dry. then it sits there. the water getting stagnant inside the BC. the bacteria multiplying. the stuff is growing. then you take your next dive, have an OOA emergency and breath it all in. ick. not sure if this medically backed up, our instructors told us about it and warned us not to do it. it would suck to survive an OOA emergency only to die from an unknown lung disease.