Endosmose is also a problem with He also, where air contamination is only parasitic of lift. The gas gets replaced in bulk, sooner if lift-force operating margin is accordingly smaller.
A new balloon is effectively gas-tight but delicate, and soon develops tiny leaks over time that relentlessly dilute the lifting gas, with no easy solution. Thermal cycles and general envelope motions help push lifting gas out and air in.
I used to store micro-blimps in a dark quiet stable temperature room, nose-up, and only replace the bottom gas, with its higher air content, from the tail-cap.