The AEROCHAMBER PLUS FLOW-VU Chamber should be replaced every 12 months. However, you should replace it if any of the following occur (1) The valves are missing or damaged. When this happens, the chamber cannot work properly and must be replaced. (2) The backpiece becomes cracked or torn. This may affect the amount of drug that is breathed in. (3) The mask does not comfortably fit the face. It may be time to use another model with a larger mask or use the AEROCHAMBER PLUS FLOW-VU aVHC with mouthpiece.