It is the media inside. Canister can be reused. There is information in the archives over at BimmerForum that details the procedure. Basically you need to get some activated charcoal granules (pet store - aquarium section) and use that. This is a very inexpensive way to do this, as the new canisters are a bit pricey. On the other hand, they usually last a very long time.
White 1993 850Ci - 251,187 miles
Black 1993 850Ci - 125,508 miles