I would suspect that most of these fume events are coming from compromised seals and/or cracked bleed valves. Substances are getting vaporized into the cabin air supply that should not be. Are they really checking all these seals and valves? Can a filter be installed somewhere downstream of all potential contamination sources? I suppose the ultimate solution is to design all aircraft environmental systems like that of the B-787 from now on. Here is just one example of an airline trying to deal with the problem (article from a few years ago): Lufthansa Working To Reduce A380 Cabin Fume Events | AWIN content from Aviation Week