Evidently, cut flowers start making some bad bacterias after a few days in water. One tip is to remove leaves so that only stem enters the water. Then, iff you keep them only for a few days the bacteria levels won't be too high. Before I knew this I used to keep flowers for a long time - the water in the vase would get murky and then I'd pitch them. I guess we shouldn't have the murky water lingering in the house. Also on a seperate note, a few CFer's have noted being allergic to lillies.
11. I use furnace/air conditioner air filters with MERV rating 11+ (last 3 months - you can get them at Home Depot - the fiberglass one's don't stop anything except big particles).