I was wondering this myself recently, as i'm usually too lazy to clean it thoroughly. I haven't noticed any ill effects, but just don't smoke solvent. Not only is it bad for you, but the whole bowl goes up in flames. You'll waste the spice, and might lose an eyebrow or two. I will say, the cleaner it is, the more pleasent it is to smoke though (both the smoothness, and the taste of the smoke).