Wrong my friend. NORML and MAPS did a study comparing non-filtered smoking devices (joints), water bongs, and vaporizers. The bongs performed the worst on filtering tar per amount of THC. Unfiltered joints were even better. To quote the study exactly:

Disappointingly, waterpipes performed uniformly worse than the unfiltered joint. The least bad waterpipe, the bong, produced 30% more tar per cannabinoids than the unfiltered joint. Ironically, the pipe with the electric mixer scored by far the worst of any device. This suggests that water filtration is actually counterproductive, apparently because water tends to absorb THC more readily than noxious tars. Like the waterpipes, the cigarette filter also performed worse than the unfiltered joint, by about 30%. Researchers speculate this is because cannabinoids are exceptionally sticky and adhere to other solids. Hence, any filtration system that picks up particulates is likely also to screen out cannabinoids.


source: MAPS/CaNORML vaporizer and waterpipe studies

Quote Originally Posted by CoMfOrTaBlYnUmB420
bongs dont filter thc dude. thc isnt water soluble so only a very small amount of thc will actually be taken away. and its proven that bongs dont filter that much tar. although some, but not a whole lot. the main purpose of a bong is to use the water to cool the smoke making for a longer, larger hit