never mind i found it....

http://www.maps.org/mmj/vaporizer.html

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.
note, this is from NORML, so it is very unlikely the information is wrong, since they are pro-marijuana afterall....