http://www.canorml.org/healthfacts/smokestudy.html

here is a snippet:
"Contrary to popular impression, waterpipes don't necessarily protect smokers from harmful tars in marijuana smoke, according to a study sponsored by California NORML (National Organization for the Reform of Marijuana Laws) and MAPS (Multidisciplinary Association for Psychedelic Studies). The reason is that waterpipes filter out proportionately more psychoactive THC than they do other tars, thereby requiring users to smoke more to reach their desired high. "

"The study focused on two key components of the smoke: (1) total solid particulates, or tars, which are noxious waste byproducts of burning leaf like those from tobacco; and (2) cannabinoids, the chemicals distinctive to marijuana, including its major psychoactive ingredient, delta-9-tetrahydrocannabinol (THC), and its two commonest chemical relatives, cannabinol (CBN) and cannabidiol (CBD), which are only weakly psychoactive but may have medical benefits."

"Surprisingly, the unfiltered joint outperformed all devices except the vaporizers, with a ratio of about 1 part cannabinoids to 13 parts tar. This disturbingly poor ratio may be explained by the low potency of the NIDA-supplied marijuana used in the study, which was around 2.3%.

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 other, noxious tars."