I'm sure a few people remember Shai Hulud's post.
Quote Originally Posted by Shai Hulud
There are a lot of misconceptions in the stoner world and most smokers in fact don't know shit except the rumors they hear from their stoner friends who also don't know shit. One of these rumors is that using bongs is healthier because it filters out the tars. It does filter out some tars, but it filters out MORE THC, meaning you have to smoke more to get the same amount of high, which translates to higher tar intake than even an unfiltered joint.

The California NORML/MAPS Smoking Device Study

Here's a quote:
The least bad waterpipe, the bong, produced 30% more tar per cannabinoids than the unfiltered joint....This suggests that water filtration is actually counterproductive, apparently because water tends to absorb THC more readily than other, noxious tars.

Bong water that has been unchanged for weeks will have A LOT of THC.