I found this at wikipedia.
http://en.wikipedia.org/wiki/Vaporizer#Factors_affecting_vaporizer_output
I am not going to say that Wikipedia is the ultimate authority -and this passage is not cited - but it makes sense to me:
"Most vaporizers deliver dry, warm vapor that can irritate the throat and upper airway tree. By running the vapor through water and/or past ice to moisture condition and cool prior to delivery irritation of the throat and upper airway can be reduced or eliminated. Raising the temperature so pyrolysis products like nitrosamine are present, is known to increase risk for cancer in exposed oral and airways epithelium, among other health effects of tobacco. A properly used vaporizer should never allow this to happen; however, many smokers making the transition to vaporization will make this mistake. Smoking means pyrolysis, so tar and noxious gas from cannabis combustion are absent in properly temperature constrained vapor. In addition, THC is a powerful expectorant, so the cilia, cough and other healthy airways defense mechanisms are activated, cleaning out any inhaled material. Vaporizers or vaporization "tool" based systems that feature rapid extraction and delivery combined with water and/or ice cooling and conditioning of the vapor deliver the best of all: clean, concentrated, cool, and moisturized vapor for maximized bio-activity and minimized impact."
It also states this, which may be where the "water filters out the good stuff" theory came from; but it says this study was flawed by using lousy mj:
"A 1996 MAPS study[12] tested two simple vaporizer models against water pipes and filtered and unfiltered cannabis cigarettes (joints). The smoke produced by each was analyzed for solid particulates (tars) and 3 major cannabinoids. The various smoking methods were then rated based on their cannabinoid-to-tar ratio.
The two tested vaporizers performed up to 25% better than unfiltered cannabis cigarettes (second best) in terms of tar delivery. However, both vaporizers produced more than ten times more tars than cannabinoids, which may partly be attributable to the low potency (2.3%) of the NIDA-supplied cannabis used in the study. Surprisingly, the same study found that water pipes (bongs) and filtered cigarettes performed 30% worse than regular, unfiltered joints. The reason was that waterpipes and filters filter out psychoactive THC with the tars, thereby requiring users to smoke more to reach their desired effect. The study did not, however, rule out the possibility that waterpipes could have other benefits, such as filtering out harmful gases such as carbon monoxide.
These studies have not measured the presence of toxic gases, such as ammonia, hydrogen cyanide and carbon monoxide, though previous studies have indicated unquantified decreases in carbon monoxide with vaporization.
Although vaporizers produce cleaner vapors than smoking, they do not completely eliminate respiratory irritation. A large puff of potent vaporized cannabis will often cause severe coughing. This is likely due to the THC itself, which is known to have a strong expectorant effect".
I guess it is possible that water filters out THC or cannabinoids, but man, I used bongs exclusively for years and I sure got plenty high off of very small amounts of mj. The part that I underlined above says that water does filter out THC. Frankly I find this hard to believe. I have always felt I got higher with a bong than with a joint, and I always burned very little mj in my bong. Big hits, yes, but from a small amount of herb. Therefore I am skeptical about the idea of water filtering out a significant amount of THC. But I'm open minded to it if I can find more evidence.
