This questions was just on a show called Manswers on TBS.
My figures may not be exact nor do I claim this source to be right, but sounds about right to me.
% of available THC: Joint=28% bong=78% vaporizer=92%
I am guessing the low #'s for a joint are due to wasted second hand smoke and as THC vaporizes far below the temp of a burning joint, it is plausible that much vaporizes and is lost before actually smoked.
I have always felt like some of what is filtered out in bong water contributes to the overall high, this is why a high from a bong feels a little different. This is just my thoughts...Now for some science!
I am quoting from my Cannabis Doc's book from Chapter 9 entitled "difference in inhaled (smoked, vapor) vs eaten cannabis"
The chapter cites a study sponsored by CA NORML and MAPS (multidisciplinary Association for psychedelic studies) which found that "contrary to popular impression, water pipes don't necessarily protect smokers from harmful tars in marijuana smoke. [...] The reason is that water pipes filter out proportionately more psychoactive THC than than they do other tars, thereby requiring users to smoke more to reach their desired theraputic range."
Translation: THC is actually partially water soluable (not very much, don't worry and don't drink your bong water!). Bongs do not filter tar very well, they actually filter a little THC. (think about if you smoke that big binger load in four or five one hitters) The reason people hear that you get more tar is you may have to smoke more bud to get high therby smoking more tar. Bongs do not somehow increase the amount of tar in cannabis.
And as for moist air/smoke causing pneaumonia...This is either caused by a bacterium or a virus. Not a rainstorm! The cooled moist smoke is one reason why you can take a bigger hit. And your vaporizer is much nicer with a moisturizer (a bubbler for the end of the whip) I bubble mine through a small clean bong. Much easier on the lungs. No yes if you have some sort of respiratory infection...dry air may be better for you.