Right.. So studies show that tar is linked to emphesyma, but not really cancer.. A new popular belief is that whats actually causing cancer is combined effect of the tar + additives... For instance, substances like radium and arsenic, definitely found in cigs, but not weed.
Nicotine itsself is toxic and possibly carcinogenic.. Ciggarettes are dangerous because they are more of a designer drug than anthing on the streets... There are hundreds of added chemicals in cigs..
Second, nicotine causes vasoconstriction. THC causes vasodilation.. Think about it, you can actually feel it.. After a cig your chest feels tight and extremeties feel cold.. After weed, you feel flushed and your chest expands causing you to explosively cough... This means that cigs are trapping in all that excess shite by closing your throat off, where weed is actually helping you cough out all that "nonexcess - excess" stuff.
There are other things too.. Obviously the fact that few smoke as much plant material as cigarette smokers.. I love it when antidrug campains say that marijuana has increased potecny 20x since the 70s, like its a bad thing... Its been showwn that pot smokers smoke to achieve a certain high, not smoke achieve a certain amount of material smoked.. So one hitter quitter is not really that bad.
As for bongs... Theyve been shown to be unhealthier... You have to injest more tar to achieve the same high from a bong than from a joint..(the water filters more THC than tar ironically).. However, cold wwater is good, as smoke burns the cilia in your throat, preventing removal of tar buildup.. Theoretically, cooler smoke would do less damage in this sense.