quotes from the Marijuana Medical Handbook(1997)(ed rosenthal):

" Marijuana smokers should probably abandon the practice of deeply inhaling and holding their breath.Studies have shown that this does little to increase thc absorption, but may substantially increase the respiratory hazards of smoking."

" Another difference between marijuana and tobacco is that tobacco smoke tends to penetrate more deeply into the lungs.For whatever reason, marijuana smoke tends to concentrate in the larger,upper air passageways of the lungs and throat, while tobacco penetrates to the smaller,lower passageways.One result of this is, apparently, that marijuana doesn't cause emphysema,a progressive degenerative disease of the lower lungs that is linked to tobacco smoking.On the other hand, marijuana may be more apt to irritate the throat than cigarettes"