Smoke from tabacco or cannabis both contain tars / particulate matter, which provoke an immune-response which can lead to a less functional alveolar lining in the lung.
There are also carcinogenic compounds in the smoke (mainly combustion products), and nowadays there seems to be some discussion going on about the amount of carcinogens present in the smoke from cannabis compared to tabacco.
I found an interesting paper about this subject, in short:
Smoke from cannabis contains more carcinogenic compounds, but is less carcinogenic, then tabbaco-smoke because the latter contains nicotine. Normally, when a cell is transformed into a cancercell (by components in the smoke), it kills itself by a proces called apoptosis. When nicotine binds to its receptor, apoptosis is inhibited, so the cancerous cell is allowed to grow into a tumor.
In addition to this, cannabinoids seem to prevent the activation of procarcinogenic compounds, while nicotine seem to stimulate this proces.
Furhtermore, nicotine promotes more processes related to cancer, while cannabinoids seem to inhibit them, like the formation of bloodvessels in tumors.
The nett result will be that smoke from cannabis is less - and from tobacco is more - carcinogenic then can be expected from chemical analysis of the smoke alone.
In conclusion:
A joint is actually the worse case scenario because it contains a higher amount of tars and carcinogens because of the cannabis, and it also contains nicotin.
A bong / waterpipe contains a higher amount of tars and carcinogens, but no nicotin.
A vaporizer contains the least amount of tars and carcinogens, and no nicotin.
So, when you do make a joint, it is better to put a lot of weed in it, that way you have to take less hits.
And when you smoke pure cannabis through a bong / waterpipe, it would be better to not inhale very deep, because cannabinoids will be absorbed in a few seconds, so holding the smoke for long in your lungs doesn't make you more stoned, it is only worse for your lungs.
I thought this might be of interest for people who wonder what is the best way of smoking your cannabis. Decide for yourself!
Source:
http://www.pubmedcentral.nih.gov/articl ... id=1277837