THC isn't a poison, it is virtually non-toxic. No one has died from marijuana where as 1:1,000 drinkers will die from alcohol overdose.
Smoking marijuana is not likely to cause emphysema because the smoke does not obstruct the small airways as tobacco smoke does.
There don't seem to be any actual reports of cancer linked directly to marijuana smoking. Americans have smoked marijuana for over 60 years, though it had been used for thousands of years prior. An important difference between marijuana and tobacco smoke is that marijuana smoke lacks radioactive compounds such as polonium 210.
Those are the main reasons I don't worry about significant lung damage from smoking shrub, eventually I'll vaporize or ingest but that is not economically feasible at the moment.
http://www.montananorml.org/med-myths.php3