Hello to all I know my statement will not change anything. However I have read the article in the new england journal of medicine pertaining to what causes cancer in cigarettes they have come to determine that it comes from burning the radioactive material in tobacco which gets into the tobacco from the pesticides in the soil they have a half life of 18 years.
The seminal article on that was published in NEJM in 1964. . .this isn't exactly news, at least not to anyone in the field.
In short, the concept is that phosphate rich fertilizers contain lots of radioactive Polonium 210, which gets trapped in tobacco leaves then inhaled in tobacco smoke. Under ordinary conditions, radioactive polonium is a harmless weak alpha emitter, but once trapped inside a smokers' impaired lungs, it gets concentrated and pushed right up to already weakened tissue where it can have a more damaging effect. Many believe that this is probably THE most important single contributing factor in causing lung cancer in smokers.
This more recent paper from Spain hypothesizes that Polonium is specifically responsible for one type of lung cancers (adenocarcinomas), and goes into good detail about the mechanisms involved. You can read the whole thing here:
http://www.hindawi.com/journals/jo/2011/860103/
Even assuming its true, polonium is still only ONE carcinogen in tobacco smoke; there are plenty of other ones:
Besides well-known organ-specific carcinogenic substances, such as polycyclic aromatic hydrocarbons, 4-(methyl-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK), 2-Naphthylaine, 4-aminobiphenyl, arsenic, and chromium. . .
At least some of THOSE substances (eg polycyclic aromatic hydrocarbons) are specifically created by burning. Vaporizing instead should preven their formation.
In terms of cannabis vs tobacco, its not just quality of the plant, its also quantity. The *average* cigarette smoker will smoke 20 cigarettes per day, averaging one per wakng hour, EVERY day, for decades. But even the heaviest cannabis smokers rarely smoke anywhere near that amount.
Again, even if we assume that cannabis cigarettes are on average several times as toxic as tobacco ones (which they probably are), someone who is only smoking 4-10 of them per week is still only getting a fraction of the dose of tar and total aggregated particle mass as a typical smoker.
That said, smoking weed definitely isn't "safe". There is plenty of medical data linking cannabis smoking to bronchitis and other respiratory diseases. Again, vaporization will probably help out there too.