It's widely accepted that inhaling smoke from combusted material is carcinogenic ...some smoke is more carcinogenic than others depending on the material being combusted, as I understand it there is no such thing as safely inhaling smoke from any kind of combustion.
It's also widely accepted that burning cannabis plant matter produces carcinogens.
If we remind ourselves of Gunky's Scientific American article:
The smoke from burning marijuana leaves contains several known carcinogens and the tar it creates contains 50 percent more of some of the chemicals linked to lung cancer than tobacco smoke. A marijuana cigarette also deposits four times as much of that tar as an equivalent tobacco one.
A rational & unbiased person might then conclude there is high probability that inhaling carcinogenic cannabis smoke is cancer causing since that smoke contains carcinogens which are inhaled into the lungs.
The premise that cannabis smoke is excluded from the above and is somehow magically non-carcinogenic on the basis that it's cannabis...is comparatively improbable.
Science say cannabis smoke contains carcinogens, as I understand it that is undisputed, if cannabis smoke does not cause cancer then Science is wrong by definition.
A carcinogen is any substance, radionuclide, or radiation that is an agent directly involved in causing cancer.
One has never been illegal...the other has always been illegal. Collecting the necessary data for one is fairly straight forward...for the other almost impossible.
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Fuck Combustion!
A rational and unbiased person who actually does the research will quickly conclude that counter to intuition (which is all you're going on, you clearly haven't done your homework) inhaled cannabis smoke does not cause cancer. For example, see Cannabis and tobacco smoke are not equally carcinogenic, Robert Melamede, Harm Reduct J. 2005; 2: 21. He references several studies, some of them locked behind paywalls but also some available as complete articles.
The number of peer-reviewed published studies describing the cancer-mitigating properties of various cannabis compounds is staggering. There's no doubt that inhaled cannabis smoke can have negative effects on the cardiopulmonary system, but just because you can't believe it doesn't cause cancer doesn't make it so. Science isn't "wrong by definition" because it doesn't confirm your intuition. Your jump from "cannabis smoke contains carcinogens" (which isn't denied) is completely unscientific in that you fail to take into account the possibility that it also contains compounds that neutralize any carcinogens.