In the Tashkin UCLA studies and the Kaiser studies about marijuana and lung cancer (you can Google these terms and read more information), they found that marijuana alone--even heavy marijuana use--didn't result in increased lung cancer likelihood. And they theorize--they don't know this but they suspect it--that the THC in cannabis seems to have an anti-cancer effect because, as this article I link below points out, cannabis smoke isn't a clean or healthy substance from a tar or carcinogenic standpoint.

Even in the groups of people who smoked both marijuana and cigarettes, the cannabis smokers had lower rates of lung and other cancers. They won't yet go so far as to conclude that cannabis lowers the risk of cancer, mostly because they need to do lots more studies. But they suspect that that's what these results mean.

You might print out some of this information and take it to your auto shop teacher so he'll know a bit more of what he's talking about! People who smoke tobacco cigarettes are at greatly increased risk of cancer. There's no disputing that. But the same does not appear to be true of cannabis smoking. And it may well be that adding cannabis into the mix with tobacco has some protective effects from the tobacco carcinogens.

Large Study Finds No Link between Marijuana and Lung Cancer: Scientific American