Oil contains carbon atoms, so when it gets heated and the molecules break down, some of the carbon gets set free. This is what makes used oil darker than the fresh stuff. And that carbon is the same as the one on burnt food, which is a known carcinogen. I’d love to find a study about the concrete amounts of probable or proven carcinogens transferred to foods cooked in seasoned cast iron vs teflon, because I’d prefer the everlasting quality of cast iron - but convincing a doctor who works with cancer patients every day requires solid evidence.