I don’t think one can make a legitimate case that smoking a cigarette is assur. All the grounds for issur are based on results of heavy smoking over long periods of time.
While there are still plenty of added risks for light smoking or social smoking, it is much more difficult to make the case that they rise to the level of issur. Furthermore, those who are only social smokers have a much easier time quitting, making it even less dangerous long term.
Maybe one could say that “being a smoker” is assur, but you can’t say that smoking a cigarette is assur.