Reply To: This will save you from a safek issur d’oraysa

And there are also other Gedolim (besides HaGaon HaRav Kanievsky) who assur smoking. Those rabbis who allowed smoking (e.g., Rav Moshe has a teshuva about this) did so because in their days, it was not yet known just how dangerous smoking was, and if they would have known, they would have assured it as well.
In any event, it is a machlokes haposkim involving a possible issur d’oraysa (on Yom Tov as well as any other day of the year). And safek d’oraysa lechumra.