In the sefer Avnei Derech [vol. 6] he has an interesting discussion as to whether smokers are pasul li-eidus because they are transgressing an איסור דאורייתא of ונשמרתם מאד לנפשותיכם according to many poskim. He quotes a letter from Rav Yosef Lieberman Shlita where he says that even though it is wrong to smoke, if we disqualify all smokers from being witnesses there will not be that many married couples left in Klal Yisrael because so many of the witnesses at weddings were smokers.
Here is a letter from Rav Asher Weiss Shlita:
