I believe that if smoking is sinful it is because it is unhealthy. I don't think it is like drinking alcohol or smoking marijuana which can be sinful because they cause the loss of the use of reason, but I think it can be sinful to willingly harm one's health.
Not that I disagree, but there are endless ways to willingly harm one's health.....The sun is said to cause skin cancer...driving a car COULD get you involved in an accident...flying in a plane that very well could crash...skydiving, shark dives, eating mcdonalds, golfing during a thunderstorm, not changing your furnace filters often enough.......