Smoking is among the most dangerous activities that a person can do to themselves, short of drunken extreme sports and doing methamphetamines. And truth be known, since nicotine is almost as addictive as meth, that particular contest may end up with a tie for both parties. Unfortunately for the smoking world, it may be too late for them by the time that they become an active addict. At that stage of the game, there may be nothing left for the non-smoking world to do for the ones they love that this illness has claimed but to mourn their passing and move on.
However, it is not too late to help out with the people who have the potential to avoid this dreaded problem in the first place. While it can be very easy to simply say that you will not allow a person to smoke in the first place (especially if you are their parent), you know that it is very easy to sneak most kinds of activities past most authority figures. You can not be everywhere that they are, and it would take absolutely all of your energy to completely control them.
So you have got to do what you can to get them to not want to do this terrible activity in the first place. You can mention all of the costs that smoking brings about, both materially and in how it impacts them on a day to day baiss (not breathing as well and smelling terrible). You can even bring up how it coats the inside of their lungs with a dark and malignant sort of death for which there is no known cure. And then you can bring up longer life.