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Smoking is one of the worst things you can do to many different organs of the human body. Your body is a temple of Christ and is to be treated as such. We belong to God and we're here to do His will in our lives so He can be glorified. We're not doing that if we think we can do what we like with our bodies, minds and life. The idea of being a Christian is to give all that free will to Christ and to hear His instruction for our every step.
It's nothing short of betrayal for us to identify ourselves as Christians and, knowingly, go and do damage to our bodies that God armed us with. The temple He has for us is to use effectively and to show others His works and ways, it is not something to be defiled from such a destructive habit.
We all fall short, however I'd venture to claim that there's no one who doesn't know the inherent dangers of smoking, that there is nothing healthy in the practice at all. Instead of dealing with one's issues in a productive manner they chose smoking as a release, regardless of the consequences, and inherently show no respect for and cause illness to the temple of Christ.
If you're a Christian and have a basic knowledge of God's word on these matters, that's more than sufficient to know how you must intervene.