Sorry… I attempted exaggeration for the sake of lighting things up a bit.

(I need to remember my winkey faces when I post!)
There are a few posts in this thread that appear to label
any amount smoking as sinful. So here is my reply without the added exaggeration… plus a bit more least I should lead someone to confusion:
Translated:
When I said “sensitive” I mean scrupulous. In America, our society tends to adopt “all or nothing” as its motto. This motto is not always compatible with Catholicism. On the subject of smoking generally people can be classified into two groups: pack-o-day-death-lovers, and anti-tobacco-hippies. Both view points can lead to unhealthy extremes.
There is a minority “in between” group that tends to get labeled with the “pack-o-day” group, or indifferent. Not everyone in the “in between” group uses tobacco!
I hesitate to make this comparison but here I go:
During the Temperance movement (slightly before ans during Prohibition) you had groups of drunkards, and groups of people who to an extreme view against alcohol. Strangely enough the temperance movement did not seem to have much to do with true temperance. As Catholics, most of us know that it is possible to drink alcohol in moderation, without incurring and sin what so ever. Everyone has their own limits.
So to finish the awkward translation… It is my opinion that people should put more (but proper) emphasis on moderation and temperance when dealing with smokers. The American mass-media has never really done that great of job when it comes to properly defining morality. (Reasons why I previously brought up the topic of abortion.)
I am not encouraging anyone to smoke, but I am encouraging those who can handle it… to do so with moderation and temperance.

Anything outside of that would be sinful… maybe even mortally sinful.