The following have approximately equal risk, according to this carefully researched source:
- one puff on a cigarette
- watching TV for four months (due to radiation)
- nibbling one extra pointy tip off a Hershey's kiss
- crossing the street
By risk, of course, I mean only the risk to your physical health. Watching four months of TV will certainly mushify and numb your brain. Crossing the street to a movie theatre or other moshav leitzim/beis z'nus will certainly adversely affect your eternal soul. None the less. the list is instructive when assessing the halachik status of smoking.
I am marching through R' Fuerst's shiurim on psak.org, and just listened to Smoking in Halacha. I almost skipped it, as I was quite confident that I already knew the salient features of the argument. But it was on the list and I am kind of stupid that way; taking the advice on what's important from people who are wiser than I even when I think I know better this time. This shiur is really a classic R' Fuerst shmuess. You'll hear all the classics: "Let me finishe, I just got started.", "I don't make this up.", "Let me finish!", and so on. Highly recommended.
R' Fuerst addressed three issues: whether smoking is mutar or assur; if it is mutar (spoiler alert: it is), would one be allowed to smoke on Yom Tov (spoiler alert: you can't say it's assur); finally, what about smoking in public?
More than once (in fact, at any opportunity), R' Fuerst reiterated that of course one should not smoke. That being said, he also stressed that you can't make up facts and you can't distort halacha... smoking is mutar. It's a terribly bad idea, but you can't say that it is halachically forbidden. As far as the health issues, see above. One puff will not kill you more than one extra nibble on a Hershey kiss. In fact, for someone who is overweight, the kiss nibble could be much more of a health risk than that one puff. Again, not recommending smoking, just noting that you have to be real and honest.
Smoking on Yom Tov is more difficult; still hard to say it is assur, but there are poskim (R' Silberstein, I believe) who say that the situation today is different -- it is no longer shava l'chol nefesh/enjoyed by all -- and so it is now forbidden. Others disagree. No one says it's a good idea.
Most fascinating to me was smoking in public. The Shulchan Aruch, Choshen Mishpat, Siman 155, syef 35 discusses doing things that affect those around you. It says you can assume that silence is acceptance; except for four things, one of which is smoking. One must assume that smoking bothers other people, unless they are explicitly mochel. More than that, even if someone has been mochel for years, he has a right to say, "Your smoking bothers me now."; and the smoker then is halachically forbidden to smoke. Even if one in a thousand says it bothers him, the smoker is forbidden to continue smoking. The Rema argues a bit, but the halacha seems to follow the m'chaber.
But you shouldn't smoke.