>But judging the "destroying your lungs" aspect is nobody's business but the person doing it.

There's a limit to how far this principle goes. If you saw someone trying to saw something out of their arm, would you think "whatever, it's their arm," or "this person is having a psychotic break and desperately needs medical attention, including restraint?" A lot of people would extend that to most forms of irrational self-harm. I don't want anyone to extend it to anything I do, though, so applying the principle of liberty even when it involves letting people destroy themselves for no reason, still seems attractive.