This summer I was doing my first few boat dives wearing doubles, and the guy I was buddied up with had a hole in his drysuit, so the dives in 46 degree water were over for him. I got instabuddied with a fellow in his early 60's who had been diving forever (lots of Mediterranean diving) and was overweight. He had a 7 ml wetsuit. I was told he didn't breathe under water. We did two dives that afternoon, first wreck 23 minutes max 107 ft, second wreck 36 minutes max 86 ft.
His buoyancy skills and in-water skills were impeccable for the dives we did.
He used less air than I did, and at the end of the dives, he lit up a cigarette, out of the way of the other divers. He also claimed not to be excessively cold, nor did he seem chilled.
Boggled my mind.
Like so many other things it just depends. My grandfather smoked all his life and did die of smoking related illness but he was almost 90 years old. A friend of my mothers just died from cancer. She was only middle-aged and was a thin, non-smoking, excersizing health nut. She had a pain in her side and decided to have a doctor look at it. It turned out that she had cancer EVERYPLACE and they gave her 6 months but she was dead in about 2 months.
None of us will get out of life alive. As I see it, the problem with smoking isn't that it might shorten your life. None of us is guaranteed a long life. The problem I see is the likelyhood that you will get sick but take a very long time in dying. To me, it's more about the quality of life than the length of life.
My mother is in her late 60's and is suffering from emphysema and has been for a long time. She has trouble just walking across the kitchen. She doesn't go anywhere or do anything. She may live quite a while yet but I don't think that she is having much fun.