You know, I don't see why people freak out over Oda's smoking habit. Miyazaki smokes and he's been making movies for years. Granted he's retired now but still.
I mean its all about playing the odds no? You could never touch a cigarette in you life and die of cancer in your 40s or chain smoke for years and live to be 100. There's no denying though that continuous exposure to carcinogens makes it more likely to end up in the former group than the later. All that being said, does One Piece really have more than a decade left in it? Fans should be more worried about errant cars than cigarette if the fear is that he might die before completing One Piece. If they are concerned with Oda's health in general (as a person they respect and like not just "that guy who makes the comic they read"), than yes being concerned about smoking at all is very reasonable.