Inhaling toxic fumes gives a probability p that a cell is harmed into a cancer cell. The probability of a hit after n trials equals 1-(1-p)^n. First order approximation (this is the Binomial approximation), this equals n*p. This is accurate for small p and large n, exactly the situation smoking would be in.
Those are very nice numbers! It's so neat and tidy! It's also a complete divergence from physical reality. You know, that boomer land where old people yell at the cloud?
For starters, I'm an advocate of total tobacco abstinence. I watched so many people shrivel up and die from tobacco as I grew up in the south, that I would never ever be a user.
Even the people that I know who are still users basically say they would really like to quit, but they're just too addicted. The withdrawal is just too intense and long lasting. One friend's mom quit because of advancing emphysema, she said she craved a cigarette every day for the remaining 10 years of her life.
Tobacco has a withdrawal on par with heroin, but you don't even get high from it, you just feel a relief from the withdrawal after using. It's one of the most insidious drugs of compulsion known to man, which is why it's such a commercial success.
But if each cigarette takes 20 minutes off of a persons life, I know a few people who should have died in the 1800s.
Almost nothing in the natural world is a linear phenomenon.