They compare the average lifespan of people who do not smoke with the average lifespans of people who do smoke compared to how many smokes they smoke.
If someome smokes X cigarettes per day for Y years and on average die Z years earlier than people who don’t smoke at all, each cigarette could be considered to contribute to those numbers.
It isn’t an exact thing. Smoking a cigarette won’t specifically make you die exactly 11 minutes before you would have otherwise. It is just statistics and is good for comparing *large* trends and helps put a bad habit into perspective.
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