we calculate that the risk for lung cancer probably returns to that of a nonsmoker somewhere between 10 and 15 years after smoking cessation. (We have less data on the [other smoking-related cancers].) But the risk that people have for smoking-related diseases is directly related to the total number of cigarettes they've smoked in their life. We measure that with something we call "pack-years": that's the average number of packs per day multiplied by the number of years they've smoked. The greater the pack-years, the greater the risk. When you're getting up around 50 pack-years and beyond, that's a lot. If people have a lot of pack-years, the risk of, say, lung cancer never goes back down to [the risk of a non-smoker].
