Pack Years.

When working with patients with smoke, it is important to calculate their age in 'pack years'.  How to calculate pack years: 

Pack years = number of packs smoked per day × number of years as a smoker

Or, if you don't know how many packs but you do know how many cigarettes, then you calculate it using the number of cigarettes smoked per day × number of years smoked... all divided by 20 (this is assuming that one pack contains 20 cigarettes).

Also... if you are a fan of living forever (aren't we all?) or the book The Blue Zones, then check out this article (thanks M for letting me know about it!): How to Live Forever.