Calculation
Number of pack years = (packs smoked per day) × (years as a smoker)
or
Number of pack years = (number of cigarettes smoked per day × number of years smoked)/20 (1 pack has 20 cigarettes).
For example: a patient who has smoked 15 cigarettes a day for 40 years has a (15/20) x 40 = 30 pack year smoking history.
A pack-year is smoking 20 cigarettes a day for one year. If someone has smoked ten cigarettes a day for six years they would have a three pack-year history. Someone who has smoked forty cigarettes daily for twenty years has a forty pack-year history.
Read more about this topic: Pack Year
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