Smoking History pack years
A pack-year is a quantification of cigarette smoking.
The pack-year is a unit for measuring the amount a person has smoked over a long period of time. It is calculated by multiplying the number of packs of cigarettes smoked per day by the number of years the person has smoked. For example, 1 pack-year is equal to smoking 20 cigarettes (1 pack) per day for 1 year, or 40 cigarettes per day for half a year, and so on.
One pack-year is the equivalent of 365.24 packs of cigarettes or 7, 305 cigarettes.
Number of pack-years = (packs smoked per day) × (years as a smoker)
Number of pack-years = (number of cigarettes smoked per day/20) × number of years smoked. (1 pack has 20 cigarettes)
Note that despite the unit being called a "pack-year, " the actual unit is simply a number of packs (as noted above).
For example: a person 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 10 cigarettes a day for 6 years they would have a 3 pack-year history. Someone who has smoked 40 cigarettes daily for 20 years has a 40 pack-year history.
Significance and usage
Quantification of pack-years smoked is important in clinical care, where degree of tobacco exposure is correlated to risk of disease such as lung cancer.