Pack year

A pack year is a quantification of cigarette smoking.

Definition
A way to measure 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 1 pack per day for 1 year, or 2 packs per day for half a year, and so on.

Calculating 'Pack Years'
Number of pack years = (number of cigarettes smoked per day x number of years smoked)/20

For example: a patient who has smoked 15 cigarettes a day for 40 years has a (15x40)/20 = 30 pack year smoking history. (1 pack has 20 cigarettes).

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.

Significance and Usage
Quantification of pack years smoked is important in clinical care where degree of tobacco exposure is closely correlated to risk of disease.