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 20 cigarettes (1 pack) per day for 1 year, or 40 cigarettes per day for half a year, and so on.^[1]

One pack year equals 365 packs of cigarettes.

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)x40 = 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.

Someone who has a 14 pack-year history of smoking (the equivalent of 1 pack of cigarettes per day for 14 years) has smoked over 100,000 cigarettes.

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.

References