Pack-year

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.^[1]

One pack-year is the equivalent of 365 packs of cigarettes or 7,300 cigarettes, in a year as smoker.

Number of pack-years = (packs smoked per day) × (years as a smoker)

or

Number of pack-years = (number of cigarettes smoked per day/20) × number of years smoked. (1 pack has 20 cigarettes in some countries)

Note that despite the unit being called a "pack-year," the actual unit is simply a number of packs (as noted above).

${\displaystyle {\begin{aligned}1{\text{ pack-year}}&={\frac {1{\text{ pack}}}{\text{day}}}\cdot 1{\text{ year}}\\&={\frac {1{\text{ pack}}}{\text{day}}}\cdot 365{\text{ days}}\\&=365{\text{ packs}}\\&=365{\text{ packs}}\cdot {\frac {20{\text{ cigarettes}}}{\text{pack}}}\\&=7,300{\text{ cigarettes}}\end{aligned}}}$ 

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.

One 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 (2 packs) daily for 20 years has a 40 pack-year history.

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^[2] and heart disease.^[3]