# Pack-year

A pack-year is a quantification of cigarette smoking.

## Definition

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 365.24 packs of cigarettes or 7,305 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 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.[2]

## References

1. ^ http://www.cancer.gov/dictionary?CdrID=306510 National Cancer Institute definition of pack year
2. ^ World Health Organization (2008), WHO Report on the Global Tobacco Epidemic 2008: The MPOWER Package, Geneva: World Health Organization, ISBN 92-4-159628-7