Pack-year

A pack-year is a quantification of cigarette smoking. It's used in a clinical context to measure a person's exposure to tobacco and assess their risk of developing lung cancer or other pathologies related to tobacco use. However, it is difficult to rely the assessment based on the pack-year due to the different nature of the packaging by different companies.

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)

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).

{\begin{aligned}1{\text{ pack-year}}&={\frac {1{\text{ pack}}}{\text{day}}}\cdot 1{\text{ year}}\\&={\frac {1{\text{ pack}}}{\text{day}}}\cdot 365.24{\text{ days}}\\&=365.24{\text{ packs}}\\&=365.24{\text{ packs}}\cdot {\frac {20{\text{ cigarettes}}}{\text{pack}}}\\&=7,305{\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.

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.^[2]

References

↑ http://www.cancer.gov/dictionary?CdrID=306510 National Cancer Institute definition of pack year
↑ World Health Organization (2008), WHO Report on the Global Tobacco Epidemic 2008: The MPOWER Package (PDF), Geneva: World Health Organization, ISBN 92-4-159628-7, retrieved June 2015 Check date values in: |access-date= (help)

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