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/20) × number of years smoked. (1 pack has 20 cigarettes)

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

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

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

1. http://www.cancer.gov/dictionary?CdrID=306510 National Cancer Institute definition of pack year