A pack year is a term used to express the amount of smoking exposure a person has had over time.
It is calculated by multiplying the number of packs of cigarettes smoked per day by the number of years of smoking.
The calculation used is:
Number of cigarettes per day / 20 x number of years of smoking = number of pack years
So, if a patient says they started smoking at the age of 14 and stopped at the age of 65, smoking an average of 15 cigarettes per day, the calculation will look like this:
15 / 20 x 51 = 38.25
We tend to round up the calculation, so this would likely be expressed as a smoking history of about 40 pack years.
If a patient has had a very variable smoking history it is possible to break this down and calculate a more accurate pack year history but often this is not clinically relevant. More than 15 pack years is considered to be potentially clinically significant.
There are also tools to help work out the conversion to pack years for smoking roll-up cigarettes, pipe-smoking, cigars and other smoking habits. Calculator tools are available.