Pack Year : A unit of quantity for cigarettes used in medicine to measure a patient's smoking history. One pack year is the equivalent of smoking one 20-cigarette pack per day for one year, that is, a total of 7300 cigarettes
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}