pack year 

A measure of cigarette consumption equivalent to smoking one pack a day for a year.

This study analyzed data on 56,042 women who averaged about 37 years old and were free of breast cancer. Nearly half of the women said they smoked or had smoked at some point. During the next 10 years, breast cancer was diagnosed in 906 women. Those who had started smoking while young and without having given birth were more apt to have breast cancer, with the risk rising with the amount smoked. ... Women who had smoked for 10 pack years before first giving birth were 78 percent more likely to have developed breast cancer than were those who had never smoked.

pack year by plushev October 6, 2007