n. A measure of cigarette consumption equivalent to smoking one pack a day for a year.
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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.
—Linda Searing, “Smoking before childbirth may increase risk,” The Washington Post, August 07, 2007
People who smoke cigarettes and drink coffee are less likely to have Parkinson's disease. The same can't be said of those who take aspirin.

Those are the conclusions of a new study co-authored by a researcher at the University of Miami School of Medicine. …

Individuals with Parkinson's were evaluated in clinics. They and their relatives were then interviewed by telephone about use of cigarettes, coffee and aspirin. Dosages were recorded in ''pack-years'' for cigarettes, ''cup-years'' for coffee and ''tablet-years'' for aspirin.
—Fred Tasker, “Cigarettes and coffee: Study finds benefits,” The Miami Herald, April 10, 2007
1982 (earliest)
The most significant changes were found in people described as having 50 pack-years of smoking — for example a pack a day for 50 years or two packs a day for 15 [sic] years.
—“Smoking may damage immune system,” United Press International, December 18, 1982
To calculate the number of pack-years that a person has smoked, multiply the number of packs smoked per day by the number of years the person has smoked. So, in the earliest citation, 50 pack-years is equivalent to smoking two packs a day for 25 years (not 15).
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