measurement unit label Pack Year A quantification of lifetime tobacco exposure defined as (number of cigarettes smoked per day x number of years smoked)/20. One pack-year is smoking 20 cigarettes a day for one year. Pack Year Pack Year: The number of cigarettes smoked per day times the number of years smoked divided by 20 (the number of cigarettes in a pack) Pack-Year pack year pack years smoked pack_years_smoked {Pack Year}