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Table 1 Pack-years of smoking

From: Clinical verification of the relationship between smoking and the immune microenvironment of breast cancer

To calculate smoking pack-years:
  Divide the number of cigarettes smoked per day by 20 (the number of cigarettes in a pack)
  Then multiply by the number of years smoked
ex. 1 (70 cigarettes/day ÷ 20 cigarettes/pack) × 10 years = 35 pack-years
ex. 2 (35 cigarettes/day ÷ 20 cigarettes/pack) × 20 years = 35 pack-years
ex. 3 (20 cigarettes/day ÷ 20 cigarettes/pack) × 20 years = 20 pack-years