- Current smokers, ex-smokers who stopped smoking in the previous 6 months, or subjects with a cumulative smoking history >10 pack-years will be excluded. (Pack-years is a way to measure the amount a person has smoked over a long period of time. It is calculated by multiplying the number of packs of cigarettes smoked per day by the number of years the person has smoked, eg, a 10 pack-year history is equal to smoking 1 pack per day for 10 years or 2 packs per day for 5 years, etc.)