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Pack-years quantify cumulative tobacco exposure by combining how much and how long a person has smoked. It multiplies the average number of cigarette packs smoked per day by the number of years the person has smoked, yielding a score that reflects lifetime exposure. This helps estimate risk for smoking-related diseases like COPD and lung cancer. For example, smoking one pack per day for 20 years equals 20 pack-years; smoking two packs per day for 15 years equals 30 pack-years. It is not a measure of nicotine level in the blood, nor simply a count of smoking events, nor an average nicotine content of cigarettes, but a composite that reflects both intensity and duration of smoking.