pack-year

A pack-year is a tidy way of putting a number on a lifetime of smoking. Smoking one pack (20 cigarettes) a day for one year equals one pack-year; smoking two packs a day for ten years, or one pack a day for twenty years, both equal twenty pack-years. It compresses two variables — how much and for how long — into a single figure that clinicians can compare.

The calculation is simply the average number of packs smoked per day multiplied by the number of years smoked. Half a pack a day for forty years is 0.5 × 40 = 20 pack-years. Because cigarette counts vary, the convention fixes one pack at 20 cigarettes.

Pack-years are used to gauge cumulative tobacco exposure and to flag people for screening or heightened concern. For example, lung-cancer screening with low-dose CT is typically offered to those with a substantial smoking history (often 20 or more pack-years) in the relevant age band. The measure is approximate — it ignores depth of inhalation, tar content and gaps in smoking — but it remains a practical, widely used shorthand.