Pack-years quantify a person's smoking history by combining how much they smoked with how long — a standard way to communicate smoking exposure and lung disease and cancer risk.
Intensity matters as much as duration. Someone who smoked two packs a day for 10 years has the same pack-year exposure as someone who smoked one pack a day for 20 years.
In the US, a 20 pack-year history in adults aged 50–80 who currently smoke or quit within 15 years meets the USPSTF criteria for annual low-dose CT screening.
Twenty cigarettes. So half a pack a day is 0.5, and 40 cigarettes a day is 2 packs per day.
No — pack-years measure cumulative lifetime exposure, so they don't decrease after quitting, though quitting still substantially lowers ongoing risk.