packyears

Pack-years is a clinical metric used to quantify an individual’s lifetime exposure to tobacco smoke. It is defined as the product of the number of packs of cigarettes smoked per day and the number of years the person has smoked. A pack-year equals smoking 20 cigarettes in one day for one year.

Calculation and examples: Pack-years = (cigarettes per day / 20) × years smoked. For example, someone who has smoked 1 pack per day for 20 years has 20 pack-years. A person who has smoked half a pack daily for 10 years has 5 pack-years. In most practice, pack-years reflect years of active smoking; time after quitting is not counted toward the pack-years themselves.

Uses: Pack-years is widely used in medicine to estimate risk for tobacco-related diseases and to guide screening and management. It is commonly employed in risk models for lung cancer and chronic obstructive pulmonary disease (COPD), and can influence eligibility for imaging-based screening programs. It also serves as a standardized way to describe smoking history in clinical notes and research.

Limitations: Pack-years is an approximate proxy and relies on self-reported data, which can be imprecise. It does not capture changes in smoking intensity over time, depth of inhalation, cigarette type, or the use of non-cigarette tobacco products. It may not fully reflect secondhand smoke exposure or other risk factors. The metric can vary in applicability across populations and guidelines, and clinicians often report it alongside current smoking status and years since quitting for a fuller assessment.