What is a pack year?

What is the definition of a pack year?

One pack year is 20 cigarettes smoked every day for one year. Over 365 days that is 20 × 365 = 7,300 cigarettes, the figure given in the site’s FAQs.

The same number of cigarettes can be spread out differently. Ten a day for two years is also one pack year, and so is 40 a day for six months.

How do you work out pack years?

Multiply the number of cigarettes smoked a day by the number of years, then divide by 20:

Pack years = cigarettes a day × years smoked ÷ 20

If the amount changed over the years, work out each period on its own and add the results together. Other tobacco is converted into cigarettes first: the calculator counts a cigar as 4 cigarettes, for example, and 25 g (1 oz) of loose tobacco a week as 50 cigarettes a week, which it divides by 7 to give a daily figure. Every figure is set out in how the calculation works.

The calculator does the sum for you. It rounds the total to the nearest whole number, and also shows the figure before rounding, to one decimal place, when the two differ.

What does a worked example look like?

Take someone who smoked 15 cigarettes a day for 12 years, then changed to 2 cigars a day for 6 years.

The calculator shows 11 pack years, with 11.4 before rounding. Its optional history step also asks how many years were smoked in total: here, 18.

Why do clinicians record pack years?

FAQ 1 explains that numbers linked to patients “can be searched easily to identify at risk groups”, like blood pressure readings, body mass index and haemoglobin A1c. FAQ 3 notes that computerised systems record only basic smoking data, such as “ex-smoker, 20 cigarettes a day, non-smoker”.

FAQ 2 describes pack years as a common single number used by clinicians, which can be recorded on most medical computer systems. FAQ 4 argues that smoking load on clinical summaries lets the information pass between clinicians and healthcare systems: radiologists who see it on chest X-ray forms “will be more sensitised to look for smoking related disease”.

NICE guideline NG115 on chronic obstructive pulmonary disease (COPD) asks clinicians to “Document an up-to-date smoking history, including pack-years smoked… for everyone with COPD” (see pack years and COPD). In the United States, a pack-year figure is part of the lung cancer screening criteria; England uses a risk score instead. See screening in the US and screening in England.

What are the limits of pack years?

FAQ 2 calls the measure “only a crude imprecise description of smoking related exposure”, because very different histories can give the same number. It recommends recording starting and stopping dates and years smoked as well: see pack years and years smoked.

Pack years are a record of past smoking, not a diagnosis (see what your result means). If you are worried about your health, speak to your GP practice or use NHS 111.