Pack years and years smoked: why record both?

What is the difference?

Pack years combine amount and time in one number: cigarettes a day multiplied by years smoked, divided by 20 (see what is a pack year?). Years smoked is how long someone smoked, leaving out any years when they had stopped completely.

Two very different histories can give the same number of pack years:

Why does FAQ 2 put weight on years smoked?

FAQ 2 of the site’s FAQs, prepared by Dr Nigel Masters and Catherine Tutt, says: “One important measure for smoking exposure in patient’s histories is smoking duration in years. Smoking duration is known to have strong links to smoking related diseases and is considered the best way to record patient smoking exposure by epidemiologists.”

It uses the example in the table: “5 cigarettes a day smoked for 20 years is the same as 20 cigarettes smoked for 5 years i.e. 5 pack years for both but the risk of smoking related ill health is very much greater in the former smoker.” The former smoker is Person A. FAQ 2 concludes that pack years are “only a crude imprecise description of smoking related exposure”.

It adds that starting and stopping dates matter, “as some smoking related diseases appear to be age dependent and decline on stopping smoking”.

The FAQs do not reject pack years. FAQ 5 calls them “a vital smoking load tool used by epidemiologists”, and FAQ 2 says “we advocate the use of numbers to record smoking history”.

What did Dr Masters write in 2023?

In a 2023 letter to the BMJ about lung cancer screening, he wrote: “The number of cigarette pack years is still widely used by clinicians but has drawbacks. Years smoked and active smoking are the most useful terms to ascertain risk.”

For the drawbacks, the letter cites the Million Women Study, a prospective study of one million women in the UK (Pirie et al, Lancet 2013). It found that “Stopping before age 40 years (and preferably well before age 40 years) avoids more than 90% of the excess mortality caused by continuing smoking”.

He also wrote: “It would be helpful for clinical work if years smoked and date ceased were visible with body mass index and blood pressure on the GP electronic summary health screen.”

Can GP systems record years smoked?

Not as a code, according to the FAQs and the 2023 letter. FAQ 2 says that in 2016 smoking duration could not be coded in NHS computer records, where smoking is documented by SNOMED CT code descriptors such as “ex-moderate smoker (10-19/ day)”. Duration can be noted as free text, but that “is not searchable or widely used by practicing medical staff”.

In 2023 Dr Masters wrote: “Years smoked, however, is not yet an available code. Smoking pack years can, however, be recorded.” FAQ 2 notes that pack years have been difficult to record because many smokers vary their smoking habits; the calculator adds up each period for you.

How do you record both?

FAQ 2 recommends that “starting and stopping dates, smoking duration in years and smoking pack years should be recorded for each patient so that smoking exposure risks can be taken into account when patients present with symptoms of disease.”

After the tobacco questions, the calculator has an optional history step. It asks whether the person still smokes, when they stopped, and how many years they smoked in total. If only one type of tobacco was entered, it fills in the total years from the periods; with more than one type, it shows a range and asks for years when types overlapped to be counted once.

The clinician version puts it all in one line for the record. For Person A, as an ex-smoker who stopped in 2015, the line begins: “Smoking pack years: 5. Cigarettes 5/day x 20 yrs. Ex-smoker, stopped 2015. Years smoked: 20.” The calculator does not ask when smoking started.

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