Smokers who smoked more than 38 pack-years of cigarettes had a risk of getting pneumonia of over 3.15 to 1. The risk of ex-smokers was similar to current smokers, about 2.14 to 1. After 5 years of no smoking, the risk of acquiring pneumonia was reduced.”
I think the units are packs/year. 38 pack-years suggests that if you smoke 1 pack over 38 years you are at high risk. Likewise 38 packs in one month is only about 3 pack years suggesting that you are at low risk.