2.3. Smoking status

Smoking status was assessed by asking about current and former smoking habits, as well as recent changes in smoking habits. Based on this, participants were classified as never smokers (smoked < 100 cigarettes in their lifetime), former smokers (daily smokers that quit during the year before commencement in the study), or current smokers. Participants having “other smoking habits” could describe these in free text and based on this were classified into one of the three groups never smokers, former smokers, or current smokers, or denoted as having a missing value for this variable.