Smoking-related data from all subjects were recorded in a nurse-administered questionnaire, which also included a detailed medical history. Subjects were categorized as “smokers” based on their answer to the question in the questionnaire “Do/did you smoke cigarettes?” A subject who never smoked is defined as a “non-smoker”. For non-smokers, we intentionally excluded those subjects younger than the age of 25. This exclusion strategy is to ensure that our control subjects (non-smokers) are unlikely to develop smoking behavior if exposed to cigarettes in future since most smokers start smoking in adolescence. For smokers, cigarette consumption information was also collected, which was the number of cigarettes smoked per day. For the purpose of our analyses, the cigarette consumption information was transformed into indexed smoking quantity (SQ) using the criterion from the Fagerström Test for ND (FTND) questionnaire: SQ = 1 if number of cigarettes smoked/day is 10 or less, SQ = 2 if the number is 11-20, SQ = 3 if the number is 21-30, and so on.
