The covariates were the participant's age, sex, smoking status (smoker or non-smoker), and comorbidities. Participants who reported smoking cigarettes at the time of the interview were defined as smokers. Comorbidities were assessed by referring to the self-reported physician's diagnosis, and included hypertension, cancer, advanced liver disease, diabetes mellitus, stroke, heart failure, arthritis, and chronic obstructive pulmonary disease.