Yun et al. BMC Public Health 2012, 12:724 http://www.biomedcentral.com/1471-2458/12/724 Methods Subjects This study was based on a community health survey performed in Jeollanam-do, South Korea, between September and November 2009. This population-based cross-sectional survey was designed to investigate the health status, health behaviors, use of medical facilities, quality of life, and socioeconomic state of the commu- nity. A complex, multistage, probability sampling design was used to select participants. The study included 20,097 subjects aged 19–105 years. Thirteen subjects were excluded from the study because of missing smok- ing status information and the Center for Epidemio- logical Studies–Depression Scale (CES-D) score. Finally, the analysis included 20,084 subjects (9,118 males and 10,966 females). No institutional review board approval was required for this study because the data are available for public use and void of identifiers. Definition of depression The CES-D is a short self-report scale designed to meas- ure depression in the general population and is widely used for epidemiologic studies on depression. The scale asks about the existence of 20 symptoms during the last week; scores range from 0 to 60, with higher scores indi- cating greater depression [19]. Cho et al.. translated the CES-D scale into Korean, paying attention to culturally different modes of expres- sion of depressive feelings and thoughts. They reported that the Korean CES-D was reliable and valid for the Korean population, and reported that the appropriate cutoff point for depression was a Korean CES-D score of 21 [20]. Here, we defined depression as a Korean CES-D score ≥21. Definition of smoking Smoking status was classified as never-smokers (smoked <100 cigarettes in their lifetime and not currently smok- ing), former smokers (smoked ≥100 cigarettes in their life- time and currently a nonsmoker), and current-smokers (smoked ≥100 cigarettes in their lifetime and currently a smoker). Status of smoking exposure was classified as light smoker (≤10 cigarettes daily), moderate smoker (≤20 cigarettes daily), and heavy smoker (>20 cigarettes daily). Pack-years of smoking were calculated by multiplying the number of years smoked by the average number of packs per day. Based on the age of starting smoking, subjects were classified as ≤19 years, 20–24 years, and ≥25 years, and compared with never-smokers. Based on the duration of smoking, age of starting smoking, and age of former smokers, we calculated the duration of smoking cessation. Former-smokers were classified as <5 years, 5–9 years, 10–19 years, and ≥20 years, and compared with current- smokers. Page 2 of 7 Covariates Household income was determined by each subject’s reported monthly household income in Korean won and categorized as: < one million won; < two million won; < three million won; and ≥ three million won. Education status was categorized as uneducated, elementary and middle school, high school, and college. Marital status was coded into two categories (married/partnered, never married/non-partnered). Job status was coded into three categories (nonmanual, manual, others) according to the Korean Standard Classification of Occupation (KSCO). Monthly drinking was defined as drinking alcohol at least once per month in the previous year. We used monthly drinking as drinking status (yes/no) in this study. Hypertension was defined as the use of medica- tion for hypertension. Diabetes was defined as the use of medication for diabetes. Body mass index (BMI) was cal- culated using self-reported height and weight. Statistical analysis All analysis in this study was performed separately for both sexes because of differences in smoking status and depression rate according to sex, as well as the inter- action effects between sex and smoking status on de- pression. The characteristics of the study population are expressed as the mean ± standard error or number (weighted percentage). Multivariate survey logistic re- gression was used to evaluate the association of smok- ing characteristics (e.g., smoking status, cumulative smoking exposure, age of starting smoking, and dur- ation of smoking cessation) with depression. The odds ratio (OR) and 95% confidence interval (CI) was calcu- lated in unadjusted and multivariate-adjusted models after adjusting for age, income status, education status, marital status, job status, drinking status, hypertension, diabetes mellitus, cerebrovascular disease (CVD), BMI, and physical activity. The multivariate-adjusted model of smoking exposure among current-smokers was fur- ther adjusted for the amount of smoking, and the multivariate-adjusted model of the duration of smoking cessation among former-smokers was further adjusted for pack-years of smoking. The multivariate-adjusted model of age of starting smoking among current- smokers was additionally adjusted for th4e amount of smoking. All statistical analyses were performed using STATA software, version 11.0 SE (STATA, College Station, TX, USA). Results Characteristics of the study population Of the 20,084subjects analyzed, 9,118 (49.3%) were male and 10,966 (50.6%) female. The point prevalence of de- pression was 4.6% in males and 8.0% in females. Subjects