The study subjects were Korean men and women aged 19 years or older who participated in KNHANES IV from 2007 to 2009. The KNHANES is a cross-sectional nationwide community health survey conducted using a stratified, multistage probability sampling design and, thus, the participants in the KNHANES stand for the Korean general population. Among 24,871 participants of the KNHANES IV, we excluded 8,064 persons because of the following reasons: younger than 18 years old (n = 6,465); previous diagnosis of cancer (n = 504); and missing data about previous CVD attack (n = 5,020). Finally, a total of 16,807 persons were included in the present study. We defined previous CVD as having been diagnosed with angina pectoris, myocardial infarction, or stroke by doctors. Among the 16,807 study participants, 683 persons reported to have experienced CVD attack. Among the 683 persons with CVD, we identified 180 persons who were smoking at the time of CVD diagnosis. The institutional review board of the Samsung Medical Center approved the present study and acquisition of informed consent form was exempted because the data of KNHANES do not contain any identifiable personal information.

The KNHANES survey is composed of a health interview survey, self-administered questionnaire survey, health examination, and nutrition survey. For the health interview survey, data was collected using a structured questionnaire regarding demographic characteristics, socioeconomic status, morbidities: age, sex (male or female), area of residence (rural or urban), years of education (≤9 years or >9 years), occupation (economically inactive or active), marital status (married or unmarried), and previous diagnosis of diabetes, hypertension, or depression. Data on mental health included subjective stress (mild and moderate to severe) and health behaviors were collected by self-administered questionnaire. Health behaviors included smoking habit, alcohol consumption, and physical activity. Smoking habit was classified into three groups (current smoker, ex-smoker, and never smoker) based on response to questions such as "Have you ever smoked 100 cigarettes or more in your entire lifetime?" and "Do you smoke cigarettes now?" We defined a person as a never smoker if he/she had smoked less than 100 cigarettes through their lifetime. Ex-smoker was a person who had smoked at least 100 cigarettes through their lifetime but did not smoke at the time of KNHANES IV. Current smoker was a person who had smoked more than 100 cigarettes during entire lifetime and continued smoking until the time of the survey. Exsmoker and current smoker were combined into ever-smoker group. For ever smokers, more detailed information was collected about the age at smoking initiation, amount of smoking, duration of smoking, presence of smokers among family members, and experience of any programs that promotes smoking cessation, such as group educational program, personal counseling, advertisement, or campaign. By using the information about age at smoking initiation, duration of smoking, and age at diagnosis of cardiovascular disease, the age at quit-smoking and the time interval with regard to the age at CVD attack were estimated. We defined 'quitter' as a person who stopped smoking after the attack of CVD. We defined 'non-quitter' as a person who continued smoking even after the attack of CVD.

We defined regular drinker as a person who drank more often than once a month. Physical activity was categorized into two groups (active and inactive). If a person was involved in any one of the following types of physical activity, then he/she was categorized into the physically 'active' group: vigorous exercise (running, climbing, riding a bicycle fast, swimming fast, playing soccer, playing basketball, jump rope, playing squash, playing a single game of tennis, or weightlifting of heavy things) for more than 20 minutes at a time at a frequency of more than three times a week; moderate intensity exercise (swimming slowly, playing a double game of tennis, playing volleyball, playing badminton, playing ping-pong, or weightlifting of light thing) for more than 30 minutes at a time at a frequency of more than five times a week; and walking (including walking to work or to school) more than 30 minutes at a time at a frequency of more than five times a week.

In a health examination survey, body weight and height were measured with a person in light clothing and no shoes. Body mass index was calculated as weight in kilograms divided by height in meters squared. Then, obesity was defined based on body mass index (≥25 kg/m²) according to the World Health Organization's standard for Asian people.¹⁹⁾ In addition, the level of glucose after a minimum 8 hours of fasting was assessed in fresh serum. Blood pressure was measured three times by trained medical staff, and the averaged value of the second and third measurement of blood pressure was used for analysis.

We identified a person with previous CVD attack based on the question asking "Have you ever been told by your doctor that you had a stroke, myocardial infarction, or angina pectoris?" We defined hypertension as follows: high systolic blood pressure (≥140 mm Hg), high diastolic blood pressure (≥90 mm Hg), or taking antihypertensive medication at the time of survey. We defined diabetes as follows: high fasting serum glucose level (≥7 mmol/L), taking oral hypoglycemic agents or insulin at the time of survey, or previous diagnosis of diabetes by a doctor.

We compared the characteristics of the 683 persons with previous CVD with the remaining 16,124 persons without CVD to evaluate factors that are associated with CVD occurrence. As the age distribution differs markedly between the two groups, the comparison was made after standardizing for age. Among the 180 persons who were smoking at the time of CVD diagnosis, we identified 66 quit smokers and 114 persistent smokers. Then, we compared the characteristics between quitter and non-quitter in order to evaluate factors that are associated with persistent smoking after CVD attack using a chi-square test and t-test. In order to evaluate factors independently associated with persistent smoking, multi-variable adjusted logistic regression analysis was performed. Age (increase by one year), sex (male or female), duration of education (≤9 years or >9 years), regular drinking (yes or no), presence of smokers among family members (yes or no), amount of smoking (number of cigarettes per day), exposure to smoking cessation program (yes or no), diabetes (yes or no), and time (increase by one year) lapse after the diagnosis of CVD were put in the model as covariates because these factors seemed to have probable association with persistent smoking in an unadjusted analysis (P < 0.1). The statistical analyses were performed with subpopulation analysis of IBM SPSS ver. 20.0 (IBM Co., Armonk, NY, USA) with consideration of sampling weights and the complex survey design.