According to the Report on the Global Tobacco Epidemic by the World Health Organization (WHO), there are approximately 1.1 billion smokers in the world, representing 1 out of 5 adults. Further, among them, approximately 0.5 billion are expected to die from smoking cigarettes [1]. Smoking causes various cancers; has a fatal impact on other cardiac, cerebrovascular, and respiratory diseases; and is known to cause emotional anxiety and depression [2]. Because of the seriousness of the physical and mental health impact of smoking, the current state and cause of smoking are being analyzed multidimensionally to lower the global smoking rate. In addition, various smoking cessation policies have been implemented, including increasing cigarette prices, issuing warning labels on cigarette packs, designating non-smoking zones, conducting smoking cessation education, and imposing advertisement and mass media restrictions. Additionally, smoking cessation programs and promotional campaigns for smoking cessation are being implemented [3]. Though South Korea has also implemented various anti-smoking policies on the national level, the smoking rate among Korean male adults was 45.0% in 2007 and 39.3% in 2017, while that among Korean female adults continued to remain steady at 5.3% in 2007 and 5.5% at 2017. These statistics suggest that the smoking rate did not decrease significantly despite the implementation of various anti-smoking policies [2]. Therefore, it is necessary to focus on hardcore smokers, who have no intention to quit smoking [4, 5].

The definition and classification of hardcore smokers varies across countries. However, in South Korea, a hardcore smoker is defined as someone who smokes an average of ≥20 cigarettes per day, is aged ≥20 years, has been smoking for ≥5 years, had not attempted to quit smoking in the past 12 months, and has no intention to do so in the future [6]. Although the percentage of hardcore smokers may vary, they generally account for approximately 5~15% of all smokers [7, 8, 9]. The most important factor in determining whether an individual is a hardcore smoker is nicotine dependence. Higher nicotine dependence makes it more difficult to quit smoking, and it has been reported that the likelihood of becoming a hardcore smoker is higher in those who started smoking at an earlier age, have been smoking for a longer duration, smoke more, and have a lower educational level [1, 10, 11]. Hardcore smokers have a relatively higher nicotine dependence than do regular smokers [12, 13, 14]. While current smoking cessation policies may have a positive impact on regular smokers with low nicotine dependence, they have been shown to have a limited impact on hardcore smokers [4]. Accordingly, the development of differentiated smoking cessation policies and interventions targeting hardcore smokers is emphasized [4].

In particular, hardcore smokers become dependent on nicotine, but various behavioral and psychological factors are also interconnected with nicotine dependence [13, 15]. The theory of smoking proposed by Leventhal and Zvolensky [16] explains that, not only is smoking affected by psychological factors such as anxiety, depression, and stress, but continued smoking also exacerbates these psychological problems. Therefore, a concurrent approach is needed to address an individual's smoking status and psychological problems.

A study on American adolescents reported that students with high levels of depression had a higher risk of smoking because of their lack of skills in coping with rejection [17]. Therefore, depression is considered a risk factor for smoking. Among the studies that identified the relationship between depression and smoking, a study on Australian adults [18] reported that there was a higher probability of hardcore smokers being diagnosed with depression as compared to others, while a study on Korean adults aged ≥40 years reported that former and current smokers showed a significantly higher rate of depression than did non-smokers [19].

On the other hand, smoking has been suggested as a predictor of depression. In a meta-analysis of studies on smoking [20], exposure to secondhand smoking was reported to be associated with depressive symptoms and psychological distress. Nicotine acts on the brain to activate nicotine acetylcholine receptors, triggering the release of large quantities of a variety of neurotransmitters such as dopamine, norepinephrine, and serotonin, which can improve one's mood, and temporarily improving one's memory and ability to perform tasks [21]. Therefore, when a smoker quits smoking, he or she may experience more severe depression or run an increased risk of recurrence due to nicotine withdrawal symptoms, while some of these neurotransmitters have been reported to be involved in the onset and retention of depression [21]. Based on these studies, it has been determined that, while smoking may be increased by negative psychological factors such as depression, excessive smoking can also act as the cause of psychological factors such as depression [16].

Considering that the incidence of smoking has not changed greatly despite continued governmental efforts, policies for hardcore smokers with high nicotine dependence are needed. Further, since psychological factors such as depression have a circular causality with smoking status, an approach based on the same is needed. However, most of the previous smoking-related studies conducted in Korea included both hardcore and regular smokers in the same smoker group [22, 23], or studies on hardcore smokers focused only on identifying the prevalence or status of smoking [8, 24]. In particular, depression and amount of smoking in hardcore smokers are factors that can change in various ways over time, but most previous studies only examined the relationship at a specific time point, which makes it difficult to accurately identify a causal relationship between depression and smoking.

Accordingly, the present study aimed to examine the stability of depression and smoking over time in Korean hardcore smokers, and to identify the directionality of the interaction and causal relationship between these two factors to provide basic data on smoking and mental health in adults.

The present study aimed to examine the stability of depression and total amount of smoking per day over time in Korean hardcore smokers and to identify the directionality of the interaction and causal relationship between these two factors to provide basic data for smoking intervention in adults. Based on the findings in the study, the following implications may be discussed.

Majority of the hardcore smokers (96.8%) were males. Although it is somewhat difficult to make a direct comparison due to varied definitions of hardcore smokers across countries, the present value is overwhelmingly greater than the 85.3% found in 2005 and 84.4% in 2008 in the two-round Thematic Household Survey conducted on hardcore smokers in Hong Kong [27]. However, it is similar to or higher than the 92.6%, 94.1%, and 95.6% found in India, Bangladesh and Thailand, respectively, in a study on smoking conducted in South East Asian countries [14]. Further, a study of Korean hardcore smokers [6] showed that the ratio of male hardcore smokers was 2.6 times higher than that of women, and this noticeable difference in hardcore smoking ratios between the sexes in South Korea is attributable to the very low female smoking ratio and socio-cultural factors such as social prejudice against female smoking. Considering the findings that men are more vulnerable to hardcore smoking, it is deemed necessary to conduct appropriate smoking cessation programs for men.

During the period in which the present study was conducted (2013~2016), South Korea increased the prices of tobacco products at a nationwide level, and actively launched anti-smoking policies such as the expansion of smoke-free areas and the enlargement of the size of pictorial warnings on tobacco packages to 30~50%. Nonetheless, individuals aged 50 years or older accounted for 50.0% among the hardcore smokers in this study. Kishore et al. [14] also reported that those aged 45 years or older accounted for 52.6% and 41.5% of the hardcore smokers in India and Thailand, respectively. This is consistent with Cho's findings [6] that older smokers were more likely to be hardcore smokers. Considering that the more the smoking amount, the more difficult it is for hardcore smokers to stop smoking due to nicotine dependence, it is estimated that the older the smokers, the more accumulative is their smoking amount. As for the educational background of hardcore smokers in this study, 33.0% had an educational level of middle-school graduate or below, which is similar to the 29.9% reported by Cho [6] in the study on the hardcore smoking status in South Korea. Thus, it is deemed necessary to conduct smoking cessation education for groups with a low educational level.

The present study also revealed that depression in hardcore smokers at a previous time point had a significant effect on depression at the four subsequent time points. This finding was similar to that reported in a study on American adolescents [17], which found that, even 3 years after smoking initiation, students with high depression maintained their depression level. The present study also found that depression in hardcore smokers was affected even after a certain amount of time had passed. An earlier study on anxiety and depression in smokers [16] reported that depression is a chronic condition in smokers. It is believed that depressive symptoms would inevitably persist because nicotine acts on the brain to affect the nerves through the secretion of neurotransmitters [21]. Therefore, because smoking itself is involved in the onset and persistence of depression, it is necessary for nurses to regularly monitor and address depression in hardcore smokers.

Further, the present study found that hardcore smokers' total amount of smoking per day had a statistically significant effect on the total amount of smoking per day at the four subsequent time points. This finding was consistent with the results reported in other studies, which confirmed that it is difficult for hardcore smokers to quit smoking because of their nicotine addiction symptoms [12, 13, 14]. In addition, a study on hardcore smokers in Hong Kong [27] reported that the number of hardcore smokers continued to increase even after the implementation of smokefree legislation, which was attributed to smoking being a habit in hardcore smokers. Such a repetitive life habit thereby leads to a strong nicotine addiction. Therefore, since repetitive and continued smoking due to nicotine addiction is the most important cause of smoking in hardcore smokers, it is necessary to implement more aggressive policies to change their life habits and to provide a social environment conducive to smoking cessation. Moreover, effective customized smoking cessation interventions, different from those for regular smokers, are also needed.

The present analysis of the cross-lagged effect between total amount of smoking per day and depression showed that the total amount of smoking per day at a previous time point had a significantly positive relationship with depression at the next time point, while depression at a previous time point had a significantly positive relationship with total amount of smoking per day at the next time point. These findings are similar to those of a study that reported that it is difficult for smokers with mental illness to quit smoking [28]. Moreover, the findings also support an earlier study that reported that smoking makes it difficult to treat depression and that it is the strongest predictor of suicidal behavior [29]. In a systematic review of the literature on the association between smoking and depression/anxiety [30], about half of the 148 studies reviewed reported that depression/anxiety is associated with smoking behavior, while more than one-third reported that smoking is associated with depression/anxiety at a later time. However, there were almost no studies that supported a bidirectional relationship between the factors, while most of the studies to date focused on the unidirectional effect of psychological/emotional factors, such as the effect of depression on smoking or that of smoking on depression. Accordingly, it was suggested to use different methodologies to examine this relationship [30].

The findings in the present study confirmed a cross interaction between depression and total amount of smoking per day in hardcore smokers, and it is believed that the causal relationship between smoking and depression constitutes a vicious cycle without easily distinguished dependent and independent variables. In South Korea, for the smoking cessation of hardcore smokers, governmental agencies, including the Ministry of Health and Welfare, are conducting smoking cessation camps, and hospitals and clinics are introducing smoking cessation counseling programs. However, these programs only perform health status assessments, including health examinations, smoking history, nicotine dependence assessment, and carbon monoxide measurement. They do not assess or intervene for psycho-emotional factors such as depression. A preceding study [27] reported that psycho-emotional problems make smoking cessation more difficult. Therefore, smoking cessation interventions and policies for hardcore smokers should be differentiated to include the regular assessment of psychological/emotional factors, including depression, as well as treatments for nicotine addiction.

The present study analyzed the cross-lagged effect to determine the preceding causality between the two factors. The results showed that the size of the regression coefficient for the effect of depression on total amount of smoking per day was relatively larger than that of the effect of total amount of smoking per day on depression. These findings confirmed that the effect of depression on total amount of smoking per day was greater than that of total amount of smoking per day on depression. This is consistent with the previous finding [16] that smoking is affected by psychological factors such as depression. Here, it is important to consider that depression increases vulnerability to smoking [17]. Further, nicotine works on the brain to secrete a large amount of neurotransmitters such as dopamine, norepinephrine, and serotonin, which lift up mood and temporarily improve memory and task performance, thereby making the smoking cessation all the more difficult [21]. Accordingly, it is important for nurses who care for hardcore smokers to assess their depression and total amount of smoking per day. Further, it is recommended that the obligatory inclusion of depression intervention in smoking cessation programs will be effective in achieving successful smoking cessation.

This study is significant for nursing practice because it considered factors that have a mutual influence on smoking cessation. This longitudinal analysis of the KOWEPS provides basic data for the development of effective interventions and policies on smoking cessation. In addition, these findings are expected to contribute to the knowledge and practice of nurses working with hardcore smokers.

However, this study has some limitations. First, it measured the variables using a limited number of items due to the nature of the large-scale panel data. Therefore, future studies should expand the model by including other variables that are expected to have theoretical relationships with depression and hardcore smoking. Next, the sampling error was large, especially when calculating the ratio of female hardcore smokers, because a small proportion of the present participants were female, and because the sample included various age groups. Thus, it will be necessary for future studies to identify the relationships between smoking and depression by focusing on female smokers or hardcore smokers of the same age groups.

Previous studies of smoking examined the relationships of relevant variables from a cross-sectional perspective. In contrast, the present study used autoregressive cross-lagged modeling to identify the interactions between smoking and depression, and their changes over time. In doing so, the study aimed to establish a basis for applying smoking-related intervention at the appropriate time. In addition, earlier studies of adult smokers did not classify the participants based on their smoking status, whereas the present study was different in that it focused on hardcore smokers with high nicotine dependence within the smoker population. Previous studies have also reported that current smoking cessation policies have a positive effect on regular smokers but a limited effect on hardcore smokers. Accordingly, the present findings can be used to develop differentiated smoking cessation policies and intervention programs targeting hardcore smokers in Korea.