Young adulthood represents a critical time of development during which the use of tobacco may begin or cease. Quitting smoking before the age of 35 years can prevent many of the harmful consequences of smoking, resulting in nearly identical survival rates compared with those who have never smoked [1,2]. Smoking behaviors among young adults are influenced by the significant life transitions that occur during this time [3,4]. Given more autonomy, young adults establish health-related behaviors that can persist until the end of their lives [5]. Moreover, they are susceptible to initiating smoking as one of their new activities. Smoking behavior among young adults is predictive of smoking in later years [6]. Most young adult smokers are intermittent smokers, but become more addicted to smoking, and eventually become established smokers [3]. This is why the tobacco industry not only focuses on targeting young adults to convert them into new smokers, but also encourages intermittent smokers to become regular smokers [5].

Given the importance of understanding smoking behavior in young adults for tobacco control, many prior studies have focused on the young adult population [7-10]. However, those studies mainly sought to identify factors associated with smoking behaviors only among young adults or to compare characteristics between young adults and other age groups; as such, little is known about differences within the young adult population. More specifically, it has not been established how transitions in smoking behavior occur, which factors increase current smoking, or even if being older is associated with more frequent cigarette use among young adults [11-13]. In Korea, the decision to attain the highest level of tertiary education [14] prolongs the time taken for young adults to complete their education, while the low employment rate among young adults [15] delays their societal beginning of life. Therefore, the transition period between adolescence and adulthood, referred to as young adulthood, is delayed and prolonged. In light of this trend among young adults in Korea, the age limit for the implementation of the Special Act on the Promotion of Youth Employment for young adults was extended to 34 years [16]. Hence, the present study defined young adults as those aged 18-34 years. In line with this definition, Newman and Newman [17] explored how best to categorize the developmental processes of young adulthood. Young adults aged 18-24 years are commonly considered to be in transition, from late adolescence or emerging adulthood to adulthood; this period involves frequent risks and explorations in regard to many aspects of life including family, education, work, and roles. Meanwhile, adults aged 25-34 years are referred to as older young adults because their physical development and maturation are considered complete by this point. Given the importance of understanding how different characteristics are associated with different phases of life, some recent studies [6,7,18,19] have identified differences in smoking behaviors between younger (those aged 18-24 years) and older (those aged 25-34 years) young adults. Younger young adults tend to use flavored tobacco products [18] and multiple tobacco products [6]. They also tend to acquire their cigarettes from their friends or other people with whom they have social relationships [7], and are less likely to utilize smoking cessation websites [19] than with older young adults.

This study is the first to examine the factors affecting current smoking among young adults based on a dichotomization of age groups using a representative national Korean adult sample. This study hypothesized that there would be strong correlations between several variables (including occupational, health-related, and socio-demographic factors) and smoking behaviors among young adults. The study also aimed to compare the patterns of smoking between younger and older young adults.

The findings of this study suggest a significant gap between younger and older young adults, with the latter being more likely to be current smokers. An almost 10%p difference was found in smoking prevalence between younger (18.3%) and older (26.0%) young adults. Consistent with previous studies, being older adults was associated with a greater likelihood of cigarette use among young adults [11-13].

Further, this study revealed a relationship between socioeconomic status and smoking among the young adult population. Low household income and low levels of education were related to a higher prevalence of current smoking in Korea [21]. In addition, smoking prevalence was higher among manual workers than among non-manual workers [22]. This discrepancy remained consistent according to education level and occupation categories in the current study.

This study demonstrated that both non-college-educated individuals and manual workers displayed a high prevalence of smoking among younger young adults. Moreover, this pattern remained consistent among older young adults. Non-college-educated young adults were 3-4 times more likely to smoke than those who were college-educated. Young adult smokers with higher levels of education might recognize the importance of tobacco cessation earlier than those with lower levels of education [23]. Low socioeconomic status might influence health inequalities in late adulthood [24]. Hence, future prospective research may help identify causal factors. To more effectively target socially disadvantaged young adults, more information and support must be administered to non-college-educated individuals in this age range to enable them to achieve and maintain better health-related behaviors. Workplaces may prove to be another potential venue for tobacco control efforts to reduce current smoking rates among young adults, in view of the fact that 27.2% of younger young adults and 24.9% of older young adults in this research were manual workers.

Of the health-related factors, perceived health status and alcohol consumption were found to be strongly related to current smoking. Lifestyle factors such as smoking status, drinking habits, food consumption, and physical activity are generally linked to perceived health status [25], and poor perceived health has previously been associated with current smoking [26]. Alcohol consumption was shown to be a major barrier to smoking cessation among young adults [9,10,27,28]. Many young adult smokers tend to be intermittent, light, or social smokers who smoke in social contexts, and this has become an increasingly common pattern among young adults [25]. Alcohol consumption has been found to be related to the occurrence of smoking, especially among infrequent smokers [26,27]. In addition, previous studies have demonstrated that young adult smokers are vulnerable to social situations in which alcohol is consumed in the presence of other smokers and those in which they spend time smoking with other smokers [28]. Summarily, smoking cessation messages that also address health-related issues and promote healthier practices, including moderate to no alcohol consumption among young adults, would enhance perceived health status. Promoting smoke-free environments in places where young adults gather, such as college campuses, restaurants, bars, and clubs is also important in this regard.

In addition, e-cigarettes, which are marketed as a smoking cessation tool in Korea, have been soaring in popularity, with sales rising as tobacco prices have increased [29]. The results of a nationally representative sample of Korean youth revealed that 9.4% of youth (aged 13-18 years) had smoked an e-cigarette [30], similar to the 10.7% of Korean young adults who reported having tried e-cigarettes in the current study. Such results may suggest that youth and young adults are key targets for the e-cigarette industry. The use of e-cigarettes among less-established smokers, including young adults, has not proven to be an effective means of smoking cessation [31]. Instead, e-cigarette use has been found to be strongly associated with reported current tobacco smoking. Further, e-cigarette use also showed a clear association with the lack of a firm intention not to smoke, which is a powerful predictor of higher risk of progressing to actual use [32]. Moreover, e-cigarette users who held positive beliefs about e-cigarette products were more likely to use e-cigarettes consistently than those who held negative beliefs [33]. Therefore, raising awareness and spreading accurate information regarding e-cigarette use, as well as the regulation of e-cigarettes, should be a high priority for tobacco control among young adults.

A previously published study revealed that respondents in their 20s showed the highest rate of exposure to at-home secondhand smoke in Korea [34]. The present study revealed that 14.4% of young adults were exposed to secondhand smoke at home, and this variable was only associated with current smoking among younger young adults. Younger young adults may share similarities with adolescents, who have been argued to be a vulnerable population to secondhand smoke [35]. However, this study lacks evidence to adequately explain this finding, and few studies have investigated this issue. Therefore, further research is needed to demonstrate why the rate of exposure to at-home secondhand smoke was high among the young adult population. In the current study, stress levels were linked to current smoking among older young adults. As prior research has suggested [36], individuals who began smoking cigarettes at an older age may be more likely than younger initiators to begin smoking in order to manage stress or to decrease depression. Individuals with personal and social stress tend to become addicted to smoking with time [37]. Thus, older young adults might have more lifetime cumulative exposure to social stress, which can affect smoking behavior, than younger young adults. Stress was also found to affect young adults’ changes in impulsivity, causing them to require more urgent and immediate coping strategies [38].

Several limitations of this study must be considered in the interpretation of its findings. First, this was a cross-sectional study. The main finding of this study is that stress was not significantly associated with smoking among younger young adults, but it was significantly associated with smoking among older young adults. However, it was impossible to detect what types of stress might impact older young adults’ current smoking. Second, this study could not compare young adult smoking patterns with other age groups’ smoking patterns. Distinct smoking patterns of young adults, which are thought to be different from those of other age groups, could not be derived because our study did not analyze smoking patterns among individuals of all ages due to its focus on young adults. Third, the analyses in this study were incapable of distinguishing between younger young adult smokers and older young adult smokers in regard to certain smoking behaviors, such as quitting attempts, due to the small sample size of the relevant sub-groups.

Despite its limitations, this study compared various factors affecting current smoking among younger and older young adults. These findings seem to be consistent with previous studies; however, to our knowledge, this study is the first to examine smoking behavior among young adults in Korea. The findings of this study are significant for future research, which should include a larger sample of subjects to confirm the strong correlations found between smoking among young adults and, in particular, socio-demographic variables. Several such factors were found to be related to young adults’ smoking. Even though young adulthood is a critical developmental stage for tobacco use and cessation, current smoking cessation programs are mostly focused on adolescents or middle-aged people. To effectively target the young adult population, the government should strive to implement a more comprehensive approach for encouraging smoking cessation inside young adults’ living quarters and support smoking cessation programs to prevent smokers from becoming heavily addicted. Universities or workplaces might be good places to effectively implement smoking cessation programs for young adults. Therefore, smoking behaviors among young adults should receive attention from the public health community.