Heated tobacco products (HTPs) are novel tobacco products that use an electronic device to heat dry tobacco at a low temperature of 350℃, delivering a nicotine-containing aerosol that users can inhale [1]. The HTP market has grown rapidly since Philip Morris International (PMI) launched the I-Quit-Ordinary-Smoking (IQOS) HTP system in 2014. Specifically, other tobacco companies have launched new products in the HTP market (e.g., Glo by British American Tobacco [BAT] and Lil by Korea Tobacco & Ginseng) [2], and recently, IQOS has been sold in over 60 countries [1, 3].

As the HTP market has expanded globally, adolescents’ HTP use has become a major public health concern owing to its adverse health effects and prevalence. Specifically, previous studies have found that adolescents’ HTP use can have negative effects on physical (e.g., cancers, allergic rhinitis, and asthma) and mental health (e.g., depression, suicidal ideation, and suicide attempts) [4, 5, 6, 7]. Moreover, HTPs have been shown to be more hepatotoxic than combustible cigarettes (CCs) and to release significantly more carcinogens than electronic cigarettes (ECs) [7, 8]. Despite these negative consequences, 3.5% of adolescents in Korea have used HTP at least once in their lives, and 1.1% were current HTP users [9]. Notably, the prevalence of HTP use has increased faster than that of EC use among Korean adolescents [10]. In Canada, the US, and the UK, 5.6%~9.1% of adolescents were aware of IQOS [11]; the proportion of current HTP users among US adolescents was 1.3% and 1.4% for middle and high school students, respectively [12]. The widespread use of HTPs among adolescents can be attributed to (a) the perception that HTPs are less harmful and more socially acceptable than CCs, (b) their appealing flavors, such as mint, menthol, and berries, and (c) their design, which is similar to popular mobile electronic devices [13].

International tobacco companies claim that a complete switch from CC to HTP significantly reduces users’ exposure to harmful chemicals [3, 14]. However, one notable aspect of adolescents’ HTP use is that a substantial number of HTP users are multiple tobacco product users. Specifically, among current HTP-using Korean adolescents, only 4.1% were single HTP users; 63.4% were triple users of CCs, ECs, and HTPs; and 32.4% were dual users of HTPs with CCs or ECs [15]. In a nationally representative study of US adolescents, 43% and 75% of current HTP users used CC and EC, respectively [16]. Moreover, current HTP users among adolescents have a greater likelihood of (a) CC, EC, or other tobacco product use and (b) frequent CC and EC use [10, 15, 16, 17]. These findings indicate that HTP use in adolescents does not necessarily imply a decrease in the frequency or quantity of CC or EC use.

To comprehensively understand the characteristics of adolescents’ HTP use and develop new interventions for their HTP use, it is necessary to consider the frequency and quantity of HTP use [16, 18]. Although both frequency and quantity of tobacco products are associated with persistent tobacco product use and nicotine dependence [19, 20], studies on the detailed characteristics of HTP use have been limited to adults [21, 22]. However, previous studies have indicated that compared to adults, adolescents are (a) more sensitive to HTP use and advertisements [11, 13, 23] and (b) more likely to continue using HTP after a single experiment [23]. Despite the differences in HTP use between adults and adolescents, most previous studies on adolescents were limited to examining the associations between lifetime and monthly use of HTPs and smoking characteristics [10, 15, 16, 17, 24]. Although the intention to quit smoking is an important phase towards quitting smoking [25], few studies have identified an association between the detailed characteristics of HTP use and intention to quit smoking. Additionally, findings on the relationship between HTP use and attempts to quit smoking are inconsistent. Specifically, one study found no significant association between HTP use and attempts to quit smoking [10], whereas another study found that dual use of HTPs and ECs was associated with attempts to quit smoking [26]. Moreover, two review studies revealed a controversy over whether HTP use for smoking cessation is effective [27, 28]. Nevertheless, a substantial number of HTP users consider HTP use helpful in quitting smoking [29]. Thus, this study aimed to investigate the associations between the frequency and quantity of HTP use and current smoking characteristics (i.e., current CC and EC use and attempts to quit smoking) among current CC-smoking adolescents.

This study found that 21.6% of current CC users reported their monthly HTP use. Similarly, in previous studies conducted in the US, the prevalence of current HTP use among current CC-smoking adolescents was 16.0% [16]. One of the notable findings of our study was that the percentages of frequent and heavy HTP use were the highest among heavy daily CC users and CC users with daily EC use. In addition, the percentage of heavy HTP users among triple users was more than three times higher than that of dual users of CCs and HTPs. To the best of our knowledge, this study is the first to conduct a detailed assessment of adolescent HTP use in terms of frequency and quantity. Thus, it is difficult to directly compare the results of this study with those of other adolescent studies. However, our results are partially similar to the EC use patterns of current CC-using adolescents in Korea and the US: the proportion of frequent EC users was higher among frequent CC users and among EC users who also used other substances, such as CC or alcohol [32, 36, 37]. As the potential health effects of HTP use at the individual and population levels are still unknown and HTPs contain nicotine and toxic chemicals that can cause nicotine addiction [1, 11], the high proportion of frequent and heavy HTP users among CC users is a matter of concern.

This study found that current CC and EC use was associated with the frequency and quantity of HTP use. First, the risks of more frequent and heavier HTP use were greater among daily and heavier CC users. Specifically, daily CC users had higher risks of more frequent and heavier HTP use than non-daily CC users. Daily CC users with more CPD were at greater risks of more frequent and heavier HTP use; and heavy daily CC users had the highest risks of frequent and heavy HTP use. Similarly, a nationally representative study of Japanese adults found that most dual users of CCs and HTPs used both daily and daily dual users used more HTP sticks per day than non-daily dual users [21]. These strong associations between CC and HTP use can be partly attributed to the high nicotine dependence among dual users of CCs and HTPs. Specifically, concurrent use of CCs and HTPs could increase nicotine intake [29], which may be linked to an increased likelihood of nicotine dependence. In addition, CC users who are highly nicotine-dependent may increase their nicotine dependence even further if they concurrently use HTPs [38]. Moreover, HTPs allow current CC users to smoke even in a smoke-free environment [39]. Specifically, an HTP can be attractive to smoking adolescents since it does not look like a tobacco product and has no smell or ash, allowing them to hide their smoking [2, 40], and dual users can switch between HTP use and CC use depending on their circumstances [18]. Therefore, when planning an intensive smoking cessation program for CC users, particularly daily users, it is necessary to consider an integrated intervention for the use of various tobacco products rather than focusing on a specific product [17]. Furthermore, there is a need to strengthen the regulations prohibiting the use of indoor tobacco products in schools.

Second, dual users of CCs and ECs were more likely to use HTPs more frequently and heavily than current CC-only users. In addition, dual users of CCs and ECs, who reported their daily EC use, had the highest risks of frequent and heavy HTP use. A possible reason for these strong associations is that multiple tobacco users may have higher levels of nicotine addiction and may underestimate the risk of using multiple tobacco products than single tobacco product users [17]. Indeed, a nationally representative study of US adolescents found that two or more tobacco product users were more likely to experience nicotine dependence symptoms than single tobacco users and frequent EC users were more likely to report nicotine dependence than less frequent EC users [41]. Furthermore, higher levels of nicotine dependence among multiple tobacco product users have been associated with frequent and heavy tobacco use [42]. These findings support the claim that HTP complements tobacco products [18]. Considering that most adolescent smoking patterns proceed to a wider variety of tobacco product use [43], frequent and excessive HTP use in dual users of CCs and ECs is unlikely to be a transient phenomenon. Rather, it may contribute to their perception of smoking behavior as normal, desirable, and socially acceptable (i.e., renormalization) [44].

Finally, we found that frequency and quantity of HTP use were not associated with attempts to quit smoking among current CC smoking adolescents. Consistent with these findings, a study of Japanese adults found an insignificant association between the frequency of HTP use in CC users and attempts to quit smoking [21]. Similarly, ever use of HTPs among Korean CC-smoking adolescents was not associated with attempts to quit smoking [10, 17]. This phenomenon contradicts advertisements by tobacco companies claiming that HTP use can help people quit smoking [40]. Specifically, CC-smoking adolescents do not use HTPs as a tool to help them quit smoking or replace CCs, but are more likely to use HTPs concurrently with CCs as another means to consume nicotine [45]. Indeed, a study of Dutch adolescents and adults indicated that the primary reasons for using HTPs were curiosity and a better taste [46]. Nevertheless, considering that HTPs are advertised as less dangerous tobacco products or smoking cessation aids, strong policies banning advertising and marketing of HTPs as well as CCs should be pursued immediately [2].

In addition, our study found that triple users of CCs, ECs, and HTPs were less likely to attempt smoking cessation than CC-only users. Given that multiple tobacco use in adolescence can cause and intensify nicotine addiction [47], these results indicate that higher nicotine dependence among triple users acts as an obstacle to smoking cessation among adolescents.

Based on our findings, strategies should be established at the government, community, and school levels. First, at the government level, HTPs should be regulated at the same level as CCs in all tobacco control areas (e.g., tax increases on tobacco products, advertisement regulation, and indoor use regulation) according to the WHO guidelines [1]. For example, to prevent and regulate adolescent smoking and increase successful smoking cessation, legislation is needed for stronger monitoring and regulation of retailer sales of tobacco products, including HTPs [17]. In addition, there is a need for stricter regulations prohibiting the sale of HTPs as substitutes for CCs or as a means of quitting smoking. Moreover, considering the continued release of new tobacco products, the Korean government, like the U.S. Food and Drug Administration, needs to establish a legal process that requires robust pre-launch evaluation of the product [2, 48].

Second, smoking cessation strategies tailored to adolescents’ use of new-generation tobacco products are needed at the community and school levels. In particular, evidence-based education programs on the adverse consequences of HTP use and associations between frequency and quantity of HTP use and smoking characteristics should be urgently developed and implemented. For example, for adolescents who use HTPs to quit smoking, it is necessary to emphasize that the frequency and quantity of HTP do not have a significant relationship with smoking cessation, but can increase the likelihood of multiple tobacco product use and nicotine addiction. Furthermore, considering adolescents’ indiscriminate exposure to HTP advertisements and HTP-related content online and on social media, efforts should be made to improve their media literacy skills to critically analyze and evaluate media messages on HTP use [49].

This study has two strengths. First, we investigated the association between adolescent HTP use and smoking characteristics, considering the frequency and quantity of HTP use in a nationally representative sample. Given that the intensity of tobacco product use is a critical factor of nicotine dependence [19], our findings may contribute to the development of policies or interventions to prevent and reduce underage smoking and nicotine dependence. Second, by incorporating the frequency and quantity of HTP use, this study found that HTP use in adolescents was more likely to reinforce tobacco product use and that multiple tobacco product use made it difficult for them to quit smoking. Despite these strengths, our study has three limitations. First, causal relationships between smoking-related characteristics and HTP use could not be established due to the use of cross-sectional data. Second, the data collected through self-reported questionnaires were analyzed, which may have caused measurement errors. For example, some adolescents may underreport their smoking behaviors due to social desirability and potential stigma. Third, it was not possible to evaluate the differences in the characteristics of HTP use based on CC users’ reasons for using HTPs because the KYRBS, from which our sample was extracted, did not measure this aspect.

Our study of CC-smoking adolescents found that the risks of more frequent and heavier HTP use were significantly associated with daily and heavier CC use and daily EC use and the frequency and quantity of HTP use were not associated with attempts to quit smoking. These detailed findings on HTP use among current CC-smoking adolescents clearly indicate that HTP use in adolescence does not effectively substitute CC use, and provides strong evidence for the need for stronger regulation of HTP use among adolescents. Additionally, regarding strategies for smoking prevention and cessation, community and school health professionals should include interventions for the use of HTPs, as well as CC and EC.

Future research should consider the following. First, to strengthen the relationships between the frequency and quantity of HTP use and smoking characteristics, repeated studies with a representative adolescent population in other countries are required. Second, further investigation of the reasons for adolescents’ HTP use and their perceptions and social norms regarding HTP are required. Finally, longitudinal studies are required to strengthen a causal relationship between detailed information on the use of HTP (e.g., frequency and quantity, reasons for use, and risk perceptions regarding HTP use) and other tobacco product use, which will contribute to establishing evidence-based strategies for regulating HTP use.