Smoking is a major cause of respiratory diseases, such as chronic obstructive pulmonary disease (COPD) and lung cancer worldwide, and is closely associated with coronary artery disease (CAD), stroke, and cancer [1]. The 2022 National Health Statistics Report estimated 34.0% of men and 6.6% of women in South Korea to be current smokers as of 2020 [2], revealing Korean men’s smoking rate to be the highest among OECD countries, despite their gradually decreasing smoking rate from the peak level of 43.7% in 2012 [3].

Although it is commonly believed that smoking helps manage stress [4], smoking is frequently associated with anxiety, depression, and agitation due to a vicious cycle of smoking, nicotine withdrawal, and smoking again to maintain the blood nicotine content [5]. Also, stress hormone increases during withdrawal period by the strong neurophysiological action of nicotine [6]. The smoking group had a 1.62-fold higher risk of stress compared with the non-smoking group in a study conducted by Bin [7]. Furthermore, smoking is closely associated with mental health problems, such as social anxiety and stress, as well as physical illness [7,8].

With increasing awareness of the harmful effects of smoking, efforts are made to implement population-level support for smoking cessation and regulation as strategies for reducing the burden of smoking-related diseases. Smoking cessation is one of the major health promotion agendas, and the South Korean government is actively pursuing smoking cessation strategies as a core task of its Health Plan 2030 [9]. In 2018, 47.3% of Korean adult smokers (≥19 years) attempted to quit smoking [10]. Although nearly half of smokers have a smoking cessation intention, only 12% to 35% of them succeed, with smoking cessation defined as continuous abstinence for over six months up to 24 months [11-13]. More than half of the respondents (52.6%) chose “stress” as the reason for the difficulty associated with smoking cessation, with “increased stress” being the main cause of smoking initiation and failure in smoking cessation [14,15]. This highlights the importance of mental health management, such as stress and anxiety, for those attempting to quit smoking.

Forest therapy is a natural therapy designed to help restore psychological and mental stability and improve physical and mental health by using a diverse forest environment to interact and communicate with nature [16]. Forest therapy suppresses sympathetic nerve activity [17,18], reduces cortisol levels [17,19], increases parasympathetic nerve activity [18], and reduces mental stress, anxiety, and anger [20,21].

Dry sauna therapy, a type of thermotherapy that has long been used in many countries to provide pleasure and relaxation [22,23], is perceived as a method to alleviate stress [22]. Charcoal kiln saunas have long been a tradition in Korea. Their health effects are attributable to the far-infrared radiation emitted from yellow ocher clay and the anions (negatively charged ions) emitted from charcoal that penetrate deep into the body and stimulates metabolism by excreting heavy metals and waste matter through sweat, stool, and urine, thus helping prevent various diseases [24]. They have the effect of relaxing the body and mind and alleviating anxiety [25]. Taken together, forest therapy and thermotherapy are effective interventions for stress and anxiety management. They can be used as alternatives to facilitate smoking cessation for those who attempt to quit smoking.

Therefore, this study aimed to examine the effects of forest-thermal combined therapy in reducing anxiety and stress by applying it to smokers with smoking cessation intentions. The results can serve as preliminary data for developing effective and economical smoking cessation programs for widespread use in communities.

Koreans have traditionally used charcoal kilns as a means of psychological and physical well-being. This study explores the psychological and physiological effects of an intervention program for smoking cessation attempters on their anxiety and stress levels using forest-thermal combined therapy, a natural therapy combining forest therapy and heat bathing in a charcoal kiln. Specifically, by examining the differences in perceived levels of stress and anxiety and physiological indicators such as HRV, serotonin, and cortisol between the experimental group, which participated in the three-day residential intervention program while the control group maintained their usual routines, it was intended to determine the applicability of the forest-thermal combined therapy as an auxiliary smoke cessation therapy to support smokers with smoke cessation intentions.

The psycho-emotional effects of the intervention program were demonstrated by the significant reductions in the perceived stress and state anxiety levels (PWI-SF and STAI-S scores, respectively) in the experimental group. These results are in line with those of previous studies on forest therapy [17,19,31] and thermotherapy [22,23]. This suggests that the proposed three-day residential program of forest-thermal combined therapy has a positive effect on stress and anxiety reduction in smokers willing to quit smoking. This may be ascribable to the stress-reducing effects of forest therapy by activating parasympathetic nerves, leading to physiological relaxation [17], as well as the anxiety-alleviating effects of thermotherapy by promoting metabolism [24] and relaxing the body and mind [25].

It has been reported that stress acts as a major determinant of the incidence and frequency of smoking [32]. Stress is often cited as the main factor that drives adolescents to attempt smoking [33], and mood improvement, that is, stress reduction, is cited as the main reason for smoking [34]. Stress was also found to be the main obstacle to smoking cessation (52.6%) [36]. In a Korean study, the moderate- and high-stress groups had 1.52- and 2.34-fold higher smoking rates, respectively, compared to the low-stress group, demonstrating a high correlation between stress level and smoking [32]. Thus, stress reduction through forest-thermal combined therapy can contribute to reducing the prevalence of smoking, which might, consequently, lead to individual health promotion and disease prevention. In addition, there is a high correlation be-tween smoking and anxiety, which can be bidirectional because smoking is initially attempted to relieve anxiety symptoms, but eventually worsens those symptoms by developing withdrawal symptoms [35]. In this study, it was found that the forest-thermal combined therapy was ef-fective for anxiety in subjects who were trying to quit smoking. Therefore, it is expected that forest-thermal combined therapy will be used as an alternative therapy that can increase the success rate of quitting smoking.

No significant differences were found in the HRV and cortisol levels, which were used as physiological indicators of stress in examining the effects of forest-thermal combined therapy. This is consistent with the finding of Jung et al. [31] that no significant effect was found in the HRV level after a three-day forest therapy program administered to female employees. In contrast, Lee et al. [17] reported a difference in LF/HF ratio, one of the HRV measures, in a study with young men who participated in a three-day forest therapy program. However, their study was conducted with nonsmokers and under substantially different conditions, in which the control group was continuously exposed to the urban environment under strict control, unlike the current study, where the control group was allowed to continue their daily routines. Similar to studies on HRV, cortisol-related results vary among studies investigating the effects of forest therapy programs. Lee et al. [17] and Ochiai et al. [19] demonstrated significant effects of forest therapy programs on cortisol. The stress-reducing effect of forest therapy is explained by the effect of phytoncide secreted by trees on the stress hormone cortisol [37]. Additionally, a forest therapy program can lead to reducing stress hormones by integrating various accompanying activities, such as exercise, yoga, and meditation, using diverse forest environmental resources [38]. Nevertheless, given the different results, such as no reduction in cortisol levels [31], as was the case with the current study, future research may focus on interstudy differences to determine the possible causes of such differences. Although there are only a very small number of studies investigating the effects of thermotherapy, there are reports of activated parasympathetic nerves, a physiological indicator of stress reduction [39,40]. In a study by Lee et al. [40], parasympathetic nerve activation induced by acupressure/heat combined massage performed on healthy men contributed to stress reduction. Jeong et al. [39] also reported a stress-relieving effect when heat and acupressure were simultaneously applied, by which physiological changes, such as HRV, were induced.

Future research on physiological changes related to stress- and anxiety-reducing effects of forest-thermal combined therapy must therefore necessary to recommend integrating frequent use of forest environment into daily living rather than participating in a short-term forest therapy camp [31], as well as investigating individual differences in the manifestation of physiological effects. That is, it is necessary to repeatedly and continuously apply the forest-thermal combined therapy to smoking cessation attempters in order to understand the long-term physiological effects of forest-thermal combined therapy. It is also necessary to examine ways to maximize the effect by running customized programs tailored to the individual characteristics of smokers willing to quit smoking.

There are several limitations to this study. First, it was conducted as a single-center study, and the number of participants was very small due to difficulties encountered amid the COVID-19 pandemic. Second, the intervention period was too short to estimate the smoking cessation success rate of the participants. Although subjects did not smoke during the intervention program, a long-term follow-up (≥1 month) is necessary to properly assess the smoking cessation success rate. To evaluate the clinical effect of the forest-thermal combined therapy for smoking cessation attempters, the scope of study needs to be broadened by conducting a long-term program followed by a long-term follow-up. Third, there is a gender bias in the participants. Whether this program is effective for women needs to be verified in the future. Despite these limitations, this study remains significant for demonstrating the effects of a forest-thermal combined therapy program in reducing stress and anxiety in smokers willing to quit smoking.

The results of this study demonstrated that the proposed forest-thermal combined therapy program has a positive effect on reducing stress and anxiety in adults with smoking cessation intentions. However, no significant changes were found in the levels of cortisol, serotonin, or HRV. This highlights the need to develop an additional intervention program capable of inducing physiological changes, given the quasi-absence of case studies on forest-thermal combined therapy intervention. Additionally, as research results are accumulated, a strategy needs to be sought to integrate forest-thermal combined therapy into a smoking cessation program.