Abstract
As the population ages and the importance of preventive medicine increases, interest in wellness tourism is growing. Wellness tourism promotes health improvement through participation in health promotion activities. In addition to soaking in hot springs, the Ministry of the Environment is promoting the 'ONSEN Stay' initiative, which includes enjoying various programmes, interacting with local people and other visitors, and revitalising both body and mind. Hot spring therapy, where people stay in hot spring areas to relieve physical and mental fatigue, has been practised in Japan since ancient times. However, there is little research into the mechanisms behind the health effects of hot spring bathing on the mind and body. Therefore, this article analyses research trends regarding hot springs and health in Japan using information from papers included in the Web of Science Core Collection (WoSCC).
Since 2006, papers included in WoSCC have been published on a regular basis, and since 2015, both the number of publications and the impact factors of the journals in which they appeared have increased. Research in fields related to daily health, such as bathing, elderly care and exercise, has increased since 2015 against the backdrop of an ageing population.
While research on hot springs and health in Japan has improved in terms of both quantity and quality, the accumulation and sharing of further evidence on health effects is essential to the more effective promotion of health enhancement through hot spring bathing and hot spring resort stays. Further promotion of international collaborative research involving foreign researchers and international students also appears to be effective.
1. Introduction
As the population ages, the importance of preventive medicine is increasing. Preventive medicine reduces risks and maintains health before diseases and health problems occur. It is hoped that preventive medicine will not only extend the period during which people can live without restrictions, but also improve quality of life, increase opportunities for social participation, and revitalise society as a whole. Against this backdrop, interest is growing in wellness tourism, which promotes health through participation in various health promotion activities. Wellness tourism in Japan involves experiences that utilise regional resources such as natural, cultural, food and human resources, and bathing in hot springs and staying in hot spring areas are experiences that make use of these resources (Sekiguchi, 2022).
The Ministry of the Environment promotes the 'ONSEN Stay' initiative, which includes bathing in hot springs and enjoying programmes that utilise the surrounding natural, historical, cultural and food resources. It also involves interacting with local people and other visitors to rejuvenate both mind and body. As part of the 'ONSEN Stay' initiative, efforts are also being made to understand the overall therapeutic effects of hot spring areas. Data has been accumulated on changes in subjective health status through visits to hot spring resorts, and a randomised controlled trial was conducted in fiscal year 2025 to verify changes in objective health status using data measured using smart rings.
In Japan, the practice of “toji," which involves staying at hot spring resorts to relieve physical and mental fatigue, has been in place since ancient times. Since the Meiji era, research on hot spring medicine has been conducted at research facilities and affiliated hospitals established at seven national universities. However, all of these entities were later abolished or restructured. While researchers belonging to the Japanese Society of Balneology, Climatology and Physical Medicine continue to study balneotherapy, their research primarily focus on the impact of hot spring components on conditions such as rheumatoid arthritis and hypertension. Conversely, comprehensive studies on the health benefits of hot spring resorts often focus on subjective changes in health condition based on survey data from the 'ONSEN Stay' initiative (e.g. Hayasaka et al., 2022; Sekiguchi & Hayasaka, 2024). Consequently, it has been noted that the mechanisms of both mental and physical health benefits of hot spring bathing have remained largely unexamined, with little scientific evidence accumulated, and no consensus has been reached (e.g. Takeda et al., 2023).
This paper therefore aims to provide an overview of research trends concerning the link between hot springs and health in Japan, understand the current state of evidence accumulation and gain insights into potential future research directions.
2. Analytical Methods
With future international comparisons in mind and building on the work of Li et al. (2024), who analysed the global trends in hot spring research, we conducted an analysis of papers included in the Web of Science Core Collection (WoSCC). This database contains papers published in major academic journals across all fields (approximately 21,000 journals). Of the 115 papers published between 1 January 1994 and 30 June 2025 containing all three keywords, 75 were selected for analysis. Papers without author keywords (32 papers) and papers analysing countries or regions other than Japan, such as the United States (eight papers), were excluded.
The analysis focused on both the quantity and quality of the research. The quantity of research was determined by the number of papers published annually, author attributes and author keywords, in an attempt to grasp each trend. The quality of the research was evaluated based on the impact factor of the journals in the Journal Citation Reports, which is an analytical tool that assesses and compares the influence of academic journals using citation data from papers. The impact factor indicates how frequently the average paper published in a specific academic journal is cited during a given year, representing the journal's influence within its field. While the quality of research is usually assessed by the number of citations each paper receives, this paper uses the impact factor of the academic journal in which the paper was published as the evaluation metric since citation counts are also influenced by the publication date.
3. Trends in the number of papers published and impact factor by publication year and author attributes
First, we examine trends in the number of papers published and the impact factor of the journals in which they appeared, categorised by publication year and author attributes (Figure 1).
The publication years are divided into three periods: 1994–2004, when papers were published irregularly (with no papers published in 2005 or 2014); 2006–2013, when papers were published regularly; and from 2015 onwards, when the number of papers published increased.
Seven papers (an average of 0.6 per year) were published between 1994 and 2004. From 2006 to 2013, 25 papers (3.1 per year) were published; in 2008 alone, six papers presenting research results from various fields appeared. Since 2015, 43 papers have been published (4.1 per year), including seven in the first half of 2025, marking a record high.
When the authors' attributes are divided into Japanese authors only, foreign authors only, and co-authorships involving both Japanese and foreign authors, the proportion of papers authored solely by Japanese authors was 71.4% from 1994 to 2004 and 72.0% from 2006 to 2013 - both over 70% - but declined to 58.1% from 2015 onwards. The proportion of papers co-authored by Japanese and foreign authors has exceeded 30% since 2015, indicating progress in international collaborative research involving foreign researchers and international students.
The average impact factor has increased from 1.48 from 1994 to 2004, to 2.58 from 2006 to 2013, to 3.85 from 2015 onwards.
4. Research fields
To see which research fields have been addressed, and whether there have been changes over time, we examined the author keywords assigned to each paper (Table 1 and Figure 2).
When we aggregated the frequency of author keywords, 'hot spring' and 'hot springs' ranked first and second, followed by 'bathing', 'hot spring water' and 'Japan'. Keywords related to legionella and radon appeared less frequently, while exercise, which is closely related to daily health, appeared three times, and stress appeared twice. Keywords related to drinking hot spring water and radon inhalation, both of which are often used in European balneotherapy, appeared twice.
Similar terms such as 'hot spring', 'hot springs' and 'spa' were consolidated to represent hot springs, and multiple words related to Legionella were aggregated. It was confirmed that, aside from the hot springs themselves, the frequencies of appearance of Legionella and Radium/Radon were high (Table 2).
Studies on Legionella include the identification of sources of pneumonia infections related to hot spring bathing (Miyamoto et al., 2007); rapid detection methods for monitoring risks (Taguri et al., 2011); the habitats of species in Japan's wild hot springs (Furuhata et al., 2013); and the distribution of species in public bathhouses (Komatsu, Tanaka and Nakanishi, 2023).
In the case of radium/radon, notable research findings include the correlation between low cancer mortality rates and low-level radon exposure in the Misasa area of Tottori Prefecture (Suzuki et al., 1994); the suppression of ethanol-induced gastric mucosal injury through the consumption of radon water and inhalation of radon (Etani et al., 2016); the therapeutic effects of radon hot spring therapy on rheumatoid arthritis symptoms (Kojima et al., 2018); the impact of radon on health, focusing primarily on sulphur-related metabolites in mouse brains (Kanzaki et al., 2022); and the alleviation of hypertension symptoms through frequent bathing (Kataoka et al., 2023).
Research on such topics as bathing, the elderly and exercise has also been conducted, with a focus on daily health. Examples include the effects and duration of health education programmes combining hot spring bathing, lifestyle education and exercise for women (Kamioka et al., 2006); the installation of hot spring water supply systems in homes in Atami City, Shizuoka Prefecture, and their association with the prevention of long-term care (Hayasaka et al., 2018); the relationship between hot spring bathing and depression among the elderly in Beppu City, Oita Prefecture (Yamasaki, Maeda and Horiuchi, 2023); and the connection between hot spring bathing habits and mental health (Takeda et al., 2023). Takeda et al. (2023) suggest that the positive mental health benefits of hot spring bathing may also be due to leaving one’s residence, interacting with nature, and the relaxation that comes with moderate exercise in a natural setting.
Next, an attempt is made to understand changes in research fields by dividing the results of re-aggregating the frequency of author keywords into the periods 1994 to 2004, 2006 to 2013, and 2015 onwards (Table 3).
Research on Legionella has been ongoing for some time. It is believed that the increase in research on radium/radon since 2015 has been influenced by heightened concerns about the safety of radioactive springs following the accident at the Fukushima Daiichi Nuclear Power Plant after the Great East Japan Earthquake in 2011. Etani et al. (2016), Kanzaki et al. (2022) and Kataoka et al. (2023) have investigated the health effects of radon.
While the absolute numbers are not high, studies related to bathing, the elderly and exercise have increased in frequency since 2015. As the population ages, research focusing on the relationship between hot springs and daily health is increasing.
5. Relationship between the quality of the research and the attributes of the authors
Section 3 mentioned that the average impact factor of the academic journals in which the papers were published has continued to increase. During this period, there has also been an increase in co-authored papers by Japanese and foreign researchers. Is there a relationship between the quality of research and the attributes of the authors?
When the average impact factor of the journals in which the papers were published was calculated, categorised by the nationality of the authors--Japanese only, foreign only, or both-the impact factors were higher for papers with foreign authors only (4.08) or Japanese and foreign authors (4.44), compared to papers with only Japanese authors (2.56). In particular, Wen et al. (2021), which was published in Applied Catalysis B: Environmental (impact factor 22.1), and Urayama et al. (2024), which was published in Nature Microbiology (impact factor 19.4), both of which appeared in academic journals with an impact factor exceeding 10, were both co-authored by Japanese and foreign researchers.
In cases of joint authorship between Japanese and foreign researchers, the fusion of diverse perspectives can enhance the quality of research and provide easier access to international networks. Research has also demonstrated that international collaborative research produces high-quality results (Murakami, 2016).
6. Conclusion
Since 2006, research papers on hot springs and health in Japan have been regularly published in academic journals, with an average of four publications per year since 2015. The average impact factor of the journals in which these papers have been published has also increased, indicating an improvement in the quantity and quality of the research. Against the backdrop of an ageing population, research related to daily health, such as bathing, exercise and the elderly, has increased since 2015. However, to further promote health improvement through hot spring bathing and staying at hot spring resorts, it is essential to accumulate and share more evidence on the health effects of hot spring bathing.
In order for balneotherapy to be covered by public health insurance as a medical treatment in the future, clinical practice guidelines must be developed that present optimal treatment methods based on evidence. Then, clinical research must also be conducted to convince the Ministry of Health, Labour and Welfare of its therapeutic validity. As a systematic review is conducted when formulating the clinical practice guidelines, rediscovery and re-evaluation of evidence are also anticipated.
Evidence is also required for healthcare services that are not categorized as medical treatments. Recognising the lack of evidence necessary for users of healthcare services to make appropriate choices and for service development providers to conduct appropriate research, the Ministry of Economy, Trade and Industry supports the collation of evidence and the formulation of guidelines by central medical societies in each disease area. Additionally, they are developing a system to promote the use of these guidelines by healthcare service developers and users through the 'Research and Development Infrastructure Project for the Social Implementation of Prevention and Health Promotion' (Healthcare Social Implementation Infrastructure Development Project).
Further accumulation and sharing of evidence would be most effectively achieved through enhanced promotion of international collaborative research involving foreign researchers and international students. According to Li et al. (2024), while Japan has the second-highest number of published papers in the field of hot spring medicine, it ranks seventh for average number of citations per paper, behind countries such as France, the United States and Italy. International collaborative research can produce high-quality results and is also expected to strengthen the dissemination of research findings abroad.
To promote international collaborative research further, it is also essential to understand global research trends. Understanding the research trends of each country and region can provide insight into which countries or regions would be the most appropriate targets for collaboration in terms of producing more effective research. One option would be to utilise WoSCC to understand the research trends of countries such as France, which have a high number of published papers and a high average number of citations per paper.
September 10, 2025
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