Abstract
Aims
While Japan's bathing culture contributes to better health, it has also become a social problem due to the high number of medical care emergencies arising from bathing‐related adverse events. In particular, bathing in hot springs may pose unique risks. This study aimed to examine the relationship between bathing‐related adverse events, including drowning, and the location of bathing, specifically hot springs and other bathing sites.
Methods
This single‐center, retrospective cohort study included 125 participants who had bathing‐related adverse events from March 1, 2012 to February 29, 2024. Participants were classified into two groups according to the bathing location: hot spring (Group H; n = 24) and other bathing (Group B; n = 101) groups. Logistic regression analysis was conducted using drowning as the objective variable and bathing in hot springs and age as explanatory variables.
Results
Only the incidence of drowning showed a significant difference between the two groups (Group H: 9/24 [37%] vs. Group B: 18/95 [18%]). Logistic regression analysis showed that bathing in hot springs was an independent factor associated with drowning, with no significant difference by age.
Conclusion
The results of this study suggest that drowning is more frequent among individuals who bathe in hot springs. Larger scale studies are needed to explore the causes of drowning during hot spring bathing and the relationship between hot spring bathing and bathing‐related adverse events.
Keywords: bath, bathing, drowning, hot spring, respiratory failure
A retrospective single‐center study investigating the characteristics of bathing‐related adverse events found that patients transported from hot springs had a higher frequency of drowning. This study may inform clinical practices and public health policies to reduce the incidence of bathing‐related emergencies.
INTRODUCTION
The Japanese bathing practice of taking a long, neck‐deep soak in a hot spring is widely practiced because it is said to improve the quality of sleep, 1 alleviate depressive symptoms, 2 and prevent hypertension, 3 among other health benefits, especially for the elderly. 4 However, there are many cases of emergency medical evacuation due to bathing‐related adverse events, and it is known that bathing‐related cardiac arrest and heat stroke occur frequently in Japan. 5 Moreover, 74% of bathing‐related drowning cases are reported to result in emergency transportation. 6 As an emergency medical center, our hospital experiences serious cases that are transported to the emergency department from hot springs. Therefore, we focused on the possibility that the type, severity, and outcome of acute illnesses may differ depending on the type of bathing location such as hot springs and other bathing, even for the same bathing‐related adverse event. Although previous literature has suggested that the majority of bathing‐related accidents can be explained by heat stroke, 6 few studies have focused on the association between bathing‐related adverse events and their location of occurrence, that is, the bathing site. The purpose of this study was to examine the relationship between bathing‐related adverse events and their location of occurrence. The hypothesis was that bathing‐related adverse events are more serious at hot springs than at other bathing sites.
MATERIALS AND METHODS
Study design and setting
Data were extracted from our clinical database and retrospectively analyzed by searching for keywords, such as “hot spring,” “drowning,” “bathing,” “Zao,” and “bath,” from March 1, 2012 to February 29, 2024.
Patient data were collected through a medical chart review, and the results are presented based on the Strengthening the Reporting of Observational Studies in Epidemiology guidelines. This study was approved by the Institutional Review Board of Yamagata University School of Medicine (approval number 2023‐323) and was conducted in accordance with the Declaration of Helsinki. Participants were free to opt out of the study, as described in the study information on our website. The need for informed consent was waived owing to the retrospective design of the study.
Study population
We identified 169 patients who were treated at our facility following bathing in hot springs, public baths, or homes. Cases that did not involve bathing‐related adverse events and cases with bathing‐related adverse events that did not have an acute onset were excluded. In addition, cases involving trauma due to falls were excluded because they were deemed less meaningful to examine and compare with those involving endogenous diseases that developed in relation to bathing. Cases involving autopsy imaging were also excluded because these cases involved deceased patients for whom autopsy imaging was performed at the request of an out‐of‐hospital facility, not at the emergency department.
Data collection
In accordance with previous literature, 4 , 5 , 6 age, sex, drowning, cardiopulmonary arrest (CPA) at presentation, alcohol consumption, comorbidities, and timing of bathing‐related adverse events were collected. Data for factors related to acute respiratory failure, including the presence or absence of tracheal intubation, were also collected. In addition, data regarding hospitalization, ICU or HCU admission, and in‐hospital mortality were collected as outcomes. Various items were selected as bathing‐related adverse events, with reference to previous literature 4 , 5 that examined adverse events related to bathing. Cardiovascular events included heart failure, aortic dissection, aortic aneurysm rupture, cardiac tamponade, and atrial fibrillation.
Outcomes and definition
The primary outcome was the incidence of drowning. Secondary outcome measures included acute respiratory disorders, other bathing‐related adverse events, hospitalization, in‐hospital mortality, and admission to the intensive care unit (ICU) or high care unit (HCU).
For emergency transport cases, we defined acute respiratory disorders as a SpO2 value <90% in room air at the time of contact with emergency personnel. For in‐hospital patients, we used the first measured SpO2 value in room air. We did not include PaO2 values in the definition of acute respiratory disorders, as they were recorded in cases involving arterial or venous blood gas analyses following oxygen administration.
Drowning was confirmed when the medical records mentioned “face submerged under water” or when aspirated water was found in the trachea or bronchi during computed tomography (CT) examination.
The final diagnosis was made by the physician in charge of the emergency department or each department after various tests in the emergency department.
Statistical analysis
Categorical variables are expressed as totals and percentages, and continuous variables are expressed as medians and interquartile ranges (IQR). Regression analysis was used to examine the causal relationship between hot spring bathing and bathing‐related adverse events. Fisher's exact test was used for comparisons between the two groups for categorical variables, and Wilcoxon's signed rank sum test was used for comparisons between the two groups for continuous variables. We used logistic regression analysis to identify the factors associated with drowning in accordance with previous studies, 4 , 5 , 6 bathing in hot springs, and age. The age of the patients was set at 70 years because a previous study showed that the frequency of bathing‐related adverse events tends to increase after the age of 70 years. 5 All tests were two‐tailed, with p < 0.05 considered significant. EZR version 1.68 was used for all statistical analyses; EZR is statistical software that extends the functionality of R and R Commander and is distributed free of charge on the website of Saitama Medical Center, Jichi Medical University. 7
RESULTS
From a total of 169 cases, 44 were excluded; 17 did not involve bathing‐related adverse events, 8 involved trauma from falls, 7 were duplicate cases, 5 required autopsy imaging, 5 did not have vital sign data or blood test results, and 2 did not involve bathing‐related events and required CT or magnetic resonance imaging examination. The remaining 125 cases were included in this study (Figure 1); all cases were transported by ambulance.
FIGURE 1.
Selection of study participants and allocation to two groups. Ai‐CT, Autopsy imaging computed tomography; MRI, magnetic resonance imaging; Zao, name of place in Yamagata Prefecture.
First, the target population was classified into 24 hot spring bathing cases (Group H) and 101 other bathing cases (Group B), and background factors, outcomes, final diagnosis, bathing‐related adverse events, vital signs, and blood sampling results were compared between the two groups. Next, the patients were classified into two groups: a drowning group (Group D) and a nondrowning group (Group N), and bathing‐related adverse events, outcomes, and age were compared between the two groups in a subgroup analysis.
Background factors and final diagnosis
Data related to background factors, drowning, CPA, tracheal intubation, and outcomes for all subjects are shown in Tables 1, 2 and 3. The final diagnoses are presented in Data S1. The age of the subjects was a median of 76 years, IQR [65–84 years], and they were predominantly male (77/125 [61%]). The incidence of drowning and the frequency of CPA at presentation were 21% (26/125) and 12% (15/125), respectively. Tracheal intubation was performed in 28% (35/125) cases. In total, 19% (24/125) cases involved hot spring bathing, 41% (52/125) involved hospitalization, and 2% (3/125) involved in‐hospital mortality. Neuromodulatory syncope was the most common final diagnosis (29/125 [23%]), followed by drowning (19/125 [15%]) and heat stroke (8/125 [6%]).
TABLE 1.
Patients' characteristics.
| Variables | All n = 125 | Group H (n = 24) | Group B (n = 101) | p values a |
|---|---|---|---|---|
| Age, year(IQR) b | 76 (65–84) | 71 (65–77) | 78 (64–84) | <0.01 |
| Sex (male), n (%) | 77 (61%) | 18 (75%) | 59 (58%) | 0.16 |
| Alcohol intake, n (%) | 11 (8%) | 2 (8%) | 9 (9%) | 1.00 |
| Comorbidities, n (%) | ||||
| Dementia | 7 (5%) | 0 (0%) | 7 (7%) | 0.34 |
| Congenital disorder | 1 (1%) | 0 (0%) | 1 (1%) | 1.00 |
| Timing of bathing‐related adverse events, n (%) | ||||
| Prebathing phase | 2 (2%) | 1 (4%) | 1 (1%) | 0.34 |
| During bathing phase | 92 (74%) | 19 (79%) | 73 (72%) | 0.61 |
| Postbathing phase | 31 (24%) | 4 (17%) | 27 (27%) | 0.43 |
| Outcomes, n (%) | ||||
| Hospitalization | 52 (41%) | 13 (52%) | 39 (39%) | 0.24 |
| Admission to ICU or HCU | 29 (23%) | 7 (28%) | 22 (22%) | 0.43 |
| In‐hospital mortality | 3 (2%) | 2 (8%) | 1 (1%) | 0.55 |
Note: Values are expressed as number (percentage) unless indicated otherwise. Subjects were divided into two groups: hot spring bathing (Group H) and other bathing (Group B).
Abbreviations: COVID‐19, coronavirus disease 2019; HCU, high care unit; ICU, intensive care unit; IQR, interquartile range.
Comparisons between the two groups were performed using Fisher's exact test for categorical variables.
Values are expressed as median (interquartile range) unless indicated otherwise.
TABLE 2.
Comparison of bathing‐related adverse events by bathing location and outcomes.
| Bathing‐related adverse events | Group H (n = 24) | Group B (n = 101) | p values a |
|---|---|---|---|
| Drowning n (%) | 9 (37%) | 17 (17%) | 0.04 |
| Cardiac pulmonary arrest at time of arrival, n (%) | 3 (12%) | 12 (12%) | 1.00 |
| Heat‐related events, n (%) | |||
| Heatstroke | 2 (8%) | 9 (9%) | 1.00 |
| Hyperthermia | 4 (17%) | 15 (15%) | 0.75 |
| CNS disorders, n (%) | |||
| Altered consciousness | 18 (75%) | 53 (52%) | 0.06 |
| Syncope | 18 (75%) | 53 (52%) | 0.06 |
| Paralysis | 2 (8%) | 5 (5%) | 0.62 |
| Dizziness | 0 (0%) | 2 (2%) | 1.00 |
| Epilepsy | 0 (0%) | 3 (8%) | 1.00 |
| Coagulopathy, n (%) | 5 (20%) | 21 (20%) | 1.00 |
| Cardiovascular events, n (%) | 3 (12%) | 7 (7%) | 0.40 |
| Other events, n (%) | |||
| Burn | 0 (0%) | 2 (2%) | 1.00 |
| Exhaustion | 3 (12%) | 28 (27%) | 0.18 |
| Chest pain | 0 (0%) | 3 (8%) | 1.00 |
| Abdominal pain | 0 (0%) | 4 (4%) | 1.00 |
| Headache | 2 (8%) | 4 (4%) | 0.58 |
| Outcomes, n (%) | |||
| Hospitalization | 13 (52%) | 39 (39%) | 0.24 |
| Admission to ICU or HCU | 7 (28%) | 22 (22%) | 0.43 |
| In‐hospital mortality | 2 (8%) | 1 (1%) | 0.09 |
Note: Values are expressed as number (percentage) unless indicated otherwise. Subjects were divided into two groups: hot spring bathing (Group H) and other bathing (Group B).
Abbreviations: CNS, central nervous system; HCU, high care unit; ICU, intensive care unit.
Comparisons between the two groups were performed using Fisher's exact test for categorical variables.
TABLE 3.
Comparison of respiratory events by bathing location.
| Group H (n = 24) | Group B (n = 101) | p values a | |
|---|---|---|---|
| Respiration events, n (%) | |||
| Acute respiratory disorders | 7 (29%) | 17 (17%) | 0.25 |
| Tracheal intubation, n (%) | 11 (46%) | 24 (24%) | 0.04 |
Note: Values are expressed as number (percentage) unless otherwise indicated. Subjects were divided into two groups: hot spring bathing (Group H) and other bathing (Group B).
Comparisons between the two groups were performed using Fisher's exact test for categorical variables.
Outcomes
The primary outcome was a significant difference in the incidence of drowning between the two groups (Group H: 9/24 [37%] vs. Group B: 17/101 [17%], p = 0.04). Secondary outcomes were not significantly different, including other bathing‐related adverse events (Tables 2 and 3).
Logistic regression analysis with drowning as the objective variable
Drowning was significantly associated with bathing in hot springs (odds ratio: 3.30, 95% CI: 1.20–9.06, p = 0.02), while age was not a significant factor (odds ratio: 1.03, 95% CI: 0.99–1.06, p = 0.09) (Table 4).
TABLE 4.
Results of logistic regression analysis with acute drowning as the objective variable and hot spring bathing and age as explanatory variables.
| Independent variables | Odds ratio (95% confidence interval) | p values |
|---|---|---|
| Type of bathing (hot spring) | 3.30 (1.20–9.06) | 0.02 |
| Age (over 70) | 1.03 (0.99–1.06) | 0.09 |
Comparison of drowning and nondrowning groups
The drowning and nondrowning groups showed significant differences in age (Group D: median 72 years, IQR [66–77 years] vs. Group N: median 77 years IQR [65–84 years], p < 0.01), acute respiratory disorders (Group D: 12/26 [46%] vs. Group N: 12/99 [12%], p < 0.01), tracheal intubation (Group D: 21/26 [80%] vs. Group N: 14/99 [14%], p < 0. 01), coagulation abnormalities (Group D: 10/26 [38%] vs. Group N: 16/99 [16%], p = 0.02), weakness (Group D: 2/26 [7%] vs. Group N: 29/99 [29%], p = 0.03), hospitalization (Group D: 19/26 [73%] vs. Group N: 33/99 [33%], p < 0.01), in‐hospital death (Group D: 3/26 [11%] vs. Group N: 0/99 [0%], p < 0.01), and emergency room death (Group D: 8/26 (30%) vs. Group N: 13/99 (13%), p = 0.04) (Table 5). No significant differences were observed in other bathing‐related adverse events.
TABLE 5.
Comparison of bathing‐related adverse events and outcomes in drowning cases.
| Bathing‐related adverse events, age, and outcomes | Group D (n = 26) | Group N (n = 99) | p values a |
|---|---|---|---|
| Age, year (IQR) b | 72 (66–77) | 77 (65–84) | <0.01 |
| Cardiac pulmonary arrest at time of arrival, n (%) | 6 (23%) | 9 (9%) | 0.02 |
| Respiration events, n (%) | |||
| Acute respiratory disorders | 12 (46%) | 12 (12%) | <0.01 |
| Tracheal intubation | 21 (80%) | 14 (14%) | <0.01 |
| Heat‐related events, n (%) | |||
| Heatstroke | 2 (8%) | 9 (9%) | 1.00 |
| Hyperthermia | 5 (19%) | 14 (14%) | 0.54 |
| CNS disorders, n (%) | |||
| Altered consciousness | 18 (75%) | 53 (52%) | 0.37 |
| Syncope | 18 (75%) | 53 (52%) | 0.26 |
| Paralysis | 1 (4%) | 6 (6%) | 1.00 |
| Dizziness | 0 (0%) | 2 (2%) | 1.00 |
| Epilepsy | 0 (0%) | 3 (3%) | 1.00 |
| Coagulopathy, n (%) | 10 (38%) | 16 (16%) | 0.02 |
| Cardiovascular events, n (%) | 1 (4%) | 9 (9%) | 0.68 |
| Other events, n (%) | |||
| Burn | 0 (0%) | 2 (2%) | 1.00 |
| Exhaustion | 2 (7%) | 29 (29%) | 0.03 |
| Chest pain | 0 (0%) | 3 (3%) | 1.00 |
| Abdominal pain | 0 (0%) | 4 (4%) | 0.58 |
| Headache | 0 (8%) | 6 (4%) | 0.42 |
| Outcomes, n (%) | |||
| Hospitalization | 19 (73%) | 33 (33%) | <0.01 |
| Admission to ICU or HCU | 10 (38%) | 19 (19%) | 0.06 |
| In‐hospital mortality | 3 (11%) | 0 (0%) | <0.01 |
| Emergency room death | 8 (30%) | 13 (13%) | 0.04 |
Note: Values are expressed as number (percentage) unless otherwise indicated. Subjects were divided into two groups: drowning (Group D) and nondrowning (Group N).
Abbreviations: CNS, central nervous system; HCU, high care unit; ICU, intensive care unit.
Comparisons between the two groups were performed using Fisher's exact test for categorical variables.
Values are expressed as median (interquartile range) unless otherwise indicated.
DISCUSSION
In the present study, among bathing‐related adverse events, drowning was the only event that showed a significant association with hot spring bathing. Furthermore, a comparison between the drowning and non‐drowning groups showed that the proportion of respiratory adverse events was higher in the drowning group.
Previous studies have shown that drowning is the most common cause of bathing‐related deaths, 4 , 5 , 8 and that bathing in hot springs may be more likely to cause drowning. Although not significantly different, acute respiratory disorders were observed in 29% of patients who bathed in hot springs, and 75% of these patients had impaired consciousness. Although this study does not suggest that bathing‐related adverse events other than drowning are associated with bathing in hot springs, future large‐scale studies are needed to determine the precise association.
The present study suggests that hot spring bathing may be associated with drowning. Previous studies have shown that drowning may be associated with bathing and that drowning is associated with impaired consciousness. 4 , 6 In all nine cases of drowning in Group H, altered consciousness was observed. This suggests that drowning may be caused by impaired consciousness due to some underlying factor during bathing, such as heat stroke or heat shock. 4 , 6 Suzuki et al. have suggested that heat stroke associated with increased body temperature is associated with the onset of impaired consciousness, and therefore, recommended bathing for less than 10 min. 5 The present study did not show an association between bathing in hot springs and heat stroke. Although this study did not show an association between the hot spring bathing group and heat stroke, the small number of patients may have been a factor, and future studies should be conducted with a larger number of participants. Japanese individuals commonly soak in the bathtub for approximately 10–15 min and most commonly spend 15–30 min in the bathroom. 8 A previous study indicated that soaking in hot water at 41°C or higher for 30 min or longer is associated with a risk for heat stroke and not soaking in hot water for 30 min or longer can prevent accidents during bathing. 6 Although the body temperature of the participants was normal in both groups (36–37°C) when they were examined, their body temperature might have dropped due to some reasons, such as during emergency transport, and they may have suffered heat stroke during bathing. In this study, we did not obtain information regarding the temperature of the hot springs; therefore, we could not examine the relationship between the temperature of the hot spring and adverse events. Moreover, the onset of heat stroke or cerebrovascular disease during hot spring bathing may lead to severe complications, including drowning. These conditions could have more serious consequences in the context of hot spring bathing, highlighting the need for further research.
Subgroup analysis suggested that drowning cases may be more prone to acute respiratory failure, and this result is consistent with previous studies. Possible causes of respiratory impairment include lung damage and pneumonia caused by aspiration of hot spring water or bathing water. Hot springs in Japan include various components such as acidic or alkaline water, depending on the location. In particular, the Zao hot spring in Yamagata Prefecture, where this study was conducted, is a strongly acidic sulfur spring with a pH of about 1.3. Some of the hot spring bathing cases included in this study include emergency cases transported from the Zao hot spring. Acute respiratory failure due to drowning may have been caused by aspiration of strongly acidic hot spring water. Most reports of acid‐induced lung injury are from animal studies, and acid injury increases the permeability of the alveolar epithelium and can persist. 9 , 10 A previous study reported one case of death due to severe acute respiratory failure caused by aspiration of highly acidic hot spring water with a pH of 1.2 11 and another case of acute respiratory distress syndrome caused by aspiration of Zao hot spring water, but the patient survived. 12 In particular, aspiration of strongly acidic hot spring water may cause severe respiratory failure, and the same may be true of the acute respiratory failure in the drowning case in this study. Aspiration of hot spring water or bathing water can also cause respiratory failure. In particular, a study reported that 54% (78/144) of drowning patients who required ICU admission developed acute respiratory failure. 13 The results of this study indicate that cases of drowning are more likely to require endotracheal intubation, which was performed in 80% of cases in the drowning group (vs. 14% in the nondrowning group; p < 0.01). Endotracheal intubation was performed in all cases of CPA, acute respiratory distress, consciousness disturbance, pneumonia, aspiration pneumonia, and intracerebral hemorrhage. The fact that patients in the drowning group were more frequently transported to the emergency department in a critical condition and required intubation is noteworthy. In addition, cases involving drowning were associated with higher rates of hospitalization, in‐hospital mortality, and emergency department mortality. These findings suggest that the prevention of drowning during hot spring bathing may contribute to the overall prevention of bathing‐related adverse events and improved patient outcomes.
Bathing has been shown to improve depression, hypertension, and sleep quality, 1 , 2 , 3 , 4 and a study conducted in 2024 indicated that nocturnal hot spring bathing may be associated with improved hypertension in the elderly. 14 Although the present study did not show that hot spring bathing was associated with any bathing‐related adverse events other than drowning, further research with a larger number of cases should be conducted to show a more precise association between hot spring bathing and bathing‐related adverse events, develop methods to prevent hot spring bathing‐related adverse events, and investigate how more people can safely take hot spring baths. In addition, to our knowledge, there are no reports showing a link between hot spring components and adverse events. In this study, we could not obtain accurate information regarding the components of the hot springs, and further investigation of the effects of hot spring components on adverse events is warranted. We will also develop methods to prevent adverse events associated with hot spring bathing and investigate ways to ensure that more people can safely enjoy hot spring bathing.
This study has some limitations. First, the sample size was limited and differed between groups, and this may have impacted the results. The number of adjusted variables was also limited. Second, the presence of potential unmeasured confounding factors cannot be ruled out. Third, the outcomes may vary depending on the type of hot spring, and it is difficult to generalize the findings to regions where different types of hot springs are prevalent. Fourth, because our institution is a high‐level emergency and critical care center, patients with mild bathing‐related conditions may have been transported to other medical facilities, potentially affecting the severity of cases included in this study (e.g., respiratory distress, consciousness disturbance, and coagulation abnormalities). Finally, only blood test results obtained upon arrival were used for analysis; we were unable to evaluate the clinical course after hospitalization; this may have limited our ability to fully assess disease severity.
CONCLUSION
The results of this study suggest that hot spring bathing is associated with an increased risk of drowning. Larger‐scale studies are needed to explore the causes of drowning during hot spring bathing and the relationship between hot spring bathing and bathing‐related adverse events.
CONFLICT OF INTEREST STATEMENT
The authors declare no conflict of interest for this article.
ETHICS STATEMENT
Approval of the research protocol: This study was approved by the Institutional Review Board of Yamagata University School of Medicine (approval number 2023‐323). Consent for the secondary use of samples or information was obtained from a previous study (Yamagata University School of Medicine Anesthesia and Intensive Care Database Construction, Yamagata University Hospital Ethics Committee Approval No. 2022‐232).
Informed consent: The need for informed consent was waived owing to the retrospective design of the study.
Registry and the registration no. of the study/trial: N/A.
Animal studies: N/A.
Supporting information
Data S1.
ACKNOWLEDGMENTS
I would like to thank the co‐authors for their guidance, and we would like to thank Editage (www.editage.jp) for English language editing.
Yokoyama R, Yarimizu K, Sakaguchi K, Onodera Y, Kobayashi T, Nakane M. Comparison of bathing‐related adverse events, including drowning, between hot spring bathing and general bathing: A single‐center retrospective study. Acute Med Surg. 2025;12:e70064. 10.1002/ams2.70064
DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available upon request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Data S1.
Data Availability Statement
The data that support the findings of this study are available upon request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.