Frequent sauna bathing may reduce chronic obstructive pulmonary disease risk: A prospective study

Chronic obstructive pulmonary disease (COPD) is a chronic respiratory disease that results in progressive and irreversible airflow obstruction.1 In 2019, COPD was the third leading cause of death globally; the Global Burden of Disease 2019 study reported 212.3 million global cases of COPD, with COPD accounting for 3.3 million deaths and 74.4 million disabilityadjusted life years.2 Apart from active smoking which is the major risk factor for COPD, other important risk factors include genetics, comorbidities, occupational exposures, indoor and outdoor air pollution, and infections.3 Chronic obstructive pulmonary disease, once established, is incurable. However, there are effective strategies which can prevent the development or slow the progression of symptoms and improve quality of life. Sauna bathing, a traditional Finnish activity mainly used for relaxation, pleasure and wellness and increasingly being used in many other populations, has several health benefits.4 Frequent sauna bathing is associated with reduced risk of adverse vascular outcomes such as hypertension, cardiovascular disease (CVD), dementia, as well as allcause mortality.4 Frequent sauna bathing may potentiate the beneficial effects of protective risk factors such as exercise and cardiorespiratory fitness5– 8 or offset the adverse effects of other risk factors.9– 11 Sauna bathing may also reduce the incidence of respiratory infections such as common colds and pneumonia, severity of COVID19, and improve the symptoms of lung diseases.4,6,12 The effect of sauna bathing on the risk of incident COPD has not been previously evaluated. Using a populationbased prospective cohort study of Finnish men, we aimed to assess the prospective association between frequency of sauna bathing and the risk of COPD.


| INTRODUCTION
Chronic obstructive pulmonary disease (COPD) is a chronic respiratory disease that results in progressive and irreversible airflow obstruction. 1 In 2019, COPD was the third leading cause of death globally; the Global Burden of Disease 2019 study reported 212.3 million global cases of COPD, with COPD accounting for 3.3 million deaths and 74.4 million disability-adjusted life years. 2 Apart from active smoking which is the major risk factor for COPD, other important risk factors include genetics, comorbidities, occupational exposures, indoor and outdoor air pollution, and infections. 3 Chronic obstructive pulmonary disease, once established, is incurable. However, there are effective strategies which can prevent the development or slow the progression of symptoms and improve quality of life.
Sauna bathing, a traditional Finnish activity mainly used for relaxation, pleasure and wellness and increasingly being used in many other populations, has several health benefits. 4 Frequent sauna bathing is associated with reduced risk of adverse vascular outcomes such as hypertension, cardiovascular disease (CVD), dementia, as well as all-cause mortality. 4 Frequent sauna bathing may potentiate the beneficial effects of protective risk factors such as exercise and cardiorespiratory fitness [5][6][7][8] or offset the adverse effects of other risk factors. [9][10][11] Sauna bathing may also reduce the incidence of respiratory infections such as common colds and pneumonia, severity of COVID-19, and improve the symptoms of lung diseases. 4,6,12 The effect of sauna bathing on the risk of incident COPD has not been previously evaluated. Using a population-based prospective cohort study of Finnish men, we aimed to assess the prospective association between frequency of sauna bathing and the risk of COPD.
University of Eastern Finland approved the study protocol (Reference number: 143/97; 1 December, 1983) and all participants gave written informed consent. Details of risk markers have been described previously. 14 Sauna bathing habits were assessed by a self-administrated questionnaire which included assessment of the weekly frequency and duration of sauna sessions based on a traditional Finnish sauna. We included all incident cases of COPD that occurred from study recruitment through 2014. Incident COPD cases were collected by linkage to the National Hospital Discharge Register and a comprehensive review of hospital records. Qualified physicians made the diagnoses of COPD which was based on clinical history, symptoms and spirometry findings (based on forced expiratory volume in 1 s (FEV1) and forced vital capacity (FVC)). 9 The current analysis included 2658 men with complete information on sauna bathing habits, relevant covariates and COPD outcomes (Supplementary Material S2). Multivariable-adjusted hazard ratios (HRs) with 95% confidence intervals (CIs) for incident COPD were estimated using Cox proportional hazard models.
All statistical analyses were conducted using Stata version MP 17 (Stata Corp).

| RESULTS
The mean (standard deviation) age of study participants at study entry was 53 (5) years ( Table 1). The frequency of sauna bathing ranged from 0 to 7 sessions/week with a median (interquartile range, IQR) of 2 (1, 2) sessions/ week. The median (IQR) duration of a single sauna session was 10 (8-15) min. Only 12 men did not use sauna at all. During a median (IQR) follow-up of 26.9 (16.6-31.0) years, 150 incident COPD cases were recorded. Compared with participants who had ≤2 sauna sessions/week, the age-adjusted HR of COPD for those who had 3-7 sauna sessions/week was 0.57 (95% CI: 0.36-0.91). After further adjustment for smoking, alcohol consumption, histories of type 2 diabetes, coronary heart disease, asthma, bronchitis, and tuberculosis, the HR was minimally attenuated to 0.62 (95% CI: 0.39-0.98) ( Figure 1).

T A B L E 1 Baseline characteristics of study participants
Given the strong and causal relationship between smoking and COPD risk, we assessed if smoking might modify the risk between frequency of sauna bathing and COPD risk using interaction analysis. The association of frequency of sauna bathing with risk of COPD was not modified by smoking status (current smokers versus nonsmokers) (p-value for interaction = .70). Compared with participants who had ≤2 sauna sessions/week, the multivariable adjusted HR of COPD for those who had 3-7 sauna sessions/week was 0.53 (95% CI: 0.20-1.36) and 0.65 (95% CI: 0.38-1.11), respectively, for nonsmokers (n = 36 COPD events) and current smokers (n = 114 COPD events).
In a subsidiary analysis, there was no evidence of an association between the duration of a single sauna bathing session and risk of COPD (Supplementary Material S3).

| DISCUSSION
Given suggestive observational and experimental evidence that passive heat therapies such as sauna bathing may improve symptoms of and reduce the risk of acute and chronic respiratory conditions, we sought to evaluate the prospective association between frequency of Finnish sauna bathing and COPD risk. Our results suggest that frequent sauna sessions (3-7 sessions/week) could potentially reduce the incidence of COPD in middle-aged and older Finnish men. The association was independent of major risk factors for COPD. The association between frequency of sauna bathing and COPD was not modified by smoking status (current and nonsmokers). Given the low event rates in these subgroups, these findings need to be interpreted with caution. Though we are unable to directly compare the current findings in the context of previous work, we have shown in a previous study that the frequency of sauna bathing was inversely associated with the future risk of respiratory diseases, defined as the combination of COPD, asthma, or pneumonia. 15 In this previous study, most of the cases were based on pneumonia outcomes. 15 There was no evidence of an association between duration of a single sauna session and COPD risk, which is consistent with previous evaluations of sauna bathing habits and other outcomes. 15,16 This suggests that the frequency of sauna bathing (sessions/week) may be more relevant than the duration of a sauna session.
Smoking is a major risk factor which contributes to the development of COPD. 3 Chronic obstructive pulmonary disease is characterized by a chronic inflammatory process; a distinct feature of COPD is the presence of acute exacerbations, which are typically associated with increased inflammation and are triggered by infections and environmental factors. 1 Major risk factors also interact together, leading to vascular abnormalities, such as loss of alveolar capillary endothelial cells, alveolar cell destruction, and enlargement of alveolar space, all of which are important contributors in the progression of COPD. 17 Sauna bathing has anti-inflammatory and antioxidant properties, 4 hence, we postulate that regular sauna baths could reduce the risk of COPD via reduction in systemic inflammation F I G U R E 1 Association between frequency of sauna bathing and risk of chronic obstructive pulmonary disease. Model 1: Adjusted for age. Model 2: Model 1 plus smoking status, alcohol consumption, histories of type 2 diabetes, coronary heart disease, asthma, bronchitis, and tuberculosis. CI, confidence interval; HR, hazard ratio; ref, reference. and oxidative stress. Sauna bathing may also exert direct effects on the lung tissue by reducing pulmonary congestion and increasing ventilation, tidal volume, vital capacity, and forced expiratory volume (FEV). 4 Though most of the health benefits of sauna are attributed to the physiologic responses to heat, emerging evidence suggests that the beneficial effects of sauna on the respiratory tract is due to the ability of sauna air to hydrate respiratory tract mucus. 18 Given the low relative humidity of sauna baths (10%-20%), the air is commonly described as "dry", but this is not the case from a physiology perspective. 18 A recent study on the associations of air pollution levels and climatological factors with hospital admissions for COPD exacerbation showed a significant effect of humidity, 19 which reflects the fact that the humidity associated with sauna baths may be implicated in the pathophysiology of respiratory tract conditions including COPD. Sauna use has increased globally beyond Nordic countries over the last decade because of the emerging evidence on its substantial health benefits. Hence, the current findings may have public health implications. The accumulating evidence suggests that 3-7 sauna sessions/week with each session lasting about 15 minutes may reduce the risk or severity of several vascular and nonvascular conditions including COPD. 4 Sauna bathing may represent an important lifestyle strategy for COPD prevention, but well-designed definitive trials and mechanistic studies are needed to confirm these observational findings and explore the mechanistic pathways underlying the associations. The strengths of this study include the novelty, utilization of a large general population-based sample of middle-aged and older men, and the prospective cohort design with long follow-up. Limitations deserving consideration include the inability to generalize the findings to women and other ethnicities, the relatively low event rate, and the potential for biases such as residual confounding. There was a potential for regression dilution bias, however, our reproducibility studies of sauna bathing habits indicate that sauna bathing habits may be relatively consistent within Finnish individuals over several years. 14 Nevertheless, findings should be interpreted with caution considering the limitations.

| CONCLUSION
Frequent sauna bathing may be associated with a reduced risk of COPD in middle-aged and older men independently of major risk factors. Further studies are required to confirm these findings. AUTHOR CONTRIBUTIONS SKK: Study design, data analysis and interpretation, drafting manuscript, and revising manuscript content and approving final version of manuscript; JAL: Study design and conduct, responsibility for the patients and data collection, and revising manuscript content and approving final version of manuscript.