Critical effects of regular dry sauna bathing

Sauna bathing is a form of whole-body thermotherapy that has been used in various forms (radiant heat, sweat lodges, etc.) for thousands of years in many parts of the world for hygiene, health, social, and spiritual purposes. Modern day sauna use includes traditional Finnish-style sauna, along with Turkish-style Hammam, Russian Banya, and other cultural variations, which can be distinguished by the style of construction, source of heating, and level of humidity. Traditional Finnish saunas are the most studied to date and generally involve short exposures (5−20 minutes) at temperatures of 80∘C–100∘C with dry air (relative humidity of 10% to 20%) interspersed with periods of increased humidity created by the throwing of water over heated rocks. In the past decade, infrared sauna cabins have become increasingly popular. These saunas use infrared emitters at different wave- lengths without water or additional humidity and generally run at lower temperatures (45–60∘C) than Finnish saunas with similar exposure times. Both traditional Finnish and infrared sauna bathing can involve rituals of cooling-of f periods and rehydration with oral fluids before, during, and/or after sauna bathing.

Sauna bathing is inexpensive and widely accessible with Finnish-style saunas more often used in family, group, and public settings and infrared saunas more commonly built and marketed for individual use. Public sauna facilities can be located within exercise facilities and the relationship between saunas and exercise, which may include synergistic hormetic responses, is an area of active research. The use of private saunas, especially involving infrared saunas, is also increasing and saunas are used for physical therapy in mas- sage clinics, health spas, beauty salons, and domestic homes. This trend is capitalising on the call for additional lifestyle interventions to enhance health and wellness particularly in populations that have difficulty exercising (e.g., obesity, chronic heart failure, chronic renal failure, and chronic liver disease).

Facilities offering sauna bathing often claim health benefits that include detoxification, increased metabolism, weight loss, increased blood circulation, pain reduction, antiaging, skin rejuvenation, improved cardiovas- cular function, improved immune function, improved sleep, stress management, and relaxation. However, rigorous medi- cal evidence to support these claims is scant and incomplete, as emphasized in a recent multidisciplinary review of sauna studies. There is considerable evidence to suggest that sauna bathing can induce profound physiological effects. Intense short-term heat exposure elevates skin temperature and core body temperature and activates thermoregula- tory pathways via the hypothalamus and CNS (central nervous system) leading to activation of the autonomic nervous system.

The activation of the sympathetic ner- vous system, hypothalamus-pituitary-adrenal hormonal axis, and the renin-angiotensin-aldosterone system leads to well- documented cardiovascular effects with increased heart rate, skin blood flow, cardiac output, and sweating. The resultant sweat evaporates from the skin surface and produces cooling that facilitates temperature homeostasis. In essence, sauna therapy capitalises on the thermoregulatory trait of homeothermy, the physiological capability of mammals and birds to maintain a relatively constant core body temperature with minimal deviation from a set point. It is currently unclear whether steam saunas invoke the same degree of physiological responses as dry saunas, as the higher humidity results in water condensation on the skin and reduced evaporation of sweat.

On a cellular level, acute whole-body thermotherapy (both wet and dry forms) induces discrete metabolic changes that include production of heat shock proteins, reduction of reactive oxygenated species, reduced oxidative stress and inflammation pathway activities, increased NO (nitric oxide) bioavailability, increased insulin sensitivity, and alterations in various endothelial-dependent vasodilatation metabolic pathways. It has been suggested that heat stress induces adaptive hormesis mechanisms similar to exercise, and there are reports of cellular effects induced by whole-body hyper- thermia in conjunction with oncology-related interventions (i.e., chemotherapy and radiotherapy); however the mechanisms by which the physiological and cellular changes induced by sauna bathing contribute to enhanced health and/or therapeutic effects is still being explored.

The following systematic review was undertaken to explore recent research on the clinical effects of repeated dry sauna bathing (Finnish-style, infrared, or other dry sauna forms) to document the full range of medical conditions saunas have been used for, as well as any associated health benefits and/or adverse effects observed. While a small number of reviews of sauna bathing and health have been conducted in the past, as far as we know, this is the first systematic review of sauna and health to include both Finnish and infrared saunas.

Principal Findings. The findings of this review suggest frequent dry sauna bathing improves a variety of subjective and objective health parameters and that frequent Finnish sauna bathing is associated with improved outcomes such as reduced overall mortality and reduced incidence of cardio- vascular events and dementia, at least in men. The most established clinical benefits of sauna bathing are asso- ciated with cardiovascular disease, yet there is also evidence to suggest that saunas, either Finnish-style or infrared, may benefit people with rheumatic diseases such as fibromyalgia, rheumatoid arthritis, and ankylosing spondylitis, as well as patients with chronic fatigue and pain syndromes, chronic obstructive pulmonary disease, and allergic rhinitis. Sauna bathing may also improve exercise performance in athletes, skin moisture barrier properties, and quality of life and is not associated with serious adverse events. There is not yet enough evidence to distinguish any particular health differences between repeat Finnish-style and repeat infrared sauna bathing. Cardiovascular disease has clearly been a focus for sauna researchers since 2000 despite Finnish-style sauna being con- sidered by some in the past as a contraindication for patients with CHF and other cardiovascular diseases, most likely because of perceived intolerance to the high temperatures. Nearly half (19 of 40) of the studies included in this review involved populations who had active cardiovascular disease or increased risk for cardiovascular disease, and all these studies demonstrated beneficial health effects. Seven of these 19 studies were randomized controlled trials (RCTs), with only one of them meeting the Cochrane criteria for an acceptably low risk of bias. This particular multicentre RCT (𝑛 = 149) reported improvements in all outcome measures except B-type natriuretic peptide (BNP) levels (namely, longer 6-minute walking distance, reduced cardio- thoracic ratio on chest X-ray, and improved NYHA (New York Health Association) classification) in the infrared sauna- treated congestive heart failure group compared to control over only 2 weeks of intervention. While sauna bathing appears to show promise as a lifestyle intervention for cardiovascular disease, a majority of the cardiovascular disease-related sauna studies (16 of 19) were conducted by the same core Japanese research group and affiliates who employed “Waon therapy”, which involved far-infrared sauna bathing. These Waon therapy studies used similar outcome measures and mostly involved hospitalised patients, which might reflect some differences in health care systems and thresholds for hospitalisation. The use of primarily hospitalised patients in these studies also brings up issues of how applicable the findings may or may not be to outpatient populations. Despite differences in sauna types, temperature, fre- quency, and duration of interventions, the far-infrared sauna studies involving cardiovascular disease and congestive heart failure patients suggest favourable outcomes that reinforce earlier findings of interventional Finnish sauna studies and cardiovascular disease. This suggests that heat stress, whether induced by infrared or Finnish-style sauna, causes significant sweating that is likely to lead to hormetic adap- tation and beneficial cardiovascular and metabolic effects. This is further supported by the two large observational studies that found striking risk reductions for sudden cardiac death (63%) and all-cause mortality (40%) as well as for dementia (66%) and Alzheimer’s disease (65%), in men who used a sauna 4−7 times per week compared to only once per week. While these large cohort studies are based on calculated hazard ratios with adjustments for common cardiac risk factors, it has been pointed out that the association between sauna activity and health outcomes may be noncausal and that sauna use is merely an indicator of “healthy lifestyle” and other socioeconomic confounding factors. Nevertheless, these findings point to the need for further study and serious consideration given to sauna bathing to address the ever-increasing individual, societal, and financial burdens of cardiovascular disease as well as dementia-related conditions in aging populations.