Hindawi Evidence-Based Complementary and Alternative Medicine Volume 2018, Article ID 1857413, 30 pages https://doi.org/10.1155/2018/1857413
Review Article Clinical Effects of Regular Dry Sauna Bathing: A Systematic Review
Joy Hussain and Marc Cohen
School of Health and Biomedical Sciences, RMIT University, Melbourne, VIC, Australia
Correspondence should be addressed to Joy Hussain; [email protected]
Received 9 October 2017; Revised 14 December 2017; Accepted 8 January 2018; Published 24 April 2018
Academic Editor: Kieran Cooley
Copyright © 2018 Joy Hussain and Marc Cohen. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Introduction. Many health benefts are claimed by individuals and facilities promoting sauna bathing; however the medical evidence to support these claims is not well established. Tis paper aims to systematically review recent research on the efects of repeated dry sauna interventions on human health. Methods. A systematic search was made of medical databases for studies reporting on the health efects of regular dry sauna bathing on humans from 2000 onwards. Risk of bias was assessed according to the Cochrane Collaboration guidelines. Results. Forty clinical studies involving a total of 3855 participants met the inclusion criteria. Only 13 studies were randomized controlled trials and most studies were small (� < 40). Reported outcome measures were heterogeneous with most studies reporting benefcial health efects. Only one small study (� = 10) reported an adverse health outcome of disrupted male spermatogenesis, demonstrated to be reversible when ceasing sauna activity. Conclusions. Regular dry sauna bathing has potential health benefts. More data of higher quality is needed on the frequency and extent of adverse side efects. Further study is also needed to determine the optimal frequency and duration of distinct types of sauna bathing for targeted health efects and the specifc clinical populations who are most likely to beneft.
1. Introduction with similar exposure times [2]. Both traditional Finnish and infrared sauna bathing can involve rituals of cooling-of Sauna bathing is a form of whole-body thermotherapy that f periods and rehydration with oral fuids before, during, has been used in various forms (radiant heat, sweat lodges, and/or afer sauna bathing. etc.) for thousands of years in many parts of the world Sauna bathing is inexpensive and widely accessible with for hygiene, health, social, and spiritual purposes. Modern Finnish-style saunas more ofen used in family, group, and day sauna use includes traditional Finnish-style sauna, along public settings and infrared saunas more commonly built and with Turkish-style Hammam, Russian Banya, and other marketed for individual use. Public sauna facilities can be cultural variations, which can be distinguished by the style located within exercise facilities and the relationship between of construction, source of heating, and level of humidity. saunas and exercise, which may include synergistic hormetic Traditional Finnish saunas are the most studied to date responses, is an area of active research [3–8]. Te use of and generally involve short exposures (5−20 minutes) at private saunas, especially involving infrared saunas, is also ∘ ∘ temperatures of 80 C–100 Cwithdryair(relativehumidityof increasing and saunas are used for physical therapy in mas- 10% to 20%) interspersed with periods of increased humidity sage clinics, health spas, beauty salons, and domestic homes. created by the throwing of water over heated rocks [1]. In the Tis trend is capitalising on the call for additional lifestyle past decade, infrared sauna cabins have become increasingly interventions to enhance health and wellness particularly popular. Tese saunas use infrared emitters at diferent wave- in populations that have difculty exercising (e.g., obesity, lengths without water or additional humidity and generally chronic heart failure, chronic renal failure, and chronic ∘ run at lower temperatures (45–60 C) than Finnish saunas liver disease) [9]. Facilities ofering sauna bathing ofen 2 Evidence-Based Complementary and Alternative Medicine claim health benefts that include detoxifcation, increased 2. Methods metabolism, weight loss, increased blood circulation, pain reduction, antiaging, skin rejuvenation, improved cardiovas- PRISMA guidelines for conducting systematic reviews were cular function, improved immune function, improved sleep, followed, including the use of validated tools to assess the risk stress management, and relaxation. However, rigorous medi- of bias in randomized controlled trials [70–72]. cal evidence to support these claims is scant and incomplete, 2.1. Eligibility Criteria. Studies of humans undergoing as emphasized in a recent multidisciplinary review of sauna repeated dry sauna bathing that reported on health measures studies [10]. were included in the review. Studies were included for initial Tere is considerable evidence to suggest that sauna review if they were published in English language from bathing can induce profound physiological efects [4, 11–17]. January 2000 onwards and involved research in humans Intense short-term heat exposure elevates skin temperature undergoing repeated dry sauna sessions with at least one andcorebodytemperatureandactivatesthermoregula- reported health outcome. Studies involving predominantly tory pathways via the hypothalamus [18] and CNS (central high-humidity (>50%) wet/steam “sauna” or immersion nervous system) leading to activation of the autonomic hydrotherapy were excluded for the potential confounding nervous system. Te activation of the sympathetic ner- mechanisms of diferential sweating rates and explicit focus vous system, hypothalamus-pituitary-adrenal hormonal axis, of this review limited to “dry sauna” interventions. Studies and the renin-angiotensin-aldosterone system leads to well- of partial body heating were excluded since proposed documented cardiovascular efects with increased heart rate, mechanisms of action may or may not be the same as whole- skin blood fow, cardiac output, and sweating [1, 11]. Te body heating. Studies reporting primarily animal-based, resultantsweatevaporatesfromtheskinsurfaceandproduces nonhuman fndings were excluded given the recognized cooling that facilitates temperature homeostasis. In essence, diferences in end-organ (skin) structure and responses sauna therapy capitalises on the thermoregulatory trait of (sweating mechanisms) between animals and humans. homeothermy, the physiological capability of mammals and Studies of “sauna” as a recruitment venue for potential sexual birds to maintain a relatively constant core body temperature activity, primarily regarding men who have sex with men with minimal deviation from a set point [19]. It is currently (MSM), were excluded since these studies lacked details unclear whether steam saunas invoke the same degree of of sauna interventions, confounding whether wet or dry physiological responses as dry saunas [20], as the higher interventions, and measured health metrics focused to sexual humidity results in water condensation on the skin and activity but not necessarily to sauna activity. reduced evaporation of sweat [21]. On a cellular level, acute whole-body thermotherapy 2.2. Search Strategy. PubMed, Web of Science, Scopus, and (both wet and dry forms) induces discrete metabolic changes Proquest were initially searched with keyword “sauna” and that include production of heat shock proteins, reduction date restrictions of January 2000–April 2017. Search dates of reactive oxygenated species, reduced oxidative stress and were chosen to focus on updated fndings refecting advanc- infammation pathway activities, increased NO (nitric oxide) ing technology in both diagnostics and physiological mon- bioavailability, increased insulin sensitivity, and alterations itoring to build upon the foundational literature of prior in various endothelial-dependent vasodilatation metabolic nonsystematic clinical reviews of sauna activity published in pathways [22]. It has been suggested that heat stress induces the early 2000s. Afer further restrictions of English language adaptive hormesis mechanisms similar to exercise, and there and humans, records were then expanded using Google are reports of cellular efects induced by whole-body hyper- Scholar, with searches for other research by key authors, thermia in conjunction with oncology-related interventions searches of citations and reference lists of original and review (i.e., chemotherapy and radiotherapy) [23]; however the articles, and other “related articles”. Additional searches with mechanisms by which the physiological and cellular changes expanded keywords relating to sauna including “interven- induced by sauna bathing contribute to enhanced health tional study”, “whole body hyperthermia”, and “whole body and/or therapeutic efects is still being explored [4, 7, 8, 24– thermotherapy” were also conducted with the same initial 27]. restrictions. Te following systematic review was undertaken to explore recent research on the clinical efects of repeated dry 2.3. Data Extraction. Abstracts of initially identifed studies sauna bathing (Finnish-style, infrared, or other dry sauna were screened by investigator JH and then the complete forms) to document the full range of medical conditions full-text articles of potentially eligible studies were carefully saunas have been used for, as well as any associated health screened by both investigators JH and MC for research benefts and/or adverse efects observed. While a small design, population descriptive data, timing and physical number of reviews of sauna bathing and health have been details of dry sauna intervention, outcome measures, key conductedinthepast[1,2,28–30],asfarasweknow,thisis results, and adverse efects. Discrepancies regarding inclusion the frst systematic review of sauna and health to include both of studies or data extraction were discussed until consensus Finnish and infrared saunas. Furthermore, this review only was reached. considers efects related to regular, multiple sessions of sauna activity rather than single sauna sessions, to better refect the 2.4. Assessment for Risk of Bias. Included randomized con- use of sauna bathing as a regular lifestyle intervention. trolledtrials(RCTs)wereassessedforriskofbiasaccordingto Evidence-Based Complementary and Alternative Medicine 3
PubMed Web of Science Scopus Proquest-Health and Medicine Jan 2000–April 2017 Jan 2000–April 2017 Jan 2000–April 2017 Jan 2000–April 2017 484 citation(s) 843 citation(s) 803 citation(s) 1155 citation(s)
906 nonduplicate citations screened
Inclusion-English language and human 738 articles excluded Exclusion-gay (MSM) focus, steam/wet sauna, hydrotherapy, aer title/abstract screen partial body heating, animal-based studies, sauna-personal name
168 articles retrieved
Same inclusion/exclusion 99 articles excluded 29 articles excluded criteria applied to full text aer full-text screen during data extraction
40 articles included
Figure 1: PRISMA fow diagram of evidence searches and inclusions/exclusions.
theCochraneCollaboration’stoolforassessingbiasandcal- repeated sauna therapy, which is the stated focus of this culated Jadad et al. scores [72]. Domains of bias assessed were review. selection bias (by looking for random sequence generation A total of 40 studies remained for inclusion in this and allocation concealment), performance bias (by published systematic review. A summary of extracted data is presented mention of blinding of participants and personnel), detection in Tables 1–7, with tables categorised according to participant bias (by documented attempts to blind outcome assessment), population. attrition bias (by evaluating for incomplete outcome data), reporting bias (by any indication of selective reporting of 3.2. Study Design. Of the forty studies, 13 were randomized outcomes), and other bias (e.g., conclusions not clearly controlled trials (RCTs), 6 were trials with nonrandomized supportedbyreportedoutcomes).Riskofbiaswasinitially control groups and 2 were prospective cohort studies. Te assessed by investigator JH as “low”, “unclear”, or “high” and remainder of studies were single-group or multigroup inter- then confrmed by investigator MC. Any discrepancies were ventional trials (without a control group) or retrospective discussed until consensus was reached. studies. Te following three levels of evidence were used to help stratify the quality of the studies.
3. Results Levels of Evidence 3.1. Literature Search. Figure 1 summarises the screening and Level I: multicentre or single-centre, randomized assessment strategies used with the search results. Of the 906 controlled trial (RCT) nonduplicate citations initially identifed, 738 were excluded Level II: controlled interventional trial; prospective afer a review of the abstracts. cohort study Afer retrieving 168 full-text articles and applying the same exclusion criteria as discussed above along with exclud- Level III: retrospective comparative study; case- ing review articles, case reports, and letters to the editor, 69 control study; pilot study. independent human studies involving dry sauna interven- tions were identifed for further analysis. 3.3. Limitations/Risk of Bias. Many studies were relatively In the data extraction step, one study was excluded since small, with limited number of participants, and a limited it was essentially a case series with two patients, mistakenly number of randomized studies were available for review. Of identifed as an interventional trial conducted by a key the 13 randomized controlled trials (RCTs) identifed, only 3 author [73]. Another 28 studies were excluded due to the of these studies (involving 343/840 participants) [31, 50, 58] intervention being only a single session of sauna and not were assessed with having a low overall risk of bias according 4 Evidence-Based Complementary and Alternative Medicine , , wt Mild Mild None None severe efects sauna but moderate/ pain during None/mild/ few sessions transient leg Adverse side hypovolemia, decreased BP, polyurination, resolved afer a decreased body ), � < 0.05 , �<0.05 � < 0.05 ), ,decreaseinleg , , , Health efects increase in 6 MWD ( � < 0.01 ), improved ABI ( � < 0.01 ), 2-fold increase in mRNA CD34/GAPDH gene expression levels ( � = 0.015 ), increases in serum nitrate and nitrite levels ( � < 0.05 )insaunagroup compared to control group Positive pain scores ( Positive/negative/negligible Positive sauna group with reduced concentration of hydroperoxide ( � < 0.001 ); reduced BNP levels ( � < 0.001 ); increased nitric oxide metabolites ( � < 0.05 ) Positive improved 6 MWD ( � < 0.05 ), reduced CTR on CXR ( improved NYHA classifcation ( � < 0.05 ) compared to control group Positive mean HR decreased ( � < 0.05 )insaunagroup compared to control group. High frequency component of HRV in setting ofblockade beta improved nitrite 6MWD measures Outcome nitrate, nitrite plasma levels of Body weight, BP, and HRV (heart rate ECHO, fasting plasma on chest X-ray, NYHA variability) parameters (ankle-brachial index), distance), PCR-CD34+ levels of BNP, uric acid, hydro-peroxide, nitrate, class, plasma BNP levels on chest X-ray, standard 6 MWD (6-min walking on chest X-ray, LVEF on levels of VEGF (vascular (6 min walking distance), levels in peripheral blood mononuclear cells, serum ECHO parameters, fasting catechol-amines and BNP; Leg pain (using VAS), ABI Body weight, BP,HR, CTR endothelial growth factor), progenitor gene expression cardio-thoracic ratio (CTR) CTR (cardio-thoracic ratio) Comparators controls standard standard standard standard medical care medical care medical care medical care Comparator/ Control group, Control group, Control group, Control group, Duration Intervention type Sauna � Table 1: Cardiovascular disease- (CVD-) related sauna studies. Study sample Advanced CHF/Japan 149 FIR 2 weeks RCT- Design Pop/country multicentre I I RCTI CHF/Japan 40 RCT FIR 4 weeks CHF/JapanIRCTPAD/Japan21FIR6weeks 54 FIR 4 weeks Level of evidence Study Characteristics Author & year 2016 Tei et al. [31] 2011 Fujita et al. [32] 2011 Kuwahata et al. [33] 2010 Shinsato et al. [34] Evidence-Based Complementary and Alternative Medicine 5 None None None None severe efects moderate/ None/mild/ Adverse side ). )and levels �<0.05 � < 0.05 ), 2� � < 0.01 ), �<0.05 � < 0.01 ), � < 0.001 ; � < 0.05 , , , , , � < 0.01 )insauna Health efects Positive sauna bathing 4 − 7 times a week associated with 66% risk reduction (hazard ratio 0.34, 95% CI) in developing dementia or Alzheimer’s compared with 1 time/week Positive fewer PVCs ( fewer couplets ( fewer episodes of VT ( � < 0.01 ), decreased CTR ( � < 0.05 ), increased HRV variability ( lowered serum levels of BNP ( treatment group compared to control group Positive/negative/negligible ( � < 0.001 ) signifcantly lower in sauna group compared to control group Positive systolic BP ( urinary 8-epi- prostaglandin F Positive BP and CTR decreased in both groups (sauna � < 0.01 , control Body wt decreased ( � < 0.0001 ); LVEF on ECHO increased ( � < 0.0001 ); plasma BNP decreased ( � < 0.001 )in sauna group compared with control group 2� measures Outcome Incidence plasma BNP questionnaire, disease and other concentrations of parameters, plasma recordings with HRV dementia/Alzheimer’s 24-hr ambulatory ECG CVD-related outcomes Body wt, HR, BP,HCT, (cardiothoracic ratio) by 8-epi-prosta-glandin F analysis (std deviation of chest X-ray, usual ECHO mean RR intervals), CTR ECHO parameters, fasting Self-assessed quality of life and glucose, urinary levels fasting plasma lipid profle BP, HR, body weight, body ratio) on chest X-ray, usual temp, CTR (cardio-thoracic catechol-amines, ANP, BNP Comparators Cfor Cfor ∘ ∘ in a in a 45 min. 45 min. placebo placebo controls standard 1 time/wk, duration of intervention intervention medical care 2-3 time/wk, Comparator/ 4–7 times/wk Control group Control group sauna bathing: Frequency and Control group- temp-controlled temp-controlled -supine on a bed -supine on a bed room at 24 room at 24 Duration Table 1: Continued. Intervention type Sauna � 28 FIR 2 weeks 30 FIR 2 weeks 2315 Finnish 20.7 years Study sample Risk/Japan CHF/Japan 188 FIR 2 weeks CHF/Japan Middle-aged males/Finland Increased CVD Cardiac arrhythmias, RCT RCT RCT Design Pop/country Prospective cohort study I I I II Level of evidence Study Characteristics Author & year 2008 Miyata et al. [35] 2004 Kihara et al. [36] 2004 Masuda et al. [37] 2016 Laukkanen et al. [38] 6 Evidence-Based Complementary and Alternative Medicine None None None None severe efects moderate/ None/mild/ Adverse side , � < 0.01 ); � < 0.05 ) � = 0.025 � = 0.035 ; �<0.05 � < 0.001 ) and � = 0.023 ; , , , , Health efects Positive sauna bathing 4–7 sessions weekly associated with 40% reductioninall-cause mortality compared with 1 session weekly, (hazard ratio 0.60, 95% CI, 0.46–0.80, Positive increased LVEF (lef ventricular ejection fraction), reduced levels of norepinephrine and BNP, � = 0.015 Positive/negative/negligible increased 6 MWT, � < 0.001 ;improvedFMD, � < 0.001 ;increased CD34+ counts, Positive decrease in VSD shunt fow ratio ( � < 0.05 ), increase24 in h urine nitrite and urine nitrate levels ( � < 0.05 ); surgical repair not necessary for 9/12 (75%) infants Positive improved indices of defect reversibility on myocardial perfusion scans ( extended treadmill times ( � < 0.01 ), improved fow-mediated dilation of brachial artery ( afer sauna therapy compared to control group artery mortality measures Outcome parameters; expression of cardiac death, including VSD standard ECHO and nitrite levels stress testing and scintigraphy with fatal CVD, all-cause norepinephrine and CD34-positive bone Incidence of sudden plasma levels of BNP, marrow-derived cells Myocardial perfusion (FMD) of the brachial fow-mediated dilation circulating CD34+ cells; vaso-dilation of brachial usual ECHO parameters artery, treadmill exercise Core body temp, HR, BP, adenosine, fow-mediated 6 MWT (6-min walk test); Doppler, 24 h urine nitrate measurements with colour fatal coronary heart disease, Comparators group group controls standard 1 time/wk, No control No control duration of medical care 2-3 time/wk, Comparator/ 4–7 times/wk sauna bathing: Frequency and Control group, Duration Table 1: Continued. Intervention type Sauna 12 FIR 4 weeks � 24 FIR 3 weeks 2315 Finnish 20.7 years Study sample Infants- CHF/Japan 41 FIR 3 weeks Middle-aged males/Finland occlusion/Japan VSD and CHF/Japan IHD with total coronary Design Pop/country Controlled Prospective Single group Single group cohort study clinical study clinical study clinical study II II II III Level of evidence Study Characteristics Author & year 2015 Laukkanen et al. [39] 2013 Sobajima et al. [40] 2003 Sugahara et al. [41] 2012 Ohori et al. [42] Evidence-Based Complementary and Alternative Medicine 7 None None None None severe efects moderate/ None/mild/ Adverse side for all) � < 0.01 ) � < 0.05 ), � < 0.05 ), � < 0.05 )with � < 0.05 ), � < 0.01 , , , , � < 0.01 ); 38% Health efects Positive/negative/negligible Positive 8/64 patients died in sauna therapy group vs 12/65 patients in control group (12.5% vs 18.5% mortality rate); Rehospitalization due to worsening CHF occurred in 20/64 (31.3%) patients in sauna group vs 44/65 (68.7%) patients in control group ( reduction in cardiac event rate in sauna therapy group compared to control group Positive decreased SBP ( improved CTR ( improved LVEF on ECHO ( � < 0.05 ), increased 6MWT( decreased plasma norepinephrine and epinephrine levels ( � < 0.01 , sauna intervention. Reduced number of hospitalisations ( one-year afer sauna intervention Positive pain scores decreased, 6MWDimproved,ABI improved, increase in visible collateral vessels in ischaemic legs with digital subtraction angiography observed afer sauna therapy ( Positive improved stress ( � = 0.042 ), fatigue ( � = 0.014 ), general health ( � = 0.037 )onSF-36 CHF measures Outcome ABI (ankle/ intervention angiography cardiac events, on bicycle ergometer; Body wt, BP,HR; 2 one-year afer sauna on chest X-ray; usual BNP, catecholamines; questionnaire; 6 MWT fow with Doppler laser (visual analogue scales) health survey) and VAS (6 min walk time); peak rehospitalisations due to analogue scale), 6 MWD (6 min walking distance), brachial index), leg blood Episodes of cardiac death, SF-36 (36-item short form Self-assessed quality of life number of hospitalisations ECHO parameters, plasma Leg pain using VAS (visual VO CTR (cardio-thoracic ratio) imaging, digital subtraction Comparators group group group controls standard No control No control No control medical care Comparator/ Control group, Duration Table 1: Continued. Intervention type Sauna � Study sample PAD/Japan 20 FIR 10weeks CHF/Japan 129 FIR 5 years CHF/Japan 15 FIR 4 weeks Type 2 diabetes/Canada 15 FIR 3 months trial trial Design Pop/country clinical clinical time series sequential, study/pilot study/pilot interrupted Single group Single group cohort study longitudinal, Single group, Retrospective III III III III Level of evidence Study Characteristics Author & year 2010 Beever [43] 2009 Kihara et al. [44] 2007 Tei et al. [45] 2005 Miyamoto et al. [46] 8 Evidence-Based Complementary and Alternative Medicine None None None severe efects moderate/ None/mild/ Adverse side � = 0.019 ), � < 0.05 ); � = 0.047 ), � < 0.05 ); body � < 0.05 ), FBG , , , � = 0.005 ), Health efects Positive SBP and DBP reduced ( � < 0.01 , wt reduced ( fasting glucose levels decreased ( � < 0.05 ); % fow mediated dilation of brachial artery improved ( � < 0.001 )insaunagroup but no statistical report of comparisons with control group Positive decreased SBP ( decreased CTR on CXR ( � = 0.002 ), decreased LVEDD (lef ventricularend-diastolic dimension) on ECHO ( decreased plasma BNP levels ( improved fow-mediated dilation of brachial artery on Doppler USS ( � = 0.0006 )insauna group compared to control Positive/negative/negligible Positive decreased body wt ( � < 0.05 ), SBP and( � DBP < 0.01 , ( � < 0.05 ); Improved fow mediated dilation of brachial artery ( � < 0.001 )insaunagroup but results compared to control not presented echocardiogram; VAS = visual analogue scale; FBG = una; VSD = ventricular septal defect; NYHA = New York Heart measures Outcome substances; Self-assessed Doppler USS quality of life Doppler USS; serumleptinlevels artery diameter and mediated dilation of brachial artery using brachial artery using on chest X-ray; usual fow-mediated dilation questionnaire; HR, BP; fasting plasma levels of Bodywt,HR,BP,HCT; substances, TNF-alpha; glucose, uric acid levels; USS; plasma ghrelin and resting arterial diameter; Body wt, HR, BP; fasting fow mediated dilation of profle, uric acid, glucose, using Doppler ultrasound fasting serum lipid profle, brachial artery on Doppler serum levels of HCT, Lipid nitroglycerin-induced fow fow mediated dilatation of catecholamines, ANP, BNP, thiobarbituric acid-reactive thiobarbituric acid-reactive CTR (cardio-thoracic ratio) ECHO parameters; brachial Comparators factors diseases controls any lifestyle medical care 10/35 control 10/30 control Comparator/ 10/35 without any CVD risk group without Control group group, standard Duration Table 1: Continued. Intervention type Sauna � 35 FIR 2 weeks 35 FIR 2 weeks Study sample Obesity, risk/Japan CHF/Japan 30 FIR 2 weeks HTN/Japan Increased CVD T2DM, smoking, hypercholesterolaemia, group group group Design Pop/country with control with control with control Clinical study Clinical study Clinical study III III III Level of evidence Study Characteristics Author & year 2003 Biro et al. [47] 2002 Kihara et al. [48] 2001 Imamura et al. [49] Association grading for CHF; temp = temperature; HR = heart rate; SBP = systolic blood pressure; DBP = diastolic blood pressure; wt = body weight; ECHO = CVD = cardiovascular disease; CHF = congestive heart failure; IHD = ischaemic heart disease; PAD = peripheral arterial disease; FIR = far-infrared sa fasting blood glucose; BNP = B-natriuretic peptide; HCT = haematocrit. Evidence-Based Complementary and Alternative Medicine 9 , and None None severe efects 2 patients Moderate moderate/ None/mild/ Adverse side claustrophobia excluded -could -acutebronchitis not tolerate sauna , ± � -test = �<0.05 � ); decrease 1.1 to 2.2 ∗ = 10.17; df = �<0.05 ± � � < 0.0001 )in ,improved , , = −2.65 , �<0.05 � � = 0.002 ) ∗ � < 0.001 ) = −2.32 , � ∗ Health efects Positive somatic complaints ( � < 0.001 ), improved hunger scores ( � < 0.0001 ), and improved relaxation scores ( sauna group compared to control group. Plasma ghrelin concentrations and daily caloric intake increased in sauna group ( in anger scoring in sauna group compared to control (4.5 1.6, Student 2-tailed respectively); 1,117; Positive 44% reduction in HA intensity in 6 weeks of treatment arm. Mean change in headache intensity between sauna andcontrolgroup=1.27 points (95% CI 0.48–2.07; Positive increased likelihood of return to work 2 years later ( Positive/ negative/ negligible and Index); measures Outcome caloric intake. for hunger and HDI (Headache Disability Index) scoring; degree of VAS for pain; pain with CMI (Cornell of ghrelin, glucose, scale); anger scoring NPRS (numeric pain Medical Index); sleep Zung SDS (self-rating catechol-amines; daily CMI (Cornell Medical (self-rating depression with simple numerical depression scale); VAS Depression Inventory), years afer intervention rating scale), BDI (Beck researchers with 11-item relaxation; plasma levels quality with simple 0–10 behaviour assessment by Somatic complaints with scoring; return to work 2 questionnaire; Zung SDS satisfaction of treatments Comparators Cand ∘ CBT, therapy therapy controls placebo, received course of programs, at 24 behavioural counselling, and exercise occupational Comparator/ in addition to received same rehabilitation, and education Control group Control group Control group 45 min bedrest the same rehab received advice postrest shower physical therapy, 8weeks Intervention cards sauna types, voucher Multiple Sauna type Duration � 37 28 FIR 4 weeks Table 2: Sauna studies of rheumatological disease/chronic pain/depression. Study sample Mild Japan depression/ Chronic tension headache/New Zealand Design Pop/country IRCT I RCT Chronic pain/Japan 46 FIR 4 weeks IRCT Level of evidence Study characteristics 2015 Kanji et al. [50] 2005 Masuda et al. [51] 2005 Masuda et al. [52] Author & year 10 Evidence-Based Complementary and Alternative Medicine - - None None Mild severe efects of sauna Moderate moderate/ − 24% scoring None/mild/ Adverse side 12 well-being scores during beginning protocol changed. uncomfortable on most participants, heat intolerance in ), )and � < 0.001 ); � = 0.008 ), � = 0.018 ), )afersauna )groups )andAS ); reduced –0.05) afer , decreased , , reduced VAS , Health efects Positive fatigue ( �=0.002 improved POMS scores for anxiety ( depression ( fatigue ( �=0.005 performance status ( �=0.005 Positive pain and stifness decreased in RA ( �<0.05 ( �<0.001 during sauna sessions only. Positive pain scores ( fewer # of tender pts ( �<0.01 symptoms based upon FIQ ( � < 0.001 ); improved quality of life on SF-36 questionnaire ( �<0.01 combined sauna + underwater exercise therapy Positive/ negative/ negligible Positive 7/9 in sauna group improved during sessions; 4/9 were still improved at follow-up 9 − 40 months aferwards; 2/9 non-responders. 3/6 controls receiving usual treatment improved at follow-up measures Outcome questionnaire questionnaire of mood states) (state-trait anxiety index); serum ESR SF-36 quality of life for depression); STAI measurement scales), Spondylitis functional impact questionnaire); (Dutch arthritis impact self-rating questionnaire exam; FIQ (fbromyalgia inventory questionnaire) VAS, EPM-ROM (Escola BASMI (Bath Ankylosing index of range of motion), fatigue and POMS (profle VAS-visual analogue scale; Spondylitis disease activity BASDAI (Bath Ankylosing Comparators Numerical rating scales for Paulista de Medicina range no. of tender pts on clinical of motion), DUTCH-AIMS SF-36 survey; SRQ-D (brief with sauna group not to groups exercise controls undergo receiving combined 6/15 chose group; two No control No control one part of Sauna only therapy; no same sauna underwater intervention intervention intervention; Comparator/ control group Duration Table 2: Continued. Intervention type Sauna 15 FIR 8 weeks 10 FIR 4 weeks � 34 FIR 4 weeks 44 FIR 12 weeks Study sample fatigue (RA) and autoimmune Females with Chronic fatigue disorders/Japan fbromyalgia and syndrome/myalgic Rheumatoid arthritis Females with chronic (AS)/Te Netherlands ankylosing spondylitis syndrome (CFS)/Japan encephalomyelitis/Japan trial side) study study group group group Single- Single- Design Pop/country control clinical clinical Clinical 2single- (side-by- pilot trials study with group, pilot intervention III III III III Level of evidence Study characteristics Author & year 2009 Oosterveld et al. [53] 2015 Amano et al. [54] 2015 Soejima et al. [55] 2011 Mat- sumoto et al. [56] FIR = Far-infrared sauna; ESR = erythrocyte sedimentation rate; VAS = visual analogue scale; CBT = cognitive behavioural therapy. Evidence-Based Complementary and Alternative Medicine 11 , Mild None severe efects moderate/ group; 1 in None/mild/ in treatment face mask in Adverse side stimulated by cough directly both groups (2 control group). ± 9.8% 46.7, ± ± 1 − 1.0 � = 0.04 , 5.7%, � = 0.003 )in , ± , � = 0.01 ,95% − 0.1), Positive/negative/ negligible (forced expiratory volume at 1 sec) improved (77.5% to 95.6% � = 0.002 )insauna groupcomparedwith control group. Negligible on day 2 only, signifcant decrease in symptom severity in treatment vs control group [ ( − 2.0– 95% CI] but was not sustained through day 3, 5, 7 assessments. Less cold medication takenonday1only[3% (1–9%) vs 15% (8–28%)] in treatment vs control group ( CI). Positive reduced high-freq component ( � = 0.003 ), increased low-freq component ( � = 0.003 ), increased low freq: high freq ratio ( HRV analysis; peak nasal inspiratory fow improved (119.2 L/s 46.4 to 161.9 L/s � = 0.002 ); FEV measures Outcome and usual spirometry parameters of intervention. inspiratory fow HRV, peak nasal cold medications scoring (0–10) on daily during week Symptom severity intake of common four diferent days; C, Comparators Health efects ∘ care control C, 20% RH received ∘ treatment Face mask 20%RHin group; Face Comparator/ breathing hot usual medical education and 24 Control group cool, dry air at dry air at 90 mask breathing in control group. Duration Intervention type Sauna � 26 Tai/Finnish 6 weeks 157 Finnish 3 days Table 3: Airway conditions and repeated sauna therapy. Study sample Pop/ country Coryza/ Tailand Germany symptoms/ common cold Allergic Rhinitis/ Single RCT – Design blinded IRCT I Level of evidence Study characteristics Author & year 2013- Kunbootsri et al. [57] 2010- Pach et al. [58] 12 Evidence-Based Complementary and Alternative Medicine None None severe efects moderate/ None/mild/ Adverse side 2 50 ); ± ± 17.2 pts, 97 s, − 0.01 L/s ± ± �=0.022 , , Positive between-group improvements in FEF Positive decreased SBP and DBP ( � = 0.002 –0.0002); improvements in RV function via increased pressure diferential ( � = 0.024 ); Pulmonary artery pressure during exercise decreased ( � = 0.028 ); increased exercise time (360 s 107sto392s � = 0.032 ); lowest SpO Positive/negative/ negligible (forced expiratory fow afer 50% of expired forced vital capacity) in sauna group [+0.08 L/s (0.01–0.212 L/s)] vs control group [ during exercise increased ( ( − 0.075–0.04 L/s)], � = 0.019 . symptom scores improved (59.7 pts 16.9 to 55.3 pts � = 0.002 )afersauna. ure; DBP = diastolic blood pressure; wt = weight; temp s = electrolytes with liver function tests. PR albumin measures Outcome by bicycle treatment. Spirometry Respiratory before/afer parameters; (St. George’s Questionnaire) BP, PR, body wt, symptom scores; body temp; usual exercise tolerance ergometer; SGRQ oxygen saturation; plasma BNP, HCT, 6 MWT (6-minute ECHO parameters; Borg dyspnea scale; walk test); modifed Comparators Health efects group control No control medical care Comparator/ received usual Control group Duration Intervention type Sauna Table 3: Continued. 13 FIR 4 weeks � 20 FIR 4 weeks Study sample Pop/ Japan Japan COPD/ country Male COPD Ex-smokers/ trial Design pilot study Controlled intervention Single group intervention, II III Level of evidence Study characteristics Author & year 2014- Kikuchi et al. [59] 2008- Umehara et al. [60] COPD = chronic obstructive pulmonary disease; FIR = far-infrared sauna; PR = pulse rate; HR = heart rate; BP = blood pressure; SBP = systolic blood press = body temperature; HRV = heart rate variability; freq = frequency; RH = relative humidity; ECHO = echocardiogram; BNP = B-natriuretic peptide; E/LFT Evidence-Based Complementary and Alternative Medicine 13 – Mild comments of “hot and very uncomfortable, but tolerable” per thermal comfort survey conducted every 5 min during sauna sessions Adverse side efects None/mild/ moderate/ severe None to by )afer 2 2 C ∘ � < 0.005 ) �<0.002 C( , increased axillary , � < 0.001 ), � < 0.001 ), ∘ Health efects Positive body temp 2.6 Positive postexercise sauna bathing increased plasma volume afer 4 days of intervention ( � < 0.01 ) course of sauna; increased pH by 0.8% ( �
Table 8: Risk of bias assessment in randomized controlled trials. Blinding of Random Blinding of Allocation participants Incomplete Selective Jadad et al. sequence outcome Other bias concealment and outcome data reporting score [72] generation assessment personnel Fujita et al. 2011 × ? ××✓✓? <3 Huppe¨ et al. × ? ✓✓✓× ? <3 2009 Kanji et al. 2015 ✓✓✓✓✓✓✓4 Kihara et al. × ? ××✓??<3 2004 Kunbootsri et al. × ? ××✓✓? <3 2013 Kuwahata et al. × ? ××✓✓? <3 2011 Masuda et al. × ? ××✓✓×<3 2004 Masuda et al. ✓ ? ××✓✓×<3 2005 -pain Masuda et al. 2005 × ? ××✓✓×<3 -depression Miyata et al. × ? ××✓✓? <3 2008 Pach et al. 2010 ✓ ? ✓✓✓✓ ?5 Shinsato et al. × ? ××✓✓? <3 2010 Tei et al. 2016 ✓ ? ×✓✓✓ ?3 ✓:lowriskofbias;×: high risk of bias; ?: unclear risk of bias.
totheCochraneCollaborationcriteria[71]andaJadadetal. studies of populations overburdened with environmental score > 3[72].Nineofthese13RCTsenrolledfewerthan50 toxicants. participants. Table 8 summarises the assessments of the RCTs for overall risk of several types of bias. 3.5. Interventions. Eleven studies investigated the use of Tefollow-uptimeofmanyofthestudieswasrelatively Finnish saunas and 25 studies utilised infrared sauna inter- short, in the order of weeks to months, thereby possibly com- ventions. Te remainder 4 studies used other forms of dry promising detectability and reporting of long-term health sauna (Tai-style or mixed). Sauna sessions varied from 5 efects over years. minutes to 20 minutes in single or multiple sessions totaling 30 minutes–4 hours daily, once to several times each week 3.4. Setting and Participant Characteristics. Te reviewed over study durations that ranged from 3 days to 5 months. studies included a total of 3855 participants living in 12 Te cohort studies followed frequent infrared sauna bathers diferent countries. Over half of the studies (22 of 40) for 5 years and frequent male Finnish sauna bathers for over originated in Japan. Te smallest study involved Australian 20 years. athletes (�=7) and the two largest studies (both prospective All of the studies involving Finnish-style saunas used ∘ cohort studies) involved the same cohort of 2315 Finnish men interventions ranging in temperature from 80 to 90 Cwith [38,39,61].Moststudieshadsmallsamplesizeswithoverhalf relative humidity levels of 10–20% except Huppe¨ et al. 2009, (21 of 40 studies) involving 30 or less participants. a study comparing detoxifcation protocols, which employed ∘ Te studies involved a range of healthy and disease a lower temperature sauna at 50–65 Cwith30%relative populations with 6 studies of healthy individuals, 19 studies humidity for 15 minutes in one intervention arm [68]. of people diagnosed with cardiovascular disease (CVD) or Of the 25 studies involving infrared sauna, all used increased risk for CVD (i.e., congestive heart failure, type 1 far-infrared types except Ross and Sternquist 2012, which or type 2 diabetes mellitus, and peripheral arterial disease), 7 employed a full-spectrum infrared sauna as part of a studies of patients diagnosed with rheumatological, chronic detoxifcation protocol for policemen [69]. All infrared ∘ pain, or mood disorders, 4 studies of patients diagnosed with sauna studies entailed sauna exposures at 60 C for 15–30 airway-related disorders (i.e., chronic obstructive pulmonary minutes with the exception of two studies: Amano et al. disease, allergic rhinitis), 2 studies of elite athletes, and 2 2015 studying the efects of sauna on patients diagnosed 20 Evidence-Based Complementary and Alternative Medicine with chronic fatigue syndrome/myalgic encephalomyelitis serum lipid profles (total cholesterol, LDL, HDL, and triglyc- ∘ ∘ (CFS/ME) using saunas set at 40 C–45 Cfor15minute erides), fasting plasma glucose levels, serum levels of uric sessions [54] and Oosterveld et al. 2009 examining the efects acid (potential marker of insulin resistance and metabolic ∘ of sauna set at 55 C for 30-minute sessions on patients syndrome), plasma levels of ghrelin, serum levels of leptin, diagnosed with Ankylosing Spondylitis and Rheumatoid plasma levels of Hb (hemoglobin) and HCT (haematocrit), Arthritis [53]. and urinary prostaglandin levels [37, 47, 49, 63, 66, 67]. All of the sauna interventions were conducted in super- Other specifc objective outcome measures performed vised settings (i.e., in-hospital, rehabilitation hospitals, health before/afer sauna include myocardial perfusion scintigraphy centres, university or medical laboratories, and outpatient with adenosine, treadmill exercise stress test results, fow- programs) except Kanji et al. 2015, which provided sauna mediated vasodilation of brachial artery, and expression voucher cards to allow participants to attend saunas of choice of CD34-positive bone marrow-derived cells in hospital attached to local swimming pools [50] and the two large patients with ischemic heart disease and total coronary cohortstudiesthatfollowedFinnishmenattendingsaunasof occlusion; standard spirometry parameters, peak nasal inspi- their choice [38, 39]. ratory fows, and ECG (electrocardiogram) with HRV (heart rate variability) parameters in participants diagnosed with 3.6. Outcome Measures. Some studies focused solely on allergic rhinitis; plasma volume changes (calculated from measuring subjective quality of life and symptom scoring hemoglobin readings), hydration status using urine specifc surrounding sauna activity such as SF-36 (36-item short gravity, exercise performance on ergometer, and ECG with form health survey); FASE (Foundation for Advancements HRV parameters in elite athletes; axillary body tempera- in Science and Education) 50-item survey of symptoms and tures, venous blood gas panels, lipid peroxidation by UV sleep, CMI (Cornell Medical Index) survey of somatic com- (ultraviolet light) and fuorescence analysis, and nitric oxide plaints; VAS (visual analogue scales) for hunger, relaxation, levels in elite athletes; transepidermal water loss, stratum and specifc types of pain (i.e., leg pain); various numeric corneum hydration, skin erythema, skin surface pH, surface rating scales for pain, fatigue, sleep quality, and common cold sebum contents, and NaCl (sodium chloride) concentrations symptoms; validated tools for depression, anxiety, headache in sweat of healthy men and women; basic physiological disability,andangersuchasPOMS(profleofmoodstates) observations (temperature, heart rate, blood pressure, and questionnaire, BDI (Beck Depression Inventory), SRQ-D body weight), calculated plasma volumes, and serum levels (self-rating questionnaire for depression), Zung SDS (self- of thyroid function (TSH (thyroid stimulating hormone), rating depression scale), STAI (state-trait anxiety inventory T3, and T4) and other hormones (human growth hor- questionnaire), and HDI (Headache Disability Index) [43, 50, mone, adrenocorticotropic hormone, and cortisol) in healthy 51,54,55,69]. women; and pre-and postintervention semen analysis includ- Other interventional studies focused on obtaining objec- ing standard sperm parameters, sperm chromatin structure tive measures related to sauna activity. For example, the studies involving CHF patients tracked combinations of analysis, sperm apoptosis, quantitative sperm heat-stress physiological changes using body weight, body temperature, gene expression levels, and plasma levels of male sex hormone HR (heart rate) or PR (pulse rate) and SBP and DBP levels (LH (luteinizing hormone), FSH (follicle stimulating (systolic and diastolic blood pressures); exercise tolerance hormone), testosterone, and inhibin) in healthy men. using the 6 MWD (6-minute walking distance) and peak VO2 Other interventional studies employed a combination of (peak/maximum volume of oxygen) on bicycle ergometer; subjective and objective measures. Shinsato et al. 2010 and cardiomegaly/heart enlargement using CTR (cardiothoracic Tei et al. 2007 compared VAS for leg pain as well as 6 MWD ratio) on CXR (chest X-ray); cardiac fow performance (6-minute walking distance), ABI (ankle/brachial index), leg using standard ECHO (echocardiogram) Doppler ultra- blood fows with Doppler laser imaging and angiography, sound parameters; overall functional state using clinician- gene expression levels of CD34+ blood cells and serum levels based NYHA (New York Heart Association) classifcation; of VEGF, and nitrates and nitrites in patients hospitalised endovascular reactivity using FMD (fow-mediated dilation with peripheral artery disease [34, 45]. Kikuchi et al. 2014 and of brachial artery); heart failure activity using plasma levels Umeharaetal.2008assessedmodifedBorgdyspnoeascale of BNP (B-natriuretic peptide); autonomic nervous system or SGRQ (St George’s Respiratory Questionnaire) in addition and immune-mediated activity using ECG (electrocardio- to basic physiological observations (temperature, BP, HR, gram) recordings with heart rate variability parameters and respiratory rate, and O2 saturation), standard spirometry and plasma levels of norepinephrine, TNF-� (tumour necrosis ECHO parameters, 6 MWD or ergometer exercise tolerance, factor-alpha), and CD34+ (cluster of diferentiation 34, bone andplasmalevelsofBNP,HCT,andalbumininhospi- marrow derived) cells; endovascular activity using plasma talisedpatientswithCOPD[59,60].Oosterveldetal.2009 levels of VEGF (vascular endothelial growth factor), nitric utilised subjective VAS and validated tools of EPM-ROM oxide metabolites (nitrate and nitrite), and reactive oxygen (Escola Paulista de Medicina-range of motion), DUTCH- metabolites (hydroperoxide) [31–33, 35, 36, 41, 42, 46, 48]. AIMS (Dutch arthritis impact measurement scales), BASMI Studies involving patients with increased cardiovascular risk (Bath Ankylosing Spondylitis functional index range of or studies of healthy patients with aims of detecting changes motion),andBASDAI(BathAnkylosingSpondylitisdisease in cardiovascular risk with sauna activity used some of the activity index), as well as serum levels of ESR (erythrocyte same physiological parameters mentioned above as well as sedimentation rate) [53]. Huppe¨ et al. 2009 used several Evidence-Based Complementary and Alternative Medicine 21 self-assessed validated scoring tools: Beschwerden-Liste 24- ankle/brachial index (ABI) (� < 0.01),andanincreaseinvis- item questionnaire of somatic symptoms, ADS-L/CES-D 20- ible collateral vessels in ischemic legs with digital subtraction item questionnaire of general depression, and SF-36 quality angiography (� < 0.01)observedafer10weeksofrepeated of life questionnaire. Objective tests of neuropsychologi- sauna therapy in twenty patients [45]. Te second study was a cal processing speed (GT-MT/ZVT scoring), concentration randomized controlled trial (�=21)whichreportedsimilar (attention test d2), memory power and speed (WL-N and decreases in VAS (visual analogue scale) leg pain scores (�< WL-G scoring, resp.), and serum levels of three diferent 0.05), increases in 6 MWD (� < 0.01), and improved ABI PCB (polychlorinated biphenyl) congeners, hexachloroben- (� < 0.01) in the sauna treatment group compared with zene, DDT (dichlorodiphenyltrichloroethane), and DDE no change in the control group that received conventional (p-dichlorodiphenylethylene) were measured before and/or medical therapy. Te investigators of this second study also afer sauna interventions [68]. demonstrated a 2-fold increase in mRNA CD34/GAPDH Te two largest prospective cohort studies (� = 2315) expression in peripheral blood mononuclear cells (� = 0.015) tracked the incidence of dementia, Alzheimer’s disease, and increases in serum nitrate and nitrite levels (� < 0.05, and other cardiovascular disease-related outcomes such as � < 0.05) in the sauna group with no respective changes in sudden cardiac death, fatal coronary artery disease, fatal the control group and no signifcant changes in serum VEGF cardiovascular disease, and all-cause mortality over 20+ levels in either group [34]. years, stratifed by sauna bathing one time each week, 2-3 Another randomized controlled trial examined the efects times each week, or 4−7 times each week [38, 39]. Te one of repeated sauna therapy on 24 ischemic heart disease retrospective cohort study (� = 129) tracked episodes of subjects with chronic total occlusion of coronary arteries cardiac death, cardiac events, and rehospitalisations due to detected on coronary angiogram who had failed or rejected congestive heart failure afer completion of an in-hospital 5- attempts at percutaneous coronary intervention or who had daysaunaprogramfollowedbytwiceweeklyoutpatientsauna vessels deemed unsuitable for operative interventions. Tis activity over 5 years [44]. study revealed that, afer 3 weeks of daily (5 times weekly) infrared sessions, the scoring indices of defect reversibility 3.7. Health Outcomes on myocardial perfusion scans (summed stress scores and summed diference scores) improved (16 ± 7to9± 6, �< 3.7.1. Cardiovascular Disease. Te fndings of the 9 studies 0.01,and7± 4to3± 2, � < 0.01) afer sauna therapy but not that researched sauna therapy for congestive heart failure in the control group that received standard medical care [40]. (CHF)inadultsculminatedinthelargestandmostrecent Te two largest studies of this review which prospectively prospective multicentred randomized controlled trial involv- followed 2315 men in Finland over 20.7 years of frequent ing 149 patients with advanced CHF that demonstrated small saunabathingforcardiovasculardisease-relatedoutcomes butimproved6-minutewalkingdistances(−44.9 m ± SD used multivariate analysis and calculated hazard ratios (HR) 49.3 m, � < 0.05), reduced cardiothoracic ratios on chest X- adjusting for confounding factors such as blood pressure, ray (−1.58% ± SD 2.81%, � < 0.05)refectingreducedheart resting heart rate, smoking status, Type 2 diabetes, previous sizes, and improved NYHA (New York Heart Association) myocardial infarction, LDL levels, and alcohol consumption. classifcations of disease (fewer class III and IV patients, �< Teir fndings included a 66% risk reduction [HR 0.34 0.05) afer 2 weeks of sauna therapy, all compared to no (0.16–0.71), � = 0.004] of dementia, a 65% risk reduction signifcant respective changes in a control group that received [HR 0.35 (0.14–0.90), � = 0.03] of Alzheimer’s disease, a 63% standard medical care [31]. risk reduction [HR 0.37 (0.18–0.75), � = 0.005] of sudden A study of 12 infants with ventricular septal defects cardiac death, and a 40% risk reduction [HR 0.60 (0.46–0.80), (VSDs) and related severe CHF (congestive heart failure) who � < 0.001]ofall-causemortality[38,39]. underwent sauna bathing for 5 minutes daily for 4 weeks demonstrated decreased VSD (ventricular septal defect) 3.7.2. Rheumatological and Immune-Mediated Disease. A shunt fow ratios (� < 0.05), which averted the need for Dutch study of 34 patients diagnosed with either rheuma- surgical repair in 9 infants [41]. toid arthritis (RA) or ankylosing spondylitis (AS) reported Another randomized controlled trial examined the efects decreased pain and stifness in the RA (� < 0.05)andAS(�< of repeated sauna therapy on ventricular arrhythmias in 30 0.001) groups during 4 weeks of sauna therapy that was not subjects with congestive heart failure and more than 200 sustainedaferthe4weeks,withnochangesindiseaseactivity premature ventricular contractions (PVCs) per 24 hours at beingdetectedineithergroupbaseduponrange-of-motion baseline and reported signifcantly fewer PVCs (mean 848 scoring and serum levels of ESR (erythrocyte sedimentation ± 415 versus baseline mean 3097 ± 1033 per 24 hours, �< rate) [53]. 0.01)afer2weeksofrepeatedsaunasessionscompared A Japanese single-group study of 44 patients diagnosed with no signifcant changes in a control group that received with fbromyalgia with or without another rheumatologi- conventional medical therapy [36]. cal disorder (i.e., systemic lupus erythematosus, systemic Two studies investigated the efects of repeated sauna sclerosis, rheumatoid arthritis, Sjogren’s syndrome, Behcet’s sessions on patients with peripheral arterial disease. Te disease, or aortitis syndrome) reported subjective improve- frst study was a pilot trial which reported decreased visual ments in VAS (visual analogue scale) pain scores (�< analogue scale (VAS) pain scores (� < 0.01), improved 6- 0.001), reduced symptoms based upon FIQ (fbromyalgia minutewalkingdistance(6MWD)(� < 0.01), improved impact questionnaire) (� < 0.001), improved quality of 22 Evidence-Based Complementary and Alternative Medicine life indicators on SF-36 (short form 36-item) questionnaire control group [−0.01 L/s (−0.075–0.04 L/s)], � = 0.019,that (� < 0.01–0.05),andobjectivefndingsoffewernumberof received usual medical care. No other changes in spirometry tender points (� < 0.01) palpated on physical exam afer parameters or 6-minute walk test distances were detected 12 weeks of combined far-infrared sauna and underwater between the two groups [59]. Te second study involved a exercise therapy [56]. single group of male, ex-smoker COPD patients (�=13) Two studies of patients diagnosed with chronic fatigue with the following fndings afer 4 weeks of sauna sessions: syndrome/myalgic encephalomyelitis reported subjective improved symptom scores (59.7 pts ± 16.9 to 55.3 pts ± 17.2 pts, improvements afer repeated sauna. Soejima et al. 2015 (�= � = 0.002); decreased pulmonary artery pressures during 10) reported decreased fatigue (� = 0.002)onnumerical exercise (� = 0.028); increased exercise times afer sauna rating scales and improved scores for anxiety (� = 0.008), exposures (360 s ± 107 s to 392 s ± 97 s, � = 0.032); and depression (� = 0.018), fatigue (� = 0.005), and performance improved oxygen saturation during exercise (� = 0.022)[60]. status (� = 0.005) on POMS (profle of mood states) ques- Te Tai randomized controlled trial (�=26)that tionnaire afer 4 weeks of infrared sauna sessions [55]. Amano investigated the efects of a 6-week rehabilitation sauna et al. 2015 (�=15) noted 77.8% of participants receiving program on patients diagnosed with symptomatic allergic 8 weeks of regular far-infrared sauna therapy improved in rhinitis reported improved peak nasal inspiratory fow rates symptomsbaseduponSF-36(shortform36-item),SRQ- (119.2 L/s ± 46.4 to 161.9 L/s ± 46.7, � = 0.002)andimproved ± D (brief self-rating questionnaire for depression), and STAI FEV1 (forced expiratory volume at 1 sec) (77.5% 9.8% to (state-trait anxiety inventory questionnaire) compared to 95.6% ± 5.7%, � = 0.002) in the sauna intervention group 50% of participants in the control group, who chose not to compared to a control group that received usual medical care. undergo sauna therapy [54]. Te researchers also examined HRV (heart rate variability) parameters but detected no signifcant diference between the sauna and control groups [57]. 3.7.3. Chronic Pain Syndromes. Two randomized controlled Another randomized controlled trial studied common trials investigated the subjective efects of repeated sauna on � = 157 �=37 cold suferers in Germany ( )sittingfor3minutes chronic pain disorders. One New Zealand study ( )of fully winter-dressed in a Finnish sauna daily over 3 days patients diagnosed with chronic tension headaches reported breathing in piped “hot dry” sauna air versus control “cool a 44% reduction in headache intensity within 6 weeks of the dry” room temperature air while wearing a face mask. sauna treatment arm, with mean change in headache intensity Only on day 2 assessment, a decrease in symptom severity between sauna and control group being 1.27 points (95% CI − � = 10.17 =1117 � = 0.002 scoring was detected in treatment versus control groups [ 1.0 0.48–2.07; ;df , ; )[50].Teother (−2.0–−0.1), � = 0.04, 95% CI] but this fnding was not Japanese randomized controlled trial of 46 patients with sustained through days 3, 5, and 7 of study. Fewer doses of chronic pain disorders detected an increased likelihood of � < 0.05 cold and fu medications were taken by the treatment group return to work 2 years afer sauna intervention ( )and on day 1 of assessment [3% (1–9%) versus 15% (8–28%), �= decreases in anger scoring (on CMI, Cornell Medical Index) 0.01, 95% CI], compared to the control group [58]. in the 4-week sauna-treated group compared to control ± ± � < 0.001 group (4.5 1.1 to 2.2 1.6, )whoreceivedthe 3.7.6. Athletes. Two small noncontrolled interventional trials same courses of behavioural/rehabilitation/exercise therapy studied the physiological efects of repeat sauna in athletes. without additional sauna therapy [51]. One study (�=7) reported that 30 minutes of daily postexercise sauna bathing for ten days was associated with 3.7.4. Depression. One randomized controlled trial that peaked expansion of plasma volume afer 4 days (+17.8%, investigated the efects of 4 weeks of sauna sessions on 28 90% CI: 7.4–29.3%), followed by a trend back to presauna patients diagnosed with mild depression reported improved levels by days 7–10 [61]. Te other study (�=16)noteda � < 0.001 ∘ somaticcomplaints( ), improved hunger scores mean postsauna increase in axillary body temp 2.6 C(�< � < 0.0001 �< ∘ ( ), and improved relaxation scores ( 0.001)aferfrstsaunaversusameanincreaseofonly1.9C 0.0001 ) based upon subjective somatic complaint, depres- (� < 0.002) afer completing a 5 months’ course of sauna sion, hunger, and relaxation scoring in the sauna group bathing. Te researchers also noted postsauna increases in as compared to the control group that received bedrest mean venous pH by 0.8% (� < 0.001), decreased mean base instead of sauna therapy. In this same study, plasma ghrelin excess by 20.3% (� < 0.001), increased mean venous O2 concentrations and daily caloric intakes also changed in the by 53.3% (� < 0.001), increased mean Hb concentration ∗ = −2.32 �< sauna group compared to control group ( � , in blood by 5.2% (� < 0.001), and right shif of oxygen- 0.05 ∗ = −2.65 � < 0.05 ∗ ,and � , ,resp.)with Student two- hemoglobin dissociation curve (decreased afnity, favours � tailed group -test [52]. release of O2 to tissues) afer the frst sauna with similar changes in specifed parameters noted afer a fnal sauna 5 3.7.5. Lungs and Airways. Two studies focused on the efects months later (� < 0.043–� < 0.005)[62]. of infrared sauna on patients diagnosed with COPD (chronic obstructive pulmonary disease). One controlled trial (�= 3.7.7. Healthy Populations. Two small uncontrolled, single- 20)reportedimprovedFEF50 (forced expiratory fow afer gender studies reported reduced total cholesterol levels (4.50 50% of expired forced vital capacity) in patients receiving 4 ± 0.66 mmol/L to 4.18 ± 0.41 mmol/L, � = 0.02)andreduced weeksofrepeatedsauna[+0.08L/s(0.01–0.212L/s)]versusa LDL (low density lipoprotein) levels (2.71 ± 0.47 mmol/L to Evidence-Based Complementary and Alternative Medicine 23
2.43 ± 0.35, � = 0.01)inhealthymen(�=16)afer4weeks several somatic well-being scores improved in both treatment of regular sauna activity involving 45 min sauna sessions [66] groups (I) and (II), as compared to group (III) with Duncan and reduced total cholesterol levels (4.47 ± 0.85 mmol/L to post hoc test suggesting signifcant diferences between group 4.25 ± 0.93 mmol/L, � < 0.05)andreducedLDLlevels (I) and group (III) (� < 0.01)andbetweengroups(I)and (2.83 ± 0.80 mmol/L to 2.69 ± 0.83 mmol/L, � < 0.05)in (II) (� < 0.05). No diferences however were seen between healthy women (�=9)afer2weeksofregularsauna groups (II) and (III) (� = 0.21) and no signifcant changes activity involving 30-minute sauna sessions [67]. Te same in neuropsychological testing scores (� > 0.10)orserum research group of both studies reported earlier fndings of concentrations of selected organochlorides (� > 0.10)were signifcant increases in heart rate, systolic blood pressure, reported between any of the groups [68]. growth hormone, adrenocorticotropic hormone, and cortisol levels along with signifcant decreases in diastolic blood 3.7.9. Spermatogenesis. One longitudinal time-course study pressure and plasma volumes afer single and repeated sauna examined the efects of Finnish sauna activity on male sessions in 20 women afer 2 weeks of either 30-min sauna sperm and fertility measures in 10 healthy men. Afer 3 sessions or 45-min sauna sessions [63, 65]. Reductions in total months of repeated sauna (15-min saunas twice weekly), the and LDL cholesterol levels along with increased HDL (high investigators reported reduced sperm counts (93 ± 27.0 × 6 6 density lipoprotein) cholesterol levels were reported in the 10 versus 223 ± 52.8 × 10 , � < 0.001); reduced sperm 6 6 45-min sauna group. concentrations (31 ± 13.1 × 10 /ml versus 89 ± 29.3 × 10 /ml, Another study of healthy men and women examined the � < 0.001); fewer motile sperm (36.1 ± 3.6% versus 58.0 ± skin physiology of regular sauna attenders (�=21)compared 7.6%, � < 0.01); abnormal sperm parameters [decrease in to newcomer sauna attenders (�=20) before and afer normal histone-protamine replacement (� < 0.05); abnormal sauna bathing. Te investigators reported a decrease in NaCl chromatin condensation (� < 0.05); altered mitochondrial (sodium chloride) sweat concentrations in the regular sauna function (� < 0.01)]; upregulation of various heat-stress group (∼200 mmol/L ±∼10 mmol/L to ∼170 mmol/L ±∼ genes [HIF-1� (� < 0.001), KDR (� < 0.001), FLT1 (�< 10 mmol/L, � = 0.0167) without any respective changes in 0.001), and VEGF (� < 0.001)]; and upregulation of HSPs the newcomer sauna group. Baseline values (presauna) of (heat shock proteins) and HSFs (heat shock factors) [HSP90 forehead sebum level were 25% lower in the regular sauna (� < 0.001), HSP70 (� < 0.001), HSF1 (� < 0.001), HSF2 group (� < 0.05) compared with newcomer group but sebum (� < 0.001), and HSFY (� < 0.001)].However,allspecifed levels decreased similarly in both groups afer sauna. Skin changes reverted back to normal 6 months afer ceasing sauna surfacepHwasgenerallymeasuredtobelowerintheregular activity and no signifcant changes in plasma sex hormones sauna group but similar scales of pH elevation were recorded from baseline were detected directly afer sauna or afer 3 or for both groups during and afer sauna activity [64]. 6months[27].
3.7.8. Detoxifcation. Populations burdened with toxicants 3.7.10. Adverse Side Efects. Of the 40 included studies, only were the subject of two studies. Both entailed multimodal eight reported any adverse symptoms from sauna bathing. therapies with sauna as a prominent but not sole intervention Six studies recorded adverse efects graded as mild, meaning and both demonstrated improved self-assessed quality of life symptoms of complaint were noted which did not alter the �=69 measures [68, 69]. Ross and Sternquist 2012 ( )docu- study protocol or incur dropouts to the study. Mild heat mented improved posttreatment SF-36 (short form 36-item discomfort was the major complaint [53, 61, 69]. Other mild health survey) scores in symptomatic policemen exposed complaints noted in one infrared sauna study of heart failure to employment-related drugs and toxicants compared to patients (� = 149) included symptomatic low blood pressure, pretreatment scores (with 2-tailed Student �-test paired scores hypovolemia, polyurination, weight loss, and, questionably, and Wilcoxon matched pairs test and sign test, � < 0.001), − acute bleeding afer a tooth extraction [31]. Another study of across all subscales afer 4 6 weeks of infrared sauna sessions �=21 with up to 4 hours of sauna bathing daily. Te FASE patients with peripheral arterial disease ( )reported (Foundation for Advancements in Science and Education) transient leg pain in one participant during a frst infrared 50-item and neurotoxicity symptom questionnaires further sauna session with the pain improving afer completing a revealed fewer “poor physical health” days (9.3 versus 1.8 few sauna sessions and disappearing altogether by the end of days, � < 0.001); fewer “sick days” (2.0 versus 0.3 days, the 6-week study [34]. Pach et al. 2010 reported coughing in � < 0.001); more sleep hours (5.8 versus 7.6h, � < 0.001); 3 of 157 Finnish-style sauna participants, stimulated by the and lessened neurotoxicity scoring (65.5 ± 24.8 versus 14/6 ± placement of a face mask in both intervention and control 11/5 points, � < 0.001)[69]. groups, with diferent temperatures of air piped through the Te other sauna detoxifcation study was a randomized masks of the respective groups [58]. controlled trial (�=36) of symptomatic individuals with Two studies recorded moderate adverse efects, defned elevated levels of lipophilic toxicants, comparing two separate assymptomcomplaintsthatledtodropoutofstudypar- sauna interventions with a control group: (I) steam sauna ticipants or led to changes in study protocols. One study, with oral and intravenous supplements, (II) dry sauna with involving ffeen women diagnosed with chronic fatigue substitute placebo oral and intravenous interventions, and syndrome/myalgic encephalomyelitis, reported enough heat (III) no sauna, no oral, and no intravenous interventions. intolerance in “most” of the participants such that the Usingmultivariateanalysisofvariance(MANOVA)methods, investigators reduced the temperature of the infrared sauna 24 Evidence-Based Complementary and Alternative Medicine
∘ ∘ intervention from 60 Cto45 C to fnish conducting the study brings up issues of how applicable the fndings may or may [54]. Another infrared sauna study (randomized controlled not be to outpatient populations. trial) of chronic pain patients (�=46) reported 2 patients Despite diferences in sauna types, temperature, fre- droppingoutofthetreatmentarmduetoacutebronchitisand quency, and duration of interventions, the far-infrared sauna claustrophobia experienced in the sauna room [51]. None of studies involving cardiovascular disease and congestive heart the included studies reported severe adverse efects involving failure patients suggest favourable outcomes that reinforce the need for emergency medical services. earlier fndings of interventional Finnish sauna studies and cardiovascular disease [75–79]. Tis suggests that heat stress, whether induced by infrared or Finnish-style sauna, causes 4. Discussion signifcant sweating that is likely to lead to hormetic adap- 4.1. Principal Findings. Tefndingsofthisreviewsuggest tation and benefcial cardiovascular and metabolic efects. frequent dry sauna bathing improves a variety of subjective Tisisfurthersupportedbythetwolargeobservational and objective health parameters and that frequent Finnish studies that found striking risk reductions for sudden cardiac sauna bathing is associated with improved outcomes such as death (63%) and all-cause mortality (40%) as well as for dementia (66%) and Alzheimer’s disease (65%), in men reduced overall mortality and reduced incidence of cardio- whousedasauna4−7timesperweekcomparedtoonly vascular events and dementia, at least in men [38, 39]. Te once per week [38, 39]. While these large cohort studies most established clinical benefts of sauna bathing are asso- are based on calculated hazard ratios with adjustments for ciated with cardiovascular disease, yet there is also evidence common cardiac risk factors, it has been pointed out that to suggest that saunas, either Finnish-style or infrared, may the association between sauna activity and health outcomes beneft people with rheumatic diseases such as fbromyalgia, may be noncausal and that sauna use is merely an indicator rheumatoid arthritis, and ankylosing spondylitis, as well as of “healthy lifestyle” and other socioeconomic confounding patients with chronic fatigue and pain syndromes, chronic factors [80]. Nevertheless, these fndings point to the need obstructive pulmonary disease, and allergic rhinitis. Sauna for further study and serious consideration given to sauna bathing may also improve exercise performance in athletes, bathing to address the ever-increasing individual, societal, skin moisture barrier properties, and quality of life and and fnancial burdens of cardiovascular disease as well as is not associated with serious adverse events. Tere is not dementia-related conditions in aging populations. yet enough evidence to distinguish any particular health diferences between repeat Finnish-style and repeat infrared 4.2. Mechanisms of Action: Sauna Bathing. Several mecha- sauna bathing. nismsofactionhavebeenproposedforthehealthefects Cardiovascular disease has clearly been a focus for sauna of frequent sauna bathing. Exposure to heat increases researchers since 2000 despite Finnish-style sauna being con- cardiac output and reduces peripheral vascular resistance sidered by some in the past as a contraindication for patients and induces other physiological changes in cardiovascular with CHF and other cardiovascular diseases, most likely parameters such as decreased systolic and/or diastolic blood because of perceived intolerance to the high temperatures pressure [31, 35, 37, 46–49, 60, 65], increased HRV (heart rate [1]. Nearly half (19 of 40) of the studies included in this variability) [33, 36, 57], improved cardiac function markers review involved populations who had active cardiovascular [31,33,35,36,40,42,46,48],andimprovedfow-mediated disease or increased risk for cardiovascular disease, and all arterio- and vasodilatation of small and/or large blood vessels these studies demonstrated benefcial health efects. Seven of [34, 40, 42, 45, 47–49]. Regarding hormonal and metabolic these 19 studies were randomized controlled trials (RCTs), models, reduced levels of epinephrine and/or norepinephrine withonlyoneofthemmeetingtheCochranecriteriafor [42, 46], increased levels of nitric oxide metabolites in blood an acceptably low risk of bias. Tis particular multicentre [32, 34] and urine [41], decreased total and LDL (low density RCT (� = 149) reported improvements in all outcome lipoprotein) cholesterol levels [63, 66, 67], increased serum measures except B-type natriuretic peptide (BNP) levels levels of growth hormone, adrenocorticotropic hormone (namely, longer 6-minute walking distance, reduced cardio- (ACTH), and cortisol [65], decreased fasting blood glucose thoracic ratio on chest X-ray, and improved NYHA (New levels [49], increased plasma ghrelin levels [52], and reduced York Health Association) classifcation) in the infrared sauna- urinary levels of prostaglandins (8-epi-prostaglandin F2�) treatedcongestiveheartfailuregroupcomparedtocontrol [37] have been detected afer regular sauna sessions. Together, over only 2 weeks of intervention [31]. these fndings support complex multipathway end-organ While sauna bathing appears to show promise as a efects on the central and autonomic nervous system, the lifestyle intervention for cardiovascular disease, a majority peripheral vascular endothelium, and the hypothalamus- of the cardiovascular disease-related sauna studies (16 of 19) pituitary-adrenal axis, as well as on the kidneys and the liver were conducted by the same core Japanese research group that are continuing to be documented [1, 11, 28, 81]. and afliates who employed “Waon therapy” [74], which Te complexity of how sauna bathing may infuence involved far-infrared sauna bathing. Tese Waon therapy cardiovascular risk factors is suggested by the report of studies used similar outcome measures and mostly involved benefcial efects on total cholesterol and LDL (low density hospitalised patients, which might refect some diferences in lipoprotein) cholesterol and conficting results on HDL (high health care systems and thresholds for hospitalisation. Te density lipoprotein) levels in healthy young men and women use of primarily hospitalised patients in these studies also [63,66,67].Tesefndings,whichneedtobeconfrmed Evidence-Based Complementary and Alternative Medicine 25 in larger studies with nonsauna control groups, may point are complex and the role of frequent sweating to promote to diferences between Finnish and infrared saunas as they excretion and improve health is still poorly defned [91]. contrast with previous similarly sized, yet better controlled In addition to having profound physiological efects, studies of infrared sauna bathing in populations at increased sauna bathing is reported to have benefcial psychological risk of cardiovascular disease [37, 47, 49]. Tese fndings efects that are refected in the many reports of improved may also be compared to the metabolic efects of exercise well-being, pain tolerance, and other self-assessed symptom- in healthy populations which include improvements in both relatedscoring[34,36,43,45,46,50–56,58,60,68,69]. LDLandHDLlipidlevels[82]. Te psychological impact of sauna bathing may be due to Whiletherearelikelytobemanymechanismsofaction a combination of factors that include release of endorphins infuencing the physiological efects of sauna bathing, it has and other opioid-like peptides such as dynorphins [81, 92], been suggested that sauna bathing may induce a general forced mindfulness, psychological stress reduction, relax- stress-adaptation response that leads to “hormetic adap- ation, improved sleep, time out from busy life schedules, tation” and the establishment of “sauna ftness,” possibly placebo efects, and other aspects of individual psychological analogous to the hormetic adaptation responses of exercise. and social interactions that likely occur around frequent Tisissupportedbynewer,single-cellanalysismethodsthat sauna activity. While it is difcult to distinguish between the suggest sauna bathing increases generation of free radicals diferent factors that produce positive psychological efects, andreactiveoxygenatedspeciesalongwithenhancedantiox- such efects may enhance other physiological and metabolic idant activities via proposed nitric oxide- (NO-) dependent benefts as they are likely to promote adherence to regular processes in blood [62] and upregulation of specifc HSPs sauna activity. (heat shock proteins) and HSFs (heat shock factors) in semen [27]. Te two studies in athletes further support sauna’s 4.3. Safety and Adverse Efects with Sauna. In the medical involvement in hormetic stress responses with the fndings literature at large, there are reports of severe adverse efects of plasma volume expansion afer 4 days of daily postexercise from saunas that include dry sauna-induced burns [93] and sauna bathing, followed by a trend back to presauna levels myocardial ischemia (especially in patients with unstable by days 7–10 in one study [61], along with mean postsauna coronary artery disease) [94], along with less frequent reports ∘ increases in axillary body temperature of 2.6 Caferafrst of syncope/falls [1], hypersensitivity pneumonitis (“sauna ∘ sauna versus mean postsauna increases of only 1.9 Cafer lung”) [95], nonexertional heatstroke [96], rhabdomyolysis the last session of a 5-month course in the other study [62]. [93], ocular irritations [97], “sauna stroke syndrome” [98], Additionally, increases in plasma lipid peroxidase levels and anddeath[99].Teriskofdeathfromsaunaswasexamined increases in free radical processes of RBCs and decreases in retrospective population studies of frequent sauna users in plasma �-tocopherol (antioxidant) levels and decreases in Sweden and Finland, with the annual death rate from in RBC catalase activity afer an initial sauna were not saunas being reported as 0.06 and 2 per 100,000 inhabitants, maintained afer 5 months of regular sauna [62], suggesting respectively,withhalformoreofallthesedeathsinvolving that sauna bathing may upregulate antioxidant defences. the use of alcohol and a common risk factor of sauna bathing Improved adaptation to stress with regular sauna bathing alone [99, 100]. may be further enhanced by excretion of toxicants through In this review, adverse signs and symptoms of both heavy sweating. Many industrial toxicants including heavy Finnish-style and infrared sauna bathing were reported as metals, pesticides, and various petrochemicals may be mild to moderate heat discomfort and intolerance in 4 of the excreted in sweat leading to an enhancement of metabolic studies [53, 54, 61, 69], low blood pressure/light-headedness pathways and processes that these toxic agents inhibit [83]. in one study [31], transient leg pain in another study [34], Sweat-induced excretion of toxic metals such as arsenic, airway irritation in two studies [51, 58], and claustrophobia in cadmium, lead, and mercury has been reported with the rates one study [51], with no severe adverse symptoms reported in of excretion matching or exceeding urinary routes [84]. Tere any studies. Detailed comparative analysis of adverse efects is also recent evidence that toxic chemicals and xenobiotics between studies was limited by small sample sizes, hetero- such as polybrominated diphenyl ether (PBDE) fame retar- geneity of sauna types and study design (many without con- dants, organochlorine pesticides, bisphenol-A (BPA), and trol groups), and inconsistent reporting of adverse side efects phthalates may be excreted via induced sweating at rates within outcome measures. Te highest intensity of adverse that exceed urinary excretion [85–88]. Te importance of efects (moderate levels of heat intolerance) occurred in sweat in excretion pathways has been further documented populations aficted with chronic fatigue syndrome, chronic by sweat-patch technology used to monitor illicit drug use pain, rheumatoid arthritis, and ankylosing spondylitis. As and is based on dozens of studies of the pharmacodynamics these conditions are all associated with infammation and and pharmacokinetics of amphetamine, cocaine, cannabis, abnormal immune responses, it may be that the heat and/or opiates, and associated metabolites [89, 90]. While sweat- increased sweating of sauna activity is modulating some of induced detoxifcation certainly occurs, studies using sauna these responses [51, 53, 54]. Te direct adverse efects of heat for detoxifcation purposes report more favourable fndings may also be responsible for the impairment of sperm counts, with subjective rather than objective measures [68, 69]. concentration, and motility and upregulation of heat-stress- Further research on sauna-based detoxifcation is warranted related genes reported in the sperm of 10 healthy men afer as the excretory functions of skin via sweating or other active, a 3-month course of Finnish-style sauna [27]. While these passive inter- and/or transcellular, and transdermal pathways fndings are based upon one identifed study of only 10 men, 26 Evidence-Based Complementary and Alternative Medicine thefndingsareconsistentwithsomeearlierresearchon better inform the physiological models of what is thought the efects of genital heat stress on semen quality [101–104]. to be happening with repeated sauna of either Finnish or All the deleterious sperm efects of the sauna intervention infrared types. Te concepts of hormetic stress and inter- mentioned in this study were observed to revert back to relating “sauna ftness” or habituation to the physiological “normal” presauna levels afer 6 months of avoiding sauna efects of repeated sauna activity might have implications for activity [27]. While this supports current recommendations preventive or therapeutic targets in the future. Conducting for men seeking to optimize fertility to avoid sauna-type more studies of repeated sauna in healthy but nonathletic activities [105], further research is required to determine participants may further help to elucidate the similarities and if similar efects on sperm occur with lower temperature diferences in metabolic pathways between repeated sauna infrared sauna bathing or if sauna bathing has any efect on activity and regular exercise. Further studies are also needed male fertility. to distinguish between the health efects of Finnish saunas, which ofen involve brief periods of increased humidity 4.4. Strengths/Limitations. To the best of our knowledge, this and dramatic cooling interventions, compared to the lower is the frst systematic review to include both Finnish-style temperature infrared saunas that typically do not have such and infrared sauna studies. However, we did not include variations. studies of steam sauna interventions and therefore may have overlooked some evidence of the efects of heat on health. 5. Conclusions Another limitation of this study is the inclusion of only English language, especially since sauna activity is frequent in Regular infrared and/or Finnish sauna bathing has the non-English speaking countries. Furthermore, the quality of potential to provide many benefcial health efects, especially the reviewed studies was variable with many studies having for those with cardiovascular-related and rheumatological small sample sizes, poorly described methodology, variable disease, as well as athletes seeking improved exercise per- useofcontrols,diferingtypesofsaunaandsaunaprotocols, formance. Te mechanisms for these efects may include variable duration and frequency of sauna interventions, and increased bioavailability of NO (nitric oxide) to vascular inconsistent mention of cooling therapies or rehydration endothelium, heat shock protein-mediated metabolic activa- protocols along with heterogeneous outcome measures. Te tion, immune and hormonal pathway alterations, enhanced great heterogeneity of studies makes meaningful compar- excretions of toxicants through increased sweating, and other isons across studies difcult and provides insufcient evi- hormetic stress responses. dence to recommend specifc temperature, frequency, or Currently there is insufcient evidence to recommend duration of sauna bathing for any specifc health outcome. specifc types of sauna bathing for specifc clinical conditions. In the months since this systematic review was conducted, While regular sauna bathing appears to be well-tolerated in a number of new research fndings have been published, the clinical setting with only minor and infrequent adverse analyzing various subsets of the same Finnish prospective efects reported, further data on the frequency and extent of cohort of over 2000 men who regularly sauna-bathed, initially adverse efects is required. Further studies are also required aged 42–60 years, followed over 20 years as part of the KIHD to explore the mechanisms by which sauna bathing exerts (Kuopio Ischemic Heart Disease) study, as detailed in two of physiological, psychological, and metabolic efects, as well the studies included in Table 1: cardiovascular disease- (CVD- as to better defne the benefts and risks of distinct types ) related sauna studies. Tese newer fndings cite reduced risk of saunas and the optimal frequency and duration of sauna of acute and chronic respiratory conditions [106], reduced bathing for benefcial health efects. risk of pneumonia [107], reduced serum levels of C-reactive protein (marker of systemic infammation) [108] with more Conflicts of Interest frequent sauna bathing, and reduced risk of hypertension [109] and additional improved all-cause mortality when Te authors declare that they have no conficts of interest. jointly associated with cardiorespiratory ftness [110]. Tese fndings add further support to the conclusions of this review. Acknowledgments 4.5. Future Research Perspectives. With the rise of single- Tismanuscriptwasdevelopedaspartofstudyconducted cell analysis and “omics” platforms of analysis such as by Dr. Joy Hussain during her Ph.D. candidature at RMIT metabolomics and transcriptomics, especially applied to University. 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