REF-1587 · Hussain & Cohen, 2018 · Evidence-Based Complementary and Alternative Medicine · Paper · PubMed · Free full text

Across 40 studies of regular dry sauna (3855 people, 2315 of them in one cohort of Finnish men, our arithmetic from the review's tables; most of the studies in patients, 22 of the 40 from Japan), reported harms were few and none severe

Mainly heat discomfort, low blood pressure and airway irritation, plus one study of 10 men in which sperm count and motility fell and then returned to normal within 6 months of stopping — but only 3 of the 13 randomised trials had low risk of bias and harms were reported inconsistently, so the review supports "well tolerated in supervised settings", not "safe for athletes after any session"

Main caveat: Search ended April 2017; English only; dry sauna only (steam and partial-body heat excluded).
moderate evidenceMeasured on non-athletes and recreational peopleRead in full

Hussain and Cohen searched for studies since 2000 of repeated dry sauna sessions and any health outcome, to map what saunas have been used for and what benefits and harms were seen. Forty studies with 3855 participants in 12 countries qualified; 22 came from Japan, and 16 of the 19 cardiovascular studies came from one Japanese group using far-infrared "Waon" therapy.

Most studies reported benefits, chiefly in heart failure and cardiovascular risk, with others in rheumatic disease, chronic pain, fatigue, depression and airway disease. Eight studies reported any adverse symptom; none was severe.

The authors judge that regular sauna has potential health benefits and seems well tolerated in clinical settings, but that higher-quality data on how often and how badly side effects occur are needed, and that the evidence does not support specific sauna types or doses for specific conditions.

Participants

40 studies, 3855 participants; 21 of 40 studies had 30 or fewer participants; the smallest was 7 Australian athletes, the largest a cohort of 2315 Finnish men (two studies).
Populations: 6 healthy, 19 cardiovascular disease or risk, 7 rheumatological, pain or mood disorders, 4 airway disease, 2 elite athletes, 2 toxicant-exposed groups.

Protocol

Designs: 13 RCTs, 6 non-randomised controlled trials, 2 prospective cohorts; the remaining 19 were uncontrolled or retrospective (our arithmetic: 40 − 13 − 6 − 2).
Saunas: 11 Finnish (80-90 °C, 10-20% humidity, one arm at 50-65 °C), 25 infrared (mostly 60 °C for 15-30 min; two at 40-45 °C and 55 °C), 4 other dry types. Sessions of 5-20 min, 30 min to 4 h a day, once to several times a week, over 3 days to 5 months; the cohorts followed users for 5 and over 20 years.
Setting: all supervised (hospitals, clinics, laboratories) except one RCT with sauna vouchers and the two Finnish cohorts.
Risk of bias: RCTs only, Cochrane domains plus Jadad score, rated by one reviewer and confirmed by the other.
Funding and interests: a PhD scholarship (Jacka Foundation); no conflicts declared.

Key Results

Risk of bias in the 13 RCTsCount
Low overall risk (Cochrane) and Jadad > 33 (343 of 840 RCT participants)
Enrolled fewer than 509
Random sequence generation at high risk (our count from the review's Table 8)9
Allocation concealment unclear (our count from the review's Table 8)12
Harms reportedWhat
Studies reporting any adverse symptom8 of 40
Mild (no protocol change, no dropout), 6 studiesheat discomfort; in 149 heart-failure patients low blood pressure, low blood volume, frequent urination, weight loss and a questionable bleed after a tooth extraction; transient leg pain in one patient; coughing in 3 of 157 people wearing a face mask
Moderate (dropout or protocol change), 2 studiesheat intolerance in most of 15 women with chronic fatigue syndrome, so the cabin was lowered from 60 to 45 °C; 2 of 46 chronic-pain patients dropped out (acute bronchitis, claustrophobia)
Severe (needing emergency care)none reported
Sperm (one study, 10 healthy men, 15-min Finnish sauna twice a week for 3 months)count 223 ± 52.8 → 93 ± 27.0 × 10^6, concentration 89 ± 29.3 → 31 ± 13.1 × 10^6/mL, motile sperm 58.0 ± 7.6% → 36.1 ± 3.6%; all back to normal 6 months after stopping; sex hormones unchanged
Wider literature cited in the discussion (not the reviewed studies): case reports of burns, myocardial ischaemia in unstable coronary disease, fainting and falls, hypersensitivity pneumonitis, non-exertional heatstroke, rhabdomyolysis and death; population studies in Sweden and Finland gave annual sauna deaths of 0.06 and 2 per 100,000 inhabitants, with alcohol involved in half or more and bathing alone a common factor.
Benefit claims: in the Finnish cohort of 2315 men followed about 20.7 years, sauna 4-7 times a week against once a week was associated with lower all-cause mortality (HR 0.60, 0.46-0.80) and sudden cardiac death (HR 0.37, 0.18-0.75) after adjustment; the authors note the association may be non-causal. The largest RCT, in 149 heart-failure patients, reported a longer 6-min walk after 2 weeks of infrared sauna.
Athletes: two small uncontrolled studies only — daily post-exercise sauna in 7 athletes expanded plasma volume by 17.8% (90% CI 7.4-29.3%) at day 4, drifting back by days 7-10; a 5-month study in 16 athletes reported blood and temperature responses.

Practical Application

Is regular sauna safe? Across 40 studies of repeated dry sauna, mostly in supervised patients, reported harms were few and mild to moderate (heat discomfort, low blood pressure, airway irritation), and the one notable effect, a fall in sperm count in 10 men, reversed within 6 months of stopping. Harms were not collected consistently and athletes after hard training were not studied, so this supports "well tolerated under supervision", not "safe after any session".
Men trying to conceive: one study of 10 men (Finnish sauna twice a week for 3 months) found falls in sperm count, concentration and motility that reversed after 6 months off; the authors say this fits current advice for men optimising fertility to avoid sauna-type heat, and that whether lower-temperature infrared does the same is unknown.
Practice, not from the reviewed studies: no alcohol before or in the sauna, and do not bathe alone — the population data the authors cite in their discussion tie half or more of sauna deaths to alcohol, with bathing alone a common factor.
Chest pain, palpitations, fainting or dizziness in or after a sauna are a reason to stop and see a doctor (base practice); the discussion cites myocardial ischaemia in people with unstable coronary disease.
Pregnancy: see REF-1403 for core temperature in sauna and other heat during pregnancy.
The longevity and heart findings are associations in one Finnish cohort of men; the authors note sauna use may simply mark a healthier lifestyle. Do not promise a longer life from sauna.

Limitations

Search ended April 2017; English only; dry sauna only (steam and partial-body heat excluded).
Mostly small, many uncontrolled; 16 of 19 cardiovascular studies from one research group, mostly in hospitalised patients.
Harms were reported inconsistently and follow-up was mostly weeks to months, so rare or long-term harms could be missed.
No pooling; heterogeneous sauna types, doses and outcomes; cooling and rehydration routines were often not described.
How this entry was checked and changed
Why this level

a systematic review with its risk of bias read, which the rubric would allow up to A-; held at B+ because that risk is serious (3 of 13 RCTs at low risk; 27 of 40 studies not randomised, our arithmetic), nothing was pooled, and harms, this entry's use, were not collected in a standard way across studies