Heat exposure

Forms of passive heat therapy differ in air or water temperature, humidity and whether the body is heated by hot air, water or infrared radiation. Finnish saunas use hot, dry air and are typically used for 5 to 20 minutes at a time, often interspersed with cooling off; a typical Finn takes a sauna at least once a week, and the average habitual frequency is 2 to 3 times a week [1]. Japanese hot tub bathing (ofuro) involves sitting in water up to the shoulders at about 37.8 to 41.0 °C [1]; the mean length of a bath in one Japanese study was 12.4 minutes [9]. Waon therapy, used in Japan mainly to treat heart failure, is a 15-minute session in a far-infrared dry sauna kept at 60 °C, followed by rest [1].

Heating the body raises skin and core temperature, increases heart rate and blood flow to the skin, and causes sweating. Blood pressure usually falls after a session: in studies summarised in a 2024 review, systolic blood pressure fell on average from 137 to 130 mmHg after a 30-minute sauna, and 10 minutes of hot tub immersion lowered it from 144 to 122 mmHg in people with treated high blood pressure [1]. Proposed long-term mechanisms of regular sauna use include improved function of the blood vessel lining (endothelium), reduced arterial stiffness, changes in the autonomic nervous system, improved blood lipids and lower blood pressure [10]. Heat also increases the production of heat shock proteins, which help prevent other proteins from clumping together [1].

The clearest experimental evidence comes from a randomised trial in which 20 sedentary young adults took either 60-minute hot water baths at 40.5 °C, 4 to 5 times a week for 8 weeks, or water immersion at a neutral temperature. Heat therapy raised flow-mediated dilation (a measure of endothelial function) from 5.6% to 10.9%, lowered aortic pulse wave velocity (a measure of arterial stiffness) from 7.1 to 6.1 m/s, reduced carotid artery wall thickness and lowered mean arterial pressure from 83 to 78 mmHg, changes the authors described as similar to or larger than those usually seen with exercise training in sedentary people [8]. A warm bath or shower at 40 to 42.5 °C taken 1 to 2 hours before bedtime was associated with better self-rated sleep quality and faster sleep onset in a review of 17 studies [11].

The main source of long-term evidence is the Kuopio Ischaemic Heart Disease Risk Factor Study in eastern Finland, which recorded the sauna habits of 2,315 men aged 42 to 60 between 1984 and 1989. Over a median of 20.7 years, 49.1% of the 601 men who took a sauna once a week died, compared with 37.8% of the 1,513 men who went 2 to 3 times a week and 30.8% of the 201 men who went 4 to 7 times. After adjustment for cardiovascular risk factors, the hazard ratio for sudden cardiac death was 0.78 (95% CI 0.57 to 1.07) at 2 to 3 sessions and 0.37 (0.18 to 0.75) at 4 to 7 sessions, and similar trends were found for fatal coronary heart disease, fatal cardiovascular disease and all-cause mortality. Sessions longer than 19 minutes were associated with a hazard ratio of 0.48 for sudden cardiac death compared with sessions under 11 minutes, but session length was not associated with all-cause mortality [2].

A later analysis of 1,688 participants aged 53 to 74, of whom 51.4% were women, found that the risk of cardiovascular death fell in a straight line with each additional weekly session, with no threshold. After adjustment for risk factors, physical activity and socioeconomic status, the hazard ratio was 0.75 (95% CI 0.52 to 1.08) at 2 to 3 sessions and 0.23 (0.08 to 0.65) at 4 to 7 sessions, and more than 45 minutes of sauna a week was associated with a hazard ratio of 0.57 compared with 15 minutes or less [6].

The same study population has been used to examine other outcomes. Men who took a sauna 4 to 7 times a week had a hazard ratio of 0.34 for dementia and 0.35 for Alzheimer's disease compared with once-a-week users [7]. Among 1,628 men and women followed for 14.9 years, 4 to 7 sessions were associated with a hazard ratio of 0.39 for stroke [12]. Among 1,621 men without high blood pressure at the start, 4 to 7 sessions were associated with a hazard ratio of 0.54 for developing it over 24.7 years [13].

Sauna use and fitness were independently associated with survival. In 2,277 men whose fitness was measured by breath gas analysis and who were followed for a median of 26.1 years, those with both high fitness and 3 to 7 sessions a week had a hazard ratio of 0.60 for death from any cause compared with men with low fitness and infrequent sauna use; the ratio was 0.63 for high fitness alone and 0.78 for frequent sauna use alone [14]. Fitness and exercise are described under exercise and longevity and VO2 max.

Full deep dive: Sauna bathing and longevity.

The largest study of hot tub bathing followed 30,076 Japanese adults aged 40 to 59, free of cardiovascular disease and cancer at the start, from 1990 to 2009 (538,373 person-years), during which 2,097 cases of cardiovascular disease occurred. After adjustment for standard cardiovascular risk factors and diet, bathing almost every day, compared with 0 to 2 times a week, was associated with hazard ratios of 0.72 (95% CI 0.62 to 0.84) for all cardiovascular disease, 0.65 (0.45 to 0.94) for coronary heart disease, 0.74 (0.62 to 0.87) for all stroke, 0.77 for stroke caused by a blocked artery and 0.54 (0.40 to 0.73) for bleeding within the brain. Bathing frequency was not associated with sudden cardiac death or subarachnoid haemorrhage (bleeding around the brain) [3]. The outcome was new disease, not death.

Smaller studies support an effect on blood vessels. In 873 middle-aged and older Japanese adults, those who took a hot bath 5 or more times a week had lower arterial stiffness, lower central pulse pressure and lower levels of BNP (a blood marker of strain on the heart) after adjustment for other factors, and hotter water was associated with lower arterial stiffness; in 164 of them followed over time, frequent bathing was associated with a smaller rise in BNP [9]. The 8-week trial of hot water immersion described above found improvements in endothelial function, arterial stiffness and blood pressure [8].

Hot tub bathing also changes blood pressure acutely. Among 1,479 older Japanese adults monitored at home, each 1 °C rise in skin temperature during a bath was associated with a 2.41 mmHg rise in systolic blood pressure and a 2.99 beats per minute rise in pulse rate, and a colder living room before bathing was associated with a higher skin temperature in the bath [15]. The authors noted that home hot tub bathing substantially increases drowning deaths among older adults in Japan [15].

Full deep dive: Hot tub bathing and health.

Infrared saunas heat the body mainly by infrared radiation rather than by hot air, and operate at lower air temperatures than Finnish saunas [1]. Far-infrared heat penetrates the tissues directly and produces sweating and cardiovascular responses at lower ambient temperatures, which may make it more tolerable for people unable to exercise [16]. None of the long-term Finnish or Japanese studies of mortality or disease incidence concerned infrared saunas, and a 2024 review described infrared saunas and other forms of passive heat therapy besides the Finnish sauna as relatively underexplored [1].

The available studies are small and short. In a randomised crossover trial (each participant tried every condition) of 10 healthy women, 45 minutes in an infrared sauna raised ear temperature by 1.05 °C more than resting and by 0.79 °C more than moderate cycling, but produced no detectable differences in blood pressure, arterial stiffness or heart rate variability [4]. Earlier studies at about 45 °C reported limited heat storage in the body [5]. At 65 °C, a single 45-minute session in 12 healthy adults raised rectal temperature by 1.4 °C, increased heart rate from 74 to 153 beats per minute, lowered mean arterial pressure by 9 mmHg immediately afterwards and reduced plasma volume by 11.6%, with a sweat rate of 1.4 litres an hour; the authors concluded that the effect of an infrared sauna depends on the heat load applied [5].

Clinical studies of far-infrared saunas cited in a 2025 review involved people with chronic heart failure and reported improvements in endothelial function, heart function, symptoms and walking distance [16]. Regular infrared sauna therapy in heart failure patients has also been reported to lower blood levels of BNP, a marker of strain on the heart [1]. A 2025 review concluded that evidence for claims that infrared saunas normalise blood pressure, lower cholesterol or reduce stress remains modest and that larger trials are needed [16]. The 2024 review's summary of the mortality evidence included only studies of Finnish saunas [1].

People with stable cardiovascular disease can generally use a sauna safely, and sauna bathing has been studied as a treatment in heart failure [1]. Sudden deaths linked to saunas have been largely associated with alcohol, which increases the risk of low blood pressure, heart complications and accidents. Sauna bathing may be harmful in unstable coronary artery disease, and caution or avoidance is generally advised after a recent heart attack and in severe aortic stenosis, uncontrolled high blood pressure or severe heart valve disease. In one 7-year retrospective study, 26% of burn cases were linked to sauna use, and alcohol was involved in 40% of those [1].

Hot tubs carry their own risks. Prolonged soaking can cause heat rash or folliculitis (infection of hair follicles), poorly maintained tubs can harbour the bacterium Mycobacterium avium and cause a lung condition known as hot tub lung, and soaking can cause dangerous drops in blood pressure [1]. Older adults face a documented risk of drowning in home baths, and a cold room before bathing is associated with larger changes in skin temperature and blood pressure [15]. Precautions for older adults and pregnant women are areas where clear guidelines are still lacking [1].

The mortality evidence for sauna bathing comes from a single Finnish population. A 2018 report from the Kuopio study described it as the only available study of sauna habits and the risk of death [6]. All participants in the 2015 analysis took a sauna at least once a week, so the comparison group was men who went once a week, not people who never went, and only 201 men were in the most frequent group [2]. Frequent sauna users may differ from others in wealth, health and lifestyle in ways that adjustment cannot fully capture, although the associations persisted after adjustment for fitness, physical activity and socioeconomic status [6, 14].

Randomised trials of sauna bathing are small and have not measured death. An 8-week trial of Finnish sauna in 41 adults with stable coronary artery disease found no significant improvement in endothelial function, arterial stiffness or blood pressure [16], and reviews note that trials of passive heat therapies have had weaknesses in randomisation and low statistical power [1]. The hot tub findings come from one Japanese study of disease incidence rather than mortality, with no association for some subtypes [3], and the supporting data on arteries are cross-sectional (measured at one point in time) or from 20 young adults [9, 8]. Other forms of heat exposure, including infrared saunas and steam rooms, have been studied far less than the Finnish sauna, so its findings cannot be assumed to apply to them [1].