It depends on the case; there is no single guideline.
8 weeks of hot baths (immersion at ~40.5 °C) improved vascular function in sedentary people: endothelium-dependent dilation nearly doubled (5.6→10.9%), and arterial stiffness, carotid intima-media thickness and mean arterial pressure (83→78 mmHg) decreased, with changes comparable to exercise. Small trial (n=20).
View the study (controlled trial) →⏱ 3 min read
Regular exposure to therapeutic heat (Finnish dry sauna, infrared sauna, hot baths, hydrotherapy) has documented cardiovascular, metabolic, and longevity effects with growing clinical support over the last two decades, driven especially by the Finnish epidemiological data from the prospective KIHD study, which documented notable associations between frequency of sauna use and reductions in cardiovascular mortality, dementia, and all-cause mortality in cohorts of thousands of adult men followed for decades; these epidemiological data, together with the physiological mechanisms identified, have positioned heat therapy as a lifestyle intervention with cardiovascular potential comparable to that of moderate physical exercise.
The Finnish KIHD study, published in several articles during 2015-2018, documented that frequency of sauna use was associated with significant reductions in cardiovascular mortality, stroke, dementia, and all-cause mortality: sauna use 4-7 times per week was associated with 50 percent reductions in cardiovascular mortality and 40 percent reductions in all-cause mortality compared with use once a week, a notable magnitude that requires independent replication but is consistent with the documented physiological effects on endothelial function, arterial stiffness, blood pressure, and autonomic modulation.
Passive heat exposure produces physiological responses similar in some respects to moderate physical exercise: a significant increase in heart rate (up to 100-150 bpm in intense sauna sessions), massive cutaneous vasodilation, profuse sweating, increased heat shock proteins (HSPs, especially HSP70) with antioxidant and cell-protective effects, and favorable modulation of chronic inflammation; these cardiovascular and cellular effects are the probable mechanistic basis for the population benefits observed.
The modalities studied with the best clinical support are: Finnish dry sauna (80-100°C, 15-30 minutes per session, 4-7 times per week in the Finnish studies); infrared sauna (40-60°C, longer sessions, more limited but consistent data); hot baths (40-42°C, 20-30 minutes, several studies with favorable results on blood pressure and cardiovascular function); contrast hydrotherapy (hot-cold alternation, with more limited but consistent data).
The Finnish study also documented significant reductions in the incidence of dementia and Alzheimer's disease with regular sauna use, an observation consistent with the cardiovascular effects (cardiovascular health is one of the main modifiable determinants of dementia) and with direct effects on brain function through HSPs and other pathways.
In mild arterial hypertension, several clinical studies have documented modest but significant reductions in blood pressure with regular use of sauna or hot baths over weeks to months, an indication consistent with the epidemiological data and with the mechanistic basis.
The reasonable evidence-based recommendations are: 2-4 weekly sessions of 15-30 minutes in a Finnish sauna at 80-90°C, or the equivalent in other heat formats; adequate hydration before, during, and after; a cool shower or bath afterward can enhance the cardiovascular stimulus; gradual introduction if not accustomed.
Safety considerations are important: people with unstable ischemic heart disease, recent myocardial infarction, advanced heart failure, or significant arrhythmias should have a cardiology evaluation beforehand; people with significant postural hypotension or a tendency to fainting should use caution; pregnant women should avoid saunas and very hot baths (especially in the first trimester because of the associated risk of neural tube defects with significant hyperthermia); people with acute skin disease or significant inflammation may experience exacerbation; people with significant alcohol consumption should not use a sauna because of the increased risk of sudden death (the Finnish data show an association between alcohol consumption and death in the sauna); people who take antihypertensive medications may require adjustments because of the significant vasodilation; in older adults, hydration is particularly critical and introduction should be gradual; the reasonable clinical position is: regular heat therapy has a promising clinical profile for cardiovascular health and longevity with interesting epidemiological support and consistent physiological mechanisms, and represents a lifestyle intervention that many people can integrate with reasonable benefit and a low risk profile when practiced with appropriate precautions.
Information, not medical advice. Scientific evidence reflects the quality of the studies.