A new study published through the American Physiological Society compared three widely used forms of passive heat therapy—hot water immersion, a traditional sauna, and an infrared sauna—to determine how each affects the cardiovascular and immune systems. Conducted in 20 healthy young adults, the experiment found that while all three methods produced beneficial physiological responses, hot water immersion triggered the strongest immediate effects on body temperature, circulation, and immune activity.
Researchers asked participants to attend separate sessions of each heat therapy. During every session, they measured core body temperature, heart rate, blood pressure, sweat production, and several immune markers to compare how the body responded to each type of heat exposure.
Hot Water Increased Core Body Temperature the Most
The largest physiological changes occurred during hot tub sessions, where participants experienced significantly higher increases in core body temperature than during either traditional or infrared sauna sessions.
Researchers explain that the body has fewer ways to release excess heat while immersed in hot water because sweat cannot evaporate effectively from the skin. In contrast, sauna users cool themselves partly through sweat evaporation, allowing the body to regulate temperature more efficiently.
Because this cooling mechanism is greatly reduced in water, body temperature rises more rapidly and reaches higher levels, producing a stronger overall heat stress response.
Stronger Cardiovascular and Immune Responses
The greater increase in core temperature during hot water immersion was accompanied by more pronounced cardiovascular effects. Participants experienced larger increases in heart rate, enhanced blood circulation, and short-term improvements in blood vessel function compared with both traditional and infrared saunas.
Only hot water immersion produced a clear activation of the immune system. Blood tests revealed increased concentrations of interleukin-6 (IL-6), an important signaling molecule involved in inflammation and tissue repair. Researchers also observed increases in immune cells, including T lymphocytes and natural killer (NK) cells, which play important roles in defending the body against viral infections and cancer.
Sauna Continues to Offer Important Health Benefits
Although hot water immersion produced stronger immediate physiological responses, the findings do not diminish the well-established health benefits of regular sauna bathing.
Large observational studies, particularly from Finland, have consistently associated frequent sauna use with lower risks of cardiovascular disease, improved blood vessel function, and better overall health and longevity.
The authors emphasize that their experiment evaluated only the body’s short-term responses after a single heat exposure. Longer clinical studies are needed to determine whether one form of heat therapy provides greater long-term health benefits than another.
Who Could Benefit Most?
The findings may be especially relevant for individuals who are unable to exercise regularly because of mobility limitations, chronic illness, or physical disability.
Although passive heat therapy cannot replace physical activity, it may provide an additional way to stimulate the cardiovascular system, improve circulation, and activate certain biological recovery mechanisms.
However, researchers caution that both hot tubs and saunas should be used carefully, especially during periods of extreme outdoor heat. Individuals with cardiovascular disease, uncontrolled high blood pressure, pregnancy, or other significant medical conditions should consult a healthcare professional before beginning intensive heat therapy.
More Research Is Needed
The researchers plan to investigate the optimal temperature, duration, and frequency of passive heat therapy in future studies. They also hope to determine how these treatments might best complement established lifestyle interventions such as regular exercise and healthy nutrition.
Until more evidence becomes available, both sauna bathing and hot water immersion appear to offer meaningful health benefits. While the current findings suggest that hot water immersion produces the strongest immediate cardiovascular and immune responses, the best option for most people will continue to depend on personal preference, accessibility, and individual medical circumstances.
