Not All Heat Is Equal: Sauna and Hot Tub Affect the Human Body in Different Ways

University of Oregon scientists discovered why sitting in hot water feels like moderate exercise.

Most people step into a hot bath to unwind. What they rarely stop to consider is that their body may be being pushed into overdrive. If heat can stress the body this way, does it matter how the heat is delivered? 

Is sweating in a sauna the same, biologically speaking, as sitting neck-deep in steaming water? 

Despite the popularity of both, researchers had never directly compared them under controlled laboratory conditions. This gap caught the attention of a team at the University of Oregon.

However, instead of asking people which felt better, they decided to measure what was happening beneath the skin — inside blood vessels, around the heart, and within the immune system.

Our “study compared thermoregulatory, cardiovascular, and immune responses to acute hot water immersion (HWI), traditional sauna (TRAD), and far infrared sauna (FIR),” the researchers note.

Turning up the heat in the lab

The experiments took place at the Bowerman Sports Science Center, where 20 healthy volunteers agreed to become human test subjects. The group included 10 men and 10 women between the ages of 20 and 28. 

All were physically active, non-smokers, and not taking medications — a clean slate for observing how a healthy body handles thermal stress. Each participant came to the lab ten times. Over those visits, they experienced three popular forms of passive heating.

One involved sitting submerged in a hot tub heated to 40.5°C (105°F) for 45 minutes. Another required three separate 10-minute sessions inside a traditional dry sauna set at 80°C (176°F). The third option was a 45-minute stay in a far-infrared sauna, where temperatures ranged from 45 to 65°C (113–149°F).

Before, during and after every session, researchers tracked core body temperature, heart rate, blood pressure, and cardiac output (the volume of blood the heart pumps each minute). Blood samples were also collected to examine immune cells and inflammation-related molecules.

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The key measurement was internal temperature. Once core temperature rises, the body reacts quickly. Blood vessels widen, the heart beats faster, and circulation increases to help move heat toward the skin. It resembles the early stages of a workout, even though the person is sitting still.

“These thermoregulatory and cardiovascular adjustments elicited by heat stress are comparable to those seen during moderate-intensity aerobic exercise,” the study authors said.

Water changes everything

All three heating methods raised the participants’ temperature, but hot water immersion stood apart.

When someone sits in hot air, sweat forms and evaporates, carrying heat away from the skin. This evaporation acts like a cooling system. Submerge the body in water, and that heat escape route largely disappears. Sweat cannot evaporate effectively, so heat accumulates faster and stays trapped longer.

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The data reflects this, showing core temperature climbed higher during hot water immersion than during either sauna condition. 

As a result, cardiovascular responses were stronger. Heart rate increased more, blood flow rose more sharply, and cardiac output was higher. The immune system reacted too. Blood tests revealed that only hot water immersion triggered a measurable rise in inflammatory cytokines and specific immune cell populations. 

“Hot water immersion gives you the most robust changes in core temperature because you can’t effectively dissipate heat as you can if you have contact with the air and you’re sweating to cool the body. When you’re submerged in water, the sweat mechanisms aren’t efficient,” Jessica Atencio, first authors of the study and a PhD candidate, said.

In everyday language, the body briefly shifted into a mild inflammatory state. This might sound alarming, but short-term inflammation is not necessarily harmful. Exercise creates a similar temporary response. Controlled stress, repeated over time, can train the body to adapt and strengthen.

The implications go beyond spa culture

Repeated exposure to heat stress has been linked in previous research to improved blood vessel function and lower blood pressure. 

By identifying which method produces the strongest physiological stimulus, the Oregon team has provided clearer guidance for future clinical research. Moreover, for individuals unable to exercise due to injury, mobility issues, or chronic illness, passive heat therapy may offer a partial alternative. 

However, the current study examined short-term responses in young, healthy adults. It didn’t investigate long-term health benefits. It also does not suggest that heat therapy can replace exercise. Physical activity remains the most reliable non-drug strategy for improving cardiovascular health.

Another important point to note here is that prolonged heat exposure can be risky for people with heart conditions or blood pressure problems. Therefore, medical clearance is essential before adopting regular heat sessions.

The study is published in the journal American Physiological Society.

This article originally appeared in March 2026 and was updated with new information.