Water conducts heat 24 times more effectively than air
Across cultures and centuries, humans have sought healing in hot water — and now science is beginning to explain why. A University of Oregon study published in the American Journal of Physiology found that hot water immersion produces stronger cardiovascular and immune responses than either traditional or infrared saunas, owing to water's far superior capacity to conduct heat into the body. The findings suggest that one of humanity's oldest and most accessible rituals may carry physiological benefits that rival more celebrated wellness technologies. For those unable to exercise regularly, the humble hot bath may offer a meaningful, if partial, substitute.
- Conventional wisdom about saunas as the gold standard of heat therapy has been quietly overturned by a controlled study comparing three heating methods in twenty healthy adults.
- Hot water immersion was the only method to trigger a measurable immune response, while also producing the largest gains in core body temperature and cardiac output — the infrared sauna, despite its premium reputation, barely moved the needle.
- The physics are unambiguous: water conducts heat 24 times more effectively than air, trapping thermal stress against the body and forcing a stronger physiological response than any air-based heat environment can achieve.
- Researchers are careful to frame heat therapy as a complement — not a replacement — for exercise, but see particular promise for people sidelined by illness, injury, or age who cannot move regularly.
- Even modest daily baths of 10 to 15 minutes may yield some benefit, though anyone with serious cardiovascular conditions is urged to seek medical guidance before attempting prolonged immersion.
A University of Oregon study has challenged long-held assumptions about heat therapy, finding that a hot bath outperforms both traditional and infrared saunas in triggering meaningful physiological responses. Published in the American Journal of Physiology, the research tracked twenty healthy adults across three heating conditions: 40.5-degree Celsius water, an 80-degree traditional sauna, and a far infrared sauna. Researchers measured body temperature, heart rate, blood pressure, and immune function throughout each session.
The results consistently favoured water immersion. It was the only method to produce a measurable immune response, and it generated the greatest increases in core body temperature and cardiac output. Lead author Dr. Jessica Atencio described hot tubs as potentially "the best, most robust modality for health." The infrared sauna, despite widespread marketing claims, produced only minimal changes in core temperature.
The explanation lies in basic physics. Water conducts heat roughly 24 times more effectively than air, meaning the body heats faster and stays hotter longer during immersion. In a sauna, sweat evaporates and carries heat away from the skin; in water, that cooling mechanism is blocked, forcing the body to work harder against sustained thermal stress. This triggers blood vessel dilation, increased circulation, and the release of nitric oxide — a compound associated with healthy vascular function.
Perhaps the study's most significant implication is that passive heat therapy appears to mimic some exercise-related physiological adaptations, including the inflammatory and anti-inflammatory cycles that help protect against chronic disease. Atencio was clear that heat is not a substitute for movement, but suggested it could be especially valuable for people unable to exercise due to illness, injury, or age.
The study used 45-minute sessions, which Atencio acknowledged as demanding. Shorter exposures — even ordinary daily baths of 10 to 15 minutes — may still offer modest benefits. She advised anyone feeling dizzy to exit immediately, stay hydrated, and for those with cardiovascular conditions, to consult a doctor first. The science of heat therapy remains young, but for now, the evidence quietly vindicates something people have known by instinct for a very long time.
A University of Oregon study has upended the conventional wisdom about heat therapy, finding that a simple hot bath delivers more powerful health benefits than either a traditional sauna or an infrared one. The research, published in the American Journal of Physiology, compared how three different forms of passive heat affected the bodies of twenty healthy adults. Participants spent time in 40.5-degree Celsius water, an 80-degree traditional sauna, and a far infrared sauna ranging from 45 to 65 degrees. Researchers tracked changes in body temperature, heart rate, blood pressure, and immune function throughout each session.
The results were striking. Hot water immersion produced the strongest physiological response across nearly every measure. It was the only heating method to trigger a measurable immune response in participants. Those in the bath also experienced the largest increases in core body temperature and cardiac output—the amount of blood the heart pumps per minute. Dr. Jessica Atencio, the study's lead author, described the findings as suggesting that hot tubs may be "the best, most robust modality for health." The infrared sauna, despite marketing claims about deeper penetration, produced only minimal changes in core temperature.
The advantage comes down to physics. Water conducts heat roughly 24 times more effectively than air. This means people immersed in hot water heat up faster and maintain that elevated temperature longer than those sitting in a sauna. As Atencio explained, the body's response to heat involves blood vessels dilating to redirect circulation from the core to the skin, which triggers the release of nitric oxide—a compound that promotes healthy blood vessel function. In a sauna, the body can sweat and release heat more readily because skin is exposed to air. In water, that heat transfer mechanism is blocked, forcing the body to work harder against the thermal stress.
One of the study's most intriguing findings is that passive heat therapy appears to mimic some of the physiological adaptations that occur during exercise. Both heat exposure and physical activity trigger an inflammatory response in the body, which then prompts an anti-inflammatory response. This cycle is significant because chronic inflammation underlies many long-term diseases. Atencio was careful to emphasize that heat therapy is not a substitute for exercise, but it could be valuable for people unable to move regularly due to illness, injury, or age.
The study used sessions of 45 minutes in 40.5-degree water—conditions Atencio acknowledged as "quite stressful and quite long." But shorter exposures appear to offer benefits too. She suggested that 30 minutes in a hot tub, when a person is actively sweating and feeling their heart rate rise, can provide meaningful stimulus. Even ordinary baths, which cool more quickly than hot tubs, may help. Daily soaks of 10 to 15 minutes could still produce beneficial adaptations, though the effect would be more modest.
The research does come with cautions. Passive heat therapy is not risk-free. People with serious cardiovascular conditions should consult a doctor before attempting prolonged hot water immersion. Atencio advised anyone feeling lightheaded or dizzy to exit the heat immediately and to drink water throughout the session. She also noted that the science of heat therapy is still evolving, and these findings, while promising, represent an early chapter in understanding how thermal stress affects human health. For New Zealanders heading into winter, the study offers a straightforward takeaway: the hot bath may be doing more for your body than you realized.
Notable Quotes
Hot tubs may be the best, most robust modality for health— Dr. Jessica Atencio, lead author
Heat therapy is not a replacement for physical activity, but could complement it, especially for those unable to exercise regularly because of illness, injury or age— Dr. Jessica Atencio
The Hearth Conversation Another angle on the story
Why does water work so much better than air at delivering heat to the body?
It's a matter of conduction. Water molecules transfer thermal energy far more efficiently than air molecules do—about 24 times more efficiently. When you're in hot water, your skin can't sweat effectively because the water blocks evaporative cooling, so your body has to absorb and process that heat rather than shed it.
So the body is essentially being forced to work harder?
Exactly. That thermal stress is what triggers the physiological response. Your blood vessels dilate, your heart works harder to pump blood to the skin, and your immune system activates. It's similar to what happens during exercise, but without the movement.
The study mentions that even 10 to 15 minutes daily might help. That seems almost too easy.
It does sound simple, but the researchers were careful about that claim. They tested 45-minute sessions, which is substantial. Shorter baths would produce weaker responses, but the principle is the same—consistent thermal stress, even mild, can accumulate into measurable adaptations over time.
What about people who can't exercise? Is this actually a replacement?
The lead researcher was explicit that it's not a replacement. But for someone recovering from surgery, or elderly, or dealing with chronic pain, a daily bath might provide some of the anti-inflammatory benefits of exercise when exercise itself isn't possible. It's complementary, not alternative.
The infrared sauna result surprised me. It's marketed as more effective.
The marketing doesn't match the data. Despite claims about deeper heat penetration, the study found it produced minimal changes in core body temperature. The hot water simply outperformed it across every measure.
Who shouldn't be doing this?
Anyone with serious heart conditions needs medical clearance first. The cardiovascular system is working hard during heat immersion—heart rate and blood pressure both rise. If your heart is already compromised, that stress could be dangerous.