Introduction
Hot tubs and saunas have long been linked to relaxation, but can they actually strengthen your immune system? A study from the University of Oregon, published in the American Journal of Physiology in July 2025, looked at how three kinds of passive heat exposure affect the body. The researchers compared hot tub immersion, a traditional dry sauna and a far infrared sauna, and measured changes in body temperature, heart function and certain immune markers. The findings are interesting, but the study included only 20 people, and the results apply only to healthy young adults who already exercise regularly.
One article even goes as far to say: “For anyone unable or unwilling to exercise, heat therapy may offer a surprisingly effective option.” But it seems that once again, the media is misinterpreting the results for clickbait, because there is a very important Caveat.
This article explains what the study found, why the results are limited, and how this pattern matches earlier sauna studies that also found immune changes mainly in trained or athletic people.
What the study found
Atencio and colleagues studied twenty adults who were young, healthy, and regularly active. Each person completed three types of heat sessions: hot water immersion, a traditional sauna, and a far infrared sauna. Among these conditions, hot water immersion produced the strongest overall response. It increased core temperature the most and led to the largest rise in heart rate and blood flow. This condition also created the biggest shifts in immune markers, including higher levels of cytokines and increases in certain circulating immune cells.
The traditional sauna and the far infrared sauna also produced measurable changes, but the effects were noticeably smaller under the same controlled conditions. These differences suggest that the type of heat exposure can influence how strongly the body reacts, at least in this very specific group of fit young adults.
The fine print that matters
Small sample and very specific group
Because the study used only twenty people, and all of them were young and fit, it is hard to know whether the results would apply to older adults, people with health conditions, or anyone who does not exercise regularly.
Short term responses only
The study had a very small sample of only twenty participants, and every person was young, healthy, and exercised regularly. Because the results come from such a narrow group, there is no basis to assume these findings apply to older adults, sedentary individuals, or anyone with health conditions. It is important to never take the results of a single study outside of the context of the scope under which it was studies (i.e. who was tested, what exactly was done, and how it was conducted).
Immune boost is not a simple idea
The study showed short term increases in some immune markers. That does not mean fewer infections or stronger immunity. It simply shows that the immune system reacted to heat stress.
This pattern matches earlier sauna research. For example, a 2023 study found that immune changes showed up in trained men after several Finnish sauna sessions. The changes did not appear in untrained men. This suggests that exercise level and fitness influence how the immune system responds to heat.
Modality and safety matter
Hot water immersion created the strongest heat load because the body cannot cool itself as easily in water. This can be stressful for the heart. People with cardiovascular or heat sensitivity issues should be careful. In some cases, these conditions may make hot water immersion unsafe.
What this means in practice
- Heat therapy can cause short term immune changes, but only in specific groups so far. it seems that athletes and people that regularly exercise are the only ones who show a noticeable immune response.
- Hot water immersion, including hot tub health benefits and sauna health benefits, may create stronger effects, but it also puts more stress on the body.
- If you are older, less active, or have medical concerns, you should not assume the results apply to you (until more research is conducted to be conclusive on this).
What research still needs to answer
- Larger studies with a wider range of people
- Studies that test repeated heat exposure over weeks or months
- Clear comparisons that test how long these immune changes last
- Real health outcomes such as infection rates, not only laboratory markers
Conclusion
While the research study itself was quite small, it created a big buzz on the internet. As usual, our penchant for miracle cures and our habit of trending towards clickbaits prevails over the fine print. The new study adds useful information about how different kinds of heat affect the body. Hot water immersion creates strong short term responses in temperature, circulation, and immune markers in healthy adults who exercise. But the study had clear limitations to how and when we can apply its findings, primarily reinforcing that heat therapy only shows a response in athletes and adults that exercise frequently. Earlier sauna research shows a similar pattern: immune changes appear mainly in people who are already trained or accustomed to heat.
The take away is simple. Heat therapy has potential, but we should be careful about claims and pay attention to the fine print.
