Study Reference:
Atencio, J. K., Reed, E. L., Needham, K. W., Lucernoni, K. M., Comrada, L. N., Halliwill, J. R., & Minson, C. T. (2025). Comparison of thermoregulatory, cardiovascular, and immune responses to different passive heat therapy modalities. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, 329, R20-R35.
https://studyfinds.org/hot-baths-heart-immune-system-saunas/
Abstract
A recent study conducted at the University of Oregon by researchers led by Jessica Atencio and Christopher Minson examined the health impacts of three different passive heat therapies: hot water immersion, traditional saunas, and infrared saunas. The research identified hot water immersion as significantly superior in enhancing cardiovascular health and immune function. Unlike sauna treatments, hot baths consistently elevated core body temperature, cardiac output, and triggered robust immune responses.
Methodology
Twenty healthy adults (10 men, 10 women, average age 24) participated in a comparative study consisting of three heat therapy sessions:
- Hot Water Immersion: Participants soaked up to chest level in 104.9°F water for 45 minutes.
- Traditional Sauna: Three rounds of 10-minute exposure at approximately 151°F, each separated by 5-minute cooling intervals.
- Far Infrared Sauna: A continuous 45-minute session starting at 115°F and increasing gradually to about 113°F on average.
The researchers measured core body temperature every five minutes, heart function before, during, and after sessions, and collected blood samples immediately after heating and again at 24 and 48 hours later for immune analysis.
Key Findings
Cardiovascular Responses
- Core Temperature: Hot water immersion raised core temperature by 1.98°F, substantially higher than traditional sauna (0.72°F) and infrared sauna (virtually no increase).
- Heart Rate: Increased by 39 bpm (beats per minute) in hot water immersion, compared to 34 bpm in traditional sauna and 25 bpm in infrared sauna.
- Cardiac Output: Elevated by 3.7 liters per minute with hot water immersion, compared to 2.3 liters in traditional saunas and 1.6 liters in infrared saunas.
- Blood Pressure: Mean arterial pressure dropped significantly by 15 mmHg with hot baths, far surpassing traditional sauna (4 mmHg) and infrared sauna (minimal change).
Immune System Enhancement
- Hot water immersion uniquely triggered notable immune responses:
- Elevated interleukin-6 immediately after the session, mirroring exercise-induced stress responses.
- Increases in natural killer cells and cytotoxic T-cells were observed 24-48 hours after hot bath sessions, indicating potential prolonged immune benefits.
Why Hot Water Immersion is Superior
- Efficient Heat Transfer: Water transfers heat to the body 24 times more effectively than air, resulting in rapid, uniform heating.
- Consistent Exposure: Unlike saunas, hot water immersion minimizes the body’s cooling mechanisms, thus maintaining elevated core temperature.
- Temperature Accuracy: Researchers discovered significant discrepancies between set and actual sauna temperatures in commercial settings, potentially limiting their therapeutic benefits.
Practical Implications
Hot baths offer a practical, affordable alternative to expensive sauna therapies, making sustained heat therapy accessible at home. Continuous heating without intermittent cooling (as seen with Finnish sauna practices) appears crucial for maximizing health benefits.
Limitations of the Study
- Limited to young, healthy, and active participants; results may vary in older populations or those with health conditions.
- Only immediate physiological responses were studied, leaving long-term adaptations unexplored.
- Variation in measurement methods (rectal thermometers vs. telemetry pills) and lack of menstrual cycle considerations may influence generalizability.
Conclusion
This study provides compelling evidence favoring hot water immersion as an efficient, cost-effective method for enhancing cardiovascular health and immune function. For individuals seeking therapeutic heat exposure benefits, hot baths emerge as a powerful, scientifically-supported alternative to saunas.