Hot Baths Vs. Saunas: Science Weighs in on What Gives Your Heart And Immune System A Bigger Boost
Key Highlights
- Soak > Sweat: A 45-min 40°C hot tub soak raised core body temperature about +1.1°C, versus only +0.4°C in a dry sauna (and almost 0°C in an infrared sauna). The hotter core kick-starts your circulation and immune response much more.
- Heart Happy: This greater heat load made hearts pump harder – cardiac output jumped 3.7 L/min in the hot tub vs. 2.3 L/min in a sauna – similar to a mild workout. Regular hot baths or sauna sessions can relax blood vessels, lower blood pressure, and improve artery function
- Health Payoff: Long-term heat therapy pays off. Finnish-style saunas (80–100°C) have been linked to lower risk of high blood pressure, heart disease, stroke and dementia. Daily hot-tub baths were tied to ~23–46% fewer cardiovascular events in one study.
What Is Passive Heat Therapy?
Passive heat therapy simply means heating your body from the outside (without exercise). Think hot tubs, steam baths, and saunas.
It’s a time-honored practice in many cultures: from Japanese onsen and Turkish hammams to Finnish saunas.
In scientific terms, passive heat therapy exposes you to high temperatures (often 70–100°C in a sauna, or warm water around 37–40°C) for a brief period.
Unlike a gym workout, you’re not moving, but your body still reacts. Your blood vessels dilate and your heart rate rises to cool you down, much like light exercise.
This simple practice has been shown to deliver real health benefits.
Hot Tub vs. Saunas: The Big Chill Study
A new University of Oregon study put three popular heat sessions to the test:
- Hot Tub (HWI): 45 minutes soaking in 40°C water.
- Traditional Sauna (TRAD): Three 10-minute bouts at 80°C with short breaks.
- Far-Infrared Sauna (FIR): 45 minutes at 45–65°C (the panels heat you, not the air).
The results were striking: hot water won on all fronts. Volunteers’ core temperatures rose about +1.1°C in the hot tub, versus just +0.4°C in the dry sauna and virtually no change in the infrared sauna.
Because your body heats up more, the heart works harder: cardiac output (blood pumped per minute) jumped by ~3.7 L/min in the hot tub, compared to ~2.3 L/min in the sauna.
And only hot water triggered a real immune reaction. Blood levels of interleukin-6 (IL-6) – a cytokine that usually spikes after exercise – increased only after the hot tub soak.
Natural killer cells and CD8+ T-cells (immune soldiers) also rose 24–48 hours later.
In plain English: the hot bath was like a mini immune workout, but the saunas barely roused these cells.
Takeaway: Immersing in hot water gives you the biggest “heat dose,” because water conducts heat much better than air. You can’t cool off by sweating as easily when submerged, so body temperature climbs higher.
Higher core temp means more blood flow and more “exercise-like” effects on vessels and immunity.
Why Heat Helps Your Heart and Blood Vessels
Raising core body temperature isn’t just cozy – it boosts circulation.
Your body responds by dilating (widening) blood vessels and increasing heart rate, to carry heat away. This surge in blood flow exerts a gentle “exercise” effect on your cardiovascular system.
In fact, hot soaking causes similar hemodynamic changes as low-intensity exercise.
In one landmark study, eight weeks of hot water immersion (about 90 min sessions, 4–5×/week) doubled the artery dilation response (flow-mediated dilation) and reduced arterial stiffness in sedentary adults – results on par with starting an exercise program.
Blood pressure dropped several mmHg, and thickening of artery walls decreased.
Simply put, frequent heat therapy makes your blood vessels more flexible and healthy. This is likely why long-term sauna lovers see less high blood pressure and heart disease.
For example, a 15-year study of Finns found that men using a sauna 4–7 times per week had about a 70–77% lower risk of dying from heart disease than those who went only once a week.
Hot tubs may offer similar perks – one analysis reported 23–46% fewer heart events for folks bathing nearly daily.
Why it works: Heat stress triggers the release of nitric oxide and other signals that make arteries relax. It also generates mild heat shock proteins and beneficial anti-inflammatory responses.
Think of each warm soak as a gentle “workout” for your blood vessels – no gym needed.
Immune Perks: Sauna vs. Bath
The new study highlighted an immune twist. IL-6 sounds like bad news (it’s a pro-inflammatory cytokine), but context matters.
When IL-6 spikes from exercise or sauna, it actually helps the body adapt and fight infections. In the hot bath study, IL-6 rose only after hot water immersion, not the saunas.
Likewise, immune cells like NK cells and T-cells got a bump. These cells are on patrol for viruses and cancer cells.
Why only the bath? Experts note that being in very warm water prevents your usual cooling sweats, so the heat stress is much stronger.
Jessica Atencio (lead author) explains: “When you’re submerged, the sweat mechanisms aren’t efficient”, so core temperature climbs higher. That extra warmth is a powerful signal to blood vessels and your immune system.
In practice, this suggests hot baths might give an extra immune nudge over a quick sauna. This could be useful for older adults trying to stay immune-healthy.
However, even dry saunas and infrared sessions have been linked to immune benefits like improved white blood cell function over the long term.
The key is moderate, repeated exposure.
Traditional Sauna & Infrared Sauna: How They Compare
Dry (Finnish) saunas and infrared saunas feel different, and science sees differences too.
A traditional Finnish sauna blasts air to 70–100°C. It makes you sweat heavily and can raise skin and core temps by about +0.9°C in a 30-min session.
Infrared saunas heat you more gently (often set ~50–60°C), warming skin muscles directly. One study showed muscle temperature rises under IR light, but core body temperature barely budged.
Bottom line: If your goal is maximum cardiovascular/immune response, hot water wins out, and dry sauna is in the middle. Infrared is the mildest on core temperature.
That said, even mild heat is good heat. Repeated IR sauna sessions have improved blood vessel function in studies, and they can be more comfortable for people who can’t handle blazing heat.
Pro Tip: If you use an infrared sauna, try to sit a bit closer to the heat source and do longer sessions (20–45 min) to warm up. If you’re after a big core-temperature jump, nothing beats water at the right temperature.
Safety Note: All heat therapies carry some risk if misused. Overheating can cause lightheadedness, dizziness, or even heat injury.
Always hydrate before and after, and never stay so hot you feel faint.
Hot tubs have unique warnings: poorly maintained ones can harbor bacteria (e.g. “hot tub lung” from bacteria).
Hot water can also drop blood pressure quickly when you stand up, so get up slowly.
It’s wise to get a doctor’s OK if you have heart disease, very high blood pressure, or other health issues. As Professor Christopher Minson says, make sure you’re cleared for heat exposure, just like exercise.
But for most healthy adults, a little sauna or soaking bath is safe and rejuvenating if done right.
How to Try It: Actionable Tips
- Start Slow and Build Up: If you’re new, begin with just 5–10 minutes at a moderate temperature, then cool down. Gradually work up to longer sessions.
- Set the Right Temperature: Aim for about 38–40°C (100–104°F) water in a hot tub or bath, which closely matches body temperature.
For dry saunas, 70–85°C (158–185°F) is typical – try 10–15 min per round. Infrared saunas can be cooler (~50°C); you might sit 20–30 min. - Stay Hydrated: Drink a glass of water before and after. You’ll sweat out fluid, so rehydrate with water or a light electrolyte drink.
- Listen to Your Body: Feeling warm and a bit sweaty is good. Feeling dizzy, nauseous or headache-y means you’ve done enough. Step out, cool down, and rest.
- Repeat Regularly: Aim for 2–4 sessions per week. Studies suggest more frequent heat use (4–7×/week) yields the strongest long-term gains.
Even 2–3 baths a week can improve blood pressure and fitness over time. - Pair with Relaxation: Use this time to unwind (read, meditate, or listen to music). The stress-relief from a warm soak helps mental health too, contributing to the overall benefit.
- Cool Down Properly: A quick cool shower or just air-cooling after the session helps normalize body temp and can tighten pores.
Quick Wins: Even one or two extra hot baths a week can start improving circulation and mood.
For example, one study in diabetic patients found daily 30-min hot baths lowered fasting blood sugar and A1C within weeks. While your mileage may vary, many people feel more relaxed, sleep better, and notice lower blood pressure with regular soaks.
The Bigger Picture: Heat for Healthspan
Why should a busy 40-, 50-, or 60-year-old care about this?
It turns out heat therapy fits nicely into a longevity-focused lifestyle. By passively “exercising” your heart and vessels, it can help extend your healthspan – the years you feel truly healthy.
Heat therapy ramps up protective mechanisms: it lowers chronic inflammation, improves insulin sensitivity, and even stimulates heat shock proteins that fend off cell damage.
Consider the evidence: Finnish researchers have shown that men who sauna 4–7 times per week live longer and have far less risk of Alzheimer’s and heart disease.
Regular hot-tub immersions improve artery function as much as some exercise programs. If you can’t hit the gym every day, a hot soak a few times a week might be the next best thing.
That’s not to say heat baths replace exercise entirely – but they can be a powerful supplement.
Dr. Minson (UO physiology guru) points out heat therapy is for anyone unable or unwilling to exercise, providing many overlapping benefits.
And on top of physiology, there’s a cultural and mental benefit: these rituals are often relaxing, meditative, and social, which boosts mood and stress resilience.
Practical Takeaway: Think of adding a hot bath or sauna to your weekly routine like taking a daily vitamin for your heart. It’s a simple, enjoyable strategy grounded in science.
And the new research says: if you want the most bang for your buck, get in the water.
References
- 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(1), R20.
- Laukkanen, J. A., & Kunutsor, S. K. (2024). The multifaceted benefits of passive heat therapies for extending the healthspan: A comprehensive review with a focus on Finnish sauna. Temperature (Austin), 11(1), 27–51.
- Laukkanen, T., Kunutsor, S. K., Zaccardi, F., et al. (2018). Sauna bathing is associated with reduced cardiovascular mortality and improves risk prediction in men and women: a prospective cohort study. BMC Medicine, 16(1), 219.
- Brunt, V. E., Howard, M. J., Francisco, M. A., Ely, B. R., & Minson, C. T. (2016). Passive heat therapy improves endothelial function, arterial stiffness and blood pressure in sedentary humans. Journal of Physiology, 594(18), 5329–5342.
- Ely, B. R., Clayton, Z. S., McCurdy, C. E., Pfeiffer, J., & Minson, C. T. (2017). Meta-inflammation and cardiometabolic disease in obesity: Can heat therapy help? Temperature (Austin), 5(1), 9–21.
