New Study Reveals the Benefits of a Hot Bath vs. Sauna for Cardiovascular and Immune Health

Hot-water immersion (like soaking in a hot tub) can significantly raise core body temperature, mimicking many of the effects of moderate aerobic exercise.

Heat therapies like saunas and hot baths have long been used for relaxation and wellbeing. But a recent study compared these methods head-to-head — pitting hot-water immersion (at 40.5 °C) against a traditional dry sauna at 80 °C and an infrared sauna (between 45 °C and 65 °C) to see which offers the greater cardiovascular and immune benefits. The results are striking: soaking in hot water raised core body temperature more, and produced a stronger cardiovascular and immune response, than any of the saunas. In practice, a hot-tub session can act as a form of "passive exercise", lowering blood pressure and increasing circulation, much like moderate physical activity.

In this article we'll look in detail at what the study found — from blood pressure and cardiac output to immune-cell activation — and what it means in practice for health. You'll see why hot-water immersion outperforms saunas in some areas, and when to choose one or the other depending on your health goals.

Key findings: hot-water immersion vs. saunas

• Rise in core body temperature: Hot-water immersion (40.5 °C for 45 minutes) produced the largest rise in core body temperature — around +1.1 °C — far higher than the +0.4 °C from the traditional sauna and almost no change with the infrared sauna. In other words, hot water produced a much greater thermal load than dry air or infrared radiation.

• Cardiovascular response: Submersion in hot water produced the largest rise in cardiac output (the volume of blood the heart pumps per minute), with an increase of around +3.7 L/min. The traditional sauna increased it by +2.3 L/min, and the infrared sauna by just +1.6 L/min. The heart worked harder in hot water — pumping more blood — much like during aerobic exercise.

• Lower blood pressure: Hot-water immersion also lowered blood pressure more than the sauna sessions.

• Immune stimulation (IL-6): Hot-water immersion was the only condition to produce a notable rise in interleukin-6 (IL-6), an inflammatory cytokine linked to physical exercise and immune-system activation.

• Immune-cell activation: 24 hours after the hot-water session, participants showed higher levels of natural killer (NK) cells and cytotoxic T lymphocytes (CD8+) — cells central to the body's defence against viruses and tumours. The effect persisted up to 48 hours for the T cells.

• Comparison summary: Overall, a single session of hot-water immersion produced the greatest thermal load, cardiovascular response and immune activation, compared with a traditional dry sauna and an infrared sauna. The infrared sauna was the mildest of the three — it failed to raise core temperature meaningfully or to produce relevant cardiovascular or immune changes.

The study: hot bath vs. traditional sauna vs. infrared sauna

The study, conducted at the University of Oregon and published in 2025 in the American Journal of Physiology, was the first to compare the immediate effects of three forms of passive heat therapy directly in the same group of participants.

Method:

• Participants: 20 healthy young adults (10 men and 10 women), active and non-smokers, aged 20 to 28.

• Conditions tested:

  • Hot-water immersion: 40.5 °C for 45 minutes (bath up to the neck).

  • Traditional dry sauna: 80 °C, three 10-minute sessions with cool-down breaks between each.

  • Infrared sauna: between 45 °C and 65 °C for 45 minutes.

Researchers measured core body temperature, heart rate, blood pressure and cardiac output throughout each session. They also took blood samples before and after each session — and again 24 and 48 hours later — to track IL-6 levels and immune-cell counts.

Hot bath: a passive workout for the heart

During the 45-minute immersion in 40.5 °C water, participants showed:

• A significant rise in core body temperature (+1.1 °C)

• Higher cardiac output (+3.7 L/min)

• Lower blood pressure

• Acute rise in IL-6

• Higher NK and CD8+ counts in the following 24–48 hours

This kind of physiological response resembles what's seen in moderate-intensity training. The heat of the water, paired with hydrostatic pressure, made the body react much like physical exercise: the heart pumps more blood, vessels dilate, sweat increases (but doesn't evaporate), and core temperature steadily rises.

Traditional sauna: intense heat, moderate effects

The dry sauna (80 °C for 3×10 minutes) also produced physiological changes:

• Moderate rise in body temperature (+0.4 °C)

• Higher cardiac output (+2.3 L/min)

• Slight drop in blood pressure

• No significant changes in IL-6 or immune cells

Although it offers cardiovascular benefits, the thermal load was lighter than in the hot bath, and the body could regulate temperature better through sweating and the breaks between sessions.

Infrared sauna: comfortable, but mild

The infrared sauna was the gentlest of the three:

• Body temperature largely unchanged

• Modest rise in cardiac output (+1.6 L/min)

• No changes in blood pressure or immune system

It feels comfortable, but the physiological impact is limited — ideal for anyone looking for relaxation without intense cardiovascular or thermal effort.

Why is hot water so effective?

• Thermal conductivity: Water transfers heat 25 times faster than air. Even at a lower temperature, it warms the body more quickly.

• Sweat doesn't cool: In water, sweat doesn't evaporate — so it doesn't cool the body. Heat keeps building up.

• Hydrostatic pressure: Water exerts even pressure on the body, helping venous return and increasing the volume of blood reaching the heart.

• Continuous session: Unlike saunas (which have breaks between rounds), the immersion ran continuously for 45 minutes, maximising the thermal effect.

Immune benefits: activating the body's natural defences

Only hot-water immersion produced:

• A rise in IL-6 (an inflammatory cytokine linked to adaptive responses)

• Higher NK and CD8+ counts, central to clearing viruses and tumour cells

These effects mirror the benefits of intense exercise. The traditional sauna and the infrared sauna didn't produce these responses, most likely because they didn't reach a high enough thermal load.

When to choose a sauna or a hot bath

Key:

✅ = Especially effective
✔✔✔ = Strong benefit
✔✔ = Moderate benefit
✔ = Slight or indirect benefit

Conclusion: heat is health

If you're looking for an effective way to support cardiovascular health, strengthen your immune system, ease stress and improve recovery, heat therapy is a natural, powerful approach.

Research shows that both hot-water immersion and sauna use produce significant short-term physiological effects, including a higher heart rate, better circulation and lower blood pressure. Practised regularly, both methods are linked to lasting health benefits, including stronger cardiovascular function, less inflammation, better sleep quality and improved overall wellbeing.

"I have no doubt that, if people are willing to do heat therapy, it will line up with better health — provided it's done in moderation."
— Dr Christopher Minson, study co-author

In the end, the best choice is the one you can fit into your routine. Whether it's the dry intensity of a traditional sauna, the gentler heat of an infrared sauna or the immersive calm of a hot bath, building a consistent heat-exposure habit can become a cornerstone of your long-term health.

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