Hot water immersion leads to the greatest thermoregulatory, cardiovascular, and immune responses compared with both traditional and far infrared saunas

[cl-user]

[Edit: Added missing charts + wrong description]

We have demonstrated that hot water immersion leads to the greatest thermoregulatory, cardiovascular, and immune responses compared with both traditional and far infrared saunas in young, healthy adults. Far infrared sauna heating was the least impactful on raising body core temperature and the resulting cardiovascular and immune responses.

How they did it:

During hot water immersion, participants were immersed to the level of the sternum in ∼40.5°C water for 45 min. Participants’ left arm was supported above the water for blood pressure measurements.

Paper: Comparison of thermoregulatory, cardiovascular, and immune responses to different passive heat therapy modalities

---

Figure 3. Total sweat loss in hot water immersion (HWI), traditional sauna (TRAD), and far infrared sauna (FIR). **P < 0.01 HWI vs. FIR and TRAD.

---

Figure 2. Core temperature (A ) at baseline (BL), throughout heating, and recovery in hot water immersion (HWI), traditional sauna (TRAD), and far infrared sauna (FIR). Skin temperature (B ) and mean body temperature (C ) at baseline and throughout heating. *P < 0.05 HWI vs. FIR, †P < 0.05 HWI vs. TRAD, ‡P < 0.05 TRAD vs. FIR.

---

Figure 4. Heart rate (A ) and mean arterial pressure (B ) at baseline and throughout heating in hot water immersion (HWI), traditional sauna (TRAD), and far infrared sauna (FIR). *P < 0.05 HWI vs. FIR, †P < 0.05 HWI vs. TRAD, ‡P < 0.05 TRAD vs. FIR.

---

Figure 6. Cardiac output (A ), systemic vascular resistance (B ), and stroke volume (C ) at baseline (BL), halfway through heating (MID), and at the end of hearting (END) in hot water immersion (HWI), traditional sauna (TRAD), and far infrared sauna (FIR). *P < 0.05 HWI vs. FIR, †P < 0.05 HWI vs. TRAD, ‡P < 0.05 TRAD vs. FIR.

Conclusions

This study indicates that a single session of hot water immersion leads to the greatest physiological strain compared with both traditional and far infrared saunas due to the unique environmental challenge imparted by whole body water immersion. The large increase in core temperature elicited by hot water immersion results in cardiovascular strain and immunological alterations, which may lead to beneficial health adaptations with repeated exposures. All physiological responses were far less robust with traditional and far infrared saunas in a commercially available sauna unit.

[Viracocha]

As Bryan Johnson said in his videos, too much temperature in your testies is horrible for their health. How would we protect then if we immerse ourselves in water?

[cl-user]

That’s a valid concern. As I had not looked at that before, I just asked Chat (GPT 5.1 thinking mode).
Here is the final summary which I find informative, for me at least.

1. How testicular temperature regulation normally works

The testes are designed to run cooler than core body temperature—typically about 2–4 °C lower (around 33–35 °C vs ~37 °C). This cooler environment is crucial for spermatogenesis, because germ cells are quite temperature-sensitive.

Several anatomical and physiological mechanisms maintain this lower temperature:

• Scrotal skin Thin, richly vascularized, with sweat glands and a large surface area to lose heat to the environment.
• Dartos muscle In the scrotal wall, it contracts to wrinkle the skin (reducing surface area and conserving heat) or relaxes to smooth and thin the scrotum (increasing heat loss).
• Cremaster muscle Raises the testes closer to the body when cold and lowers them when warm, adjusting their distance from the warmer core.
• Pampiniform plexus A venous network surrounding the testicular artery that works as a counter-current heat exchanger, cooling incoming arterial blood before it reaches the testis.

In normal conditions, air flow, evaporative cooling, and loose clothing further support this system, keeping intratesticular temperature in the narrow range needed for optimal sperm production.

2. Why hot-water immersion overrides this regulation

During hot-water immersion (HWI)—for example, in a hot bath or hot tub at ~39–40+ °C—the testes are surrounded by water that is at or above core temperature. This changes the physics completely:

• Water conducts heat far more efficiently than air, so heat is driven into the scrotum and testes faster than they can get rid of it.
• Evaporative cooling is essentially shut down; the scrotal skin is covered by water, not air.
• Even if the cremaster and dartos try to adjust position or surface area, they cannot move the testes out of the uniformly hot environment if the pelvis is immersed.
• As a consequence, intratesticular temperature rises toward bath temperature, especially during prolonged immersions.

In short: the body’s usual mechanisms to keep testes cooler are engineered for air, not for being submerged in hot water, so HWI partially “disables” this cooling system.

3. Short- and long-term effects of higher temperature on sperm

Short-term effects

Spermatogenesis is highly temperature-sensitive. Even a sustained 1–2 °C increase at the testicular level can disturb the process:

• Acute or repeated heat stress can damage meiotic and post-meiotic germ cells in the seminiferous tubules.
• This manifests as lower sperm count, reduced motility, and a higher proportion of abnormal forms in the weeks following the heat exposure.
• There may also be increases in DNA fragmentation and mitochondrial dysfunction in sperm after repeated heating.

After a period of frequent hot baths, hot tubs, or saunas, studies often show significantly reduced sperm concentration and total count, sometimes by 20–50% or more, depending on intensity and duration.

Long-term consequences and reversibility

The important nuance is that, in most healthy men, this is functionally significant but biologically reversible:

• The damage primarily affects ongoing spermatogenic waves , not the underlying spermatogonial stem cell pool .
• A full spermatogenic cycle takes about 74 days, plus 2–3 weeks for epididymal maturation and transit, so recovery is on the order of 3 months.
• When intense scrotal heating is reduced or stopped, many studies show partial or complete normalization of:
  • Sperm count
  • Motility
  • Morphology

So, chronic or frequent heat can measurably impair semen quality, but in the typical “hot tub / sauna user” scenario, these effects are substantial yet generally reversible over 3–6 months once exposure is reduced.

4. Effect of higher temperature on testosterone and gonadal hormones

The picture for testosterone is quite different from sperm.

Acute hormonal responses

During and shortly after heat exposure (sauna or HWI), you may see:

• Small, transient fluctuations in testosterone (sometimes down, sometimes up), largely driven by:
  • Hemodynamic changes (plasma volume shifts)
  • Stress hormones (cortisol, catecholamines)
• These changes are usually modest and brief, normalizing within hours.

Long-term testosterone effects (including Finnish sauna data)

Looking at regular sauna users, especially from Finnish cohorts where sauna use is habitual (several times per week):

• Studies show clear, reversible changes in semen parameters , but no strong evidence of sustained low testosterone attributable to sauna use.
• Basal testosterone generally remains within normal ranges in frequent sauna users.
• LH and FSH (pituitary signals to the testes) are usually preserved, indicating that the hypothalamic–pituitary–gonadal axis remains intact.
• Leydig cells (which produce testosterone) appear less temperature-sensitive than germ cells within the wellness-range of heat exposure (sauna 2–7×/week, moderate session lengths).

In other words:

• Spermatogenesis is the fragile part;
• Testosterone production is comparatively robust under typical sauna/HWI conditions studied in humans.

For men not planning to conceive, the main real-world consequence of frequent hot-water immersion is:

• Significant but reversible reduction in sperm quality ,
• With little evidence of clinically meaningful long-term suppression of testosterone , particularly in Finnish-style heat practices.

5. Big-picture summary

• The testes are kept cooler than core by a sophisticated combination of anatomy and thermoregulation designed to protect spermatogenesis.
• Hot-water immersion bypasses much of this protection by immersing the scrotum in a highly conductive, hot environment, pushing testicular temperature upward.
• This produces notable short- and medium-term declines in sperm quantity and quality , which are usually reversible over a few spermatogenic cycles once heat exposure is reduced.
• By contrast, testosterone levels and the gonadal hormonal axis are far less affected by typical sauna/HWI use; population data (e.g., Finnish cohorts) do not suggest a major hypogonadal signal from frequent sauna practices.
• So the primary cost of repeated testicular heating is on fertility, not on androgen status, which matters a lot when deciding how worried to be if you’re no longer planning to conceive.

[Tom_Clark]

I intermittantly do home hot baths. I dial it up by heating a big pot of water on the stove. I’ve started using an ice pack for “the boys”, simple frozen plastic bags. I go through several for 20-30 minutes. Not perfect, but noticably cooler, cheap and good enough.

[AustraliaLongevity]

My first thoughts. Also does anyone know of a good ice pack to use in the sauna?

[relaxedmeatball]

Great study! I’ve only tried proper HWI a couple of times and it’s VERY miserable IMO. Far less pleasant than the sauna. It’s no surprise that it’s more effective because it’s literally just sous vide cooking you!

The sweat loss is really interesting to see though, because obviously you don’t see or feel sweaty whatsoever compared to sauna.

And yeah, cooking your testicles is a definite limitation of water immersion. Maybe good for younger guys who aren’t looking to have kids?

[RapAdmin]

@AustraliaLongevity

Also does anyone know of a good ice pack to use in the sauna?

Coming soon to your Blueprint shopping network: Bryan Johnson Approved Testical Coolers:

Have to say though, you may get kicked out of the sauna at the gym if you walk in wearing these…

[AustraliaLongevity]

They won’t come close enough to me to kick me out.

[cl-user]

Looks like it’s not so easy to fill a and keep a bathtub at 40.5ºC (105F). I could not go over 38.5ºC (101F) even with periodic refills and my core temperature only reached 37.6ºC(99.7F)

Before that I did a 4km walk and my core temp reached 37.9ºC (100.2F). On a long run I can easily reach 38.6ºC(101.5F) even on a normal cool day.

[LukeMV]

I’m just imagining someone walking in on me in the sauna while I have an ice pack on my balls

[Phillipe]

I regularly float in my large spa enjoying the view of the cosmos above. Incredible what one sees on a clear evening (getting close to being a UFO believer). It has a max temp setting of 104F but with the insulated cover it hits 105.

[AlbertN]

The far better results for traditional sauna vs infrared sauna might be partially attributed to the fact that traditional saunas were run at 176 degrees while the far infrared were run at an average of only 113 degrees. The researchers thought that continuous operation of infrared heaters was more important than total heat.

Thus, the FIR saunas might be significantly better than what is shown in this paper (as is also evidenced by other papers they cite.)

[relaxedmeatball]

That was my thought too. The issue was that the IR sauna was large and the panels couldn’t run at 100% the whole time. If you had an infrared sauna blanket which can go up to 70˚C (158˚F) and you’re enclosed in there, that’s definitely going to elevate body temperature.

[AlbertN]

I think their problem is that the panels turn off once the sauna reaches the desired temperature and they wanted their panels to run continuously (in preference to keeping the temperature constant). In my infrared sauna I cover the temperature sensor with a wet cloth to allow higher temps to be reached, and the panels run continuously.

From my experience even 155 degrees is not hot enough. 113 degrees is so low that I’m not surprised that the physiological changes were minimal.

[AustraliaLongevity]

Gotta own it

[Olafurpall]

I think that’s irrelevant for genereal health and longevity and only matters if you want to get someone pregnant soon. Note that Bryan tends to exaggerate things by attempting to be youthful in all areas, even those that don’t matter for health and longevity (like e.g. his hair loss focus).

[Viracocha]

Mechanistically, it makes sense that not only your sperm but also your testosterone worsens with frequent hot bath use. That’s something worth testing. I would love to help but I don’t have a bath.

[Olafurpall]

It’s certainly possible that testosterone production is also effected, but I doubt it has significant effect on testosterone production. The cells that produce testosterone are not the same as those that produce sperm. Sperm producing cells appear to be unusually sensitive to heat.

[AgentSmith]

@AlbertN

The far better results for traditional sauna vs infrared sauna might be partially attributed to the fact that traditional saunas were run at 176 degrees while the far infrared were run at an average of only 113 degrees.

176 is also quite cool for a traditional sauna. I feel cold when temperatures are below 180 and prefer 190-200 with löyly.

The heart rate curves are correspondingly low. My RHR dips below 50 overnight, is in the 50s when I wake up, my VO2 max came in at 50.3 mL/kg/min at an exercise physiology lab last month, yet I know my HR gets up to at least 120 bpm in the sauna.

This is another study with suboptimal design.

@relaxedmeatball

I’ve only tried proper HWI a couple of times and it’s VERY miserable IMO.

Yes, I have trouble getting past 30 min in the 108-110 deg range.

@Tom_Clark

I’ve started using an ice pack for “the boys”, simple frozen plastic bags.

Agreed. I just fill a small Ziplock bag with ice cubes and wrap it in a paper towel. Dispose of after.