Hot water immersion is an effective way to mimic moderate intensity aerobic exercise and improve blood pressure, similar to what has been shown with traditional sauna.

In a new study, people who engaged in hot water immersion (105°F for 45 mins):

• improved blood pressure by 14 mmHg

• increased cardiac output by 3.7 L/min

• increased heart rate by 39 bpm

Hot water immersion also stimulated immune cells (natural killer and cytotoxic T cells) for up to 48 hours and acutely elevated IL-6, a pro-inflammatory cytokine linked to exercise-induced benefits.

The study also investigated two types of sauna: traditional and infrared.

Traditional sauna (176°F, 3 x 10 mins with a 5-minute cooling period between each) elevated heart rate by 34 bpm, increased cardiac output by 2.3 L/min, and reduced blood pressure by 4 mmHg; while far infrared sauna had the mildest effects on heart rate (+26 bpm), cardiac output (+1.6 L/min), and blood pressure (-1 mmHg).

Why the big difference? Hot water immersion raised core body temperature far more effectively, triggering stronger cardiovascular, inflammatory, and immune responses, likely due to the longer exposure used and the rapid heat transfer from water conductivity.

However, this doesn't mean saunas are less effective overall—the study's protocols influenced these outcomes significantly, and in most contexts a 10-minute sauna session is too short to induce thermal and cardiovascular effects.

Let's break it down.

Putting Hot Tubs and Sauna Head to Head

To compare the cardiovascular, inflammatory, and immune system effects of different heating modalities, researchers recruited 20 healthy adults and put them through three different passive heat exposure protocols in a randomly-assigned order with a 1-week washout period between each:
 

1. Hot water immersion: Participants sat in a hot tub with a temperature of 40.5℃/105℉ for 45 minutes
2. Traditional sauna: 80℃/176℉ for 3 x 10-minute sessions (a 5-minute cooling break separated each exposure)
3. Far infrared sauna: 46℃/115℉ increasing to 65℃/149℉ for 45 minutes
    

These might not be the exact sauna protocols you've heard me discuss or that you've used personally, but the researchers had their reasons for choosing them. Mainly, they wanted to simulate the most common real-world applications of each protocol and match each session for time (~45 minutes). I'll discuss why that matters later on in this email.
 

Among all of the heating modalities, hot water immersion elicited the most robust responses. Core body temperature increased by 1.1℃/2℉ from baseline compared to a modest 0.4℃/0.7℉ increase during the traditional sauna session and no change in temperature during the far infrared sauna session.
 

The robust change in body temperature corresponded with several cardiovascular, inflammatory, and immune responses. Cardiac output increased by 3.7 liters per minute, heart rate reached a peak of 108 beats per minute (a 39 beat per minute increase from baseline), blood pressure decreased by 14 mmHg, and stroke volume increased by 12 mL. 

Additionally, hot water immersion was the only method to stimulate inflammatory and immune responses: IL-6, cytotoxic T cells (CD8+), and natural killer (NK) cells increased while T helper cells (CD4+) decreased after hot water immersion. Some of these responses, like the increase in NK and CD8+ cells and the decrease in CD4+ cells, were present for 24–48 hours after heat exposure.
 

Traditional sauna elicited more moderate responses than hot water immersion, leading to a 2.3 liter per minute increase in cardiac output, a 34 beats per minute increase in heart rate, a 4 mmHg reduction in blood pressure, and a 1 mL increase in stroke volume.
 

Far infrared sauna had the least impact of all three exposures—increasing cardiac output by a modest 1.6 liters per minute, elevating heart rate by 26 beats per minute, reducing blood pressure by just 1 mmHg, and increasing stroke volume by 3 mL.
 

Not only did hot water immersion produce the greatest physiological strain, it also led to the greatest perceptual responses. The participants reported that hot water immersion felt hotter, was more uncomfortable, and was the most physically taxing of all three modalities.

Sauna use as a lifestyle practice to extend healthspan. Dr. Rhonda Patrick and Teresa Johnson. DOI: 10.1016/j.exger.2021.111509

Final thoughts

My takeaway from this study isn't that one method of heat exposure is better than another. Rather, it's that any modality that increases core body temperature significantly will yield cardiovascular benefits—and the hotter you get, the greater those benefits will be. But you have to get hot!
 

This means that when using heat exposure for your own health, the protocol you use should be modality specific. You might need to stay in the sauna for 15–30 minutes at a hotter temperature to get the benefits of a 30–45-minute hot tub session. The duration (and temperature) will determine the robustness of your results. In my opinion, this study didn't perfectly match the conditions for heat exposure, which explained the divergence of the responses between the methods and with previous literature on sauna's effects.
 

Personally, I'm a fan of the traditional (Finnish) sauna, not only because it feels better and is more enjoyable, but also because of the robust literature linking its use to impressive reductions in cardiovascular disease, dementia, and all-cause mortality.
 

My routine? 20–30 minutes at around 186℉ and 10–20% humidity. 

Sometimes I'll use the sauna immediately after a high-intensity Peloton workout and sometimes I'll use it without exercising beforehand. In the case of the latter, I'll stay in for at least 30 minutes. I try to use the sauna 2–5 times per week.
 

The good news is that if you don't have access to a sauna, hot water immersion using a bath or a hot tub can be an incredibly effective way to achieve similar cardiovascular adaptations. 

Not to mention, it's a great way to relax alone or with a partner while achieving health benefits at the same time.