Does a sauna or cold plunge boost your immune system?

There is a real signal here, but it is smaller and more specific than the claim on the studio wall. The best cold-exposure trial found people took fewer sick days, not that they got sick less often. The heat evidence is weaker and mostly comes from small or observational studies.

So the honest answer is that regular heat or cold probably nudges something, most likely how you feel and function when mildly ill, rather than arming your immune system against infection.

If someone promises you a boosted immune system, they are ahead of the data.

Why is “boost your immune system” the wrong frame?

Because a boosted immune system is not actually a good thing.

Your immune system is not a dial that goes higher. It is a balance. Turn it up indiscriminately and you get allergy, autoimmunity, and inflammation. What you actually want is an immune system that is well regulated, that responds when it should and stands down when it should not.

That is why serious immunologists almost never say “boost.” They talk about modulation, regulation, or specific measures like antibody response or inflammatory markers.

So when a cold plunge is marketed as an immune booster, the phrase itself is a tell. The interesting question is narrower: does regular heat or cold change how often you get ill, or how badly?

What does the cold-exposure evidence show?

One good trial, with a result that is often misreported.

The largest study is by Geert Buijze and colleagues (PLoS One, 2016), a randomized trial of over 3,000 people in the Netherlands. Participants finished their normal shower with 30, 60, or 90 seconds of cold water for a month. The group that did the cold showers reported about 29 percent fewer days of sickness absence from work.

Here is the part that gets left out. They did not report fewer illness episodes. They got sick just as often, they just called in sick less. And it was self-reported, unblinded, in people who had signed up for a cold shower study.

That is a genuinely interesting result about resilience, energy, or perceived coping. It is not evidence of fewer infections. Our cold plunge benefits guide covers what else does and does not hold up.

What about the Wim Hof endotoxin study?

Real, striking, small, and widely overstated.

Matthijs Kox and colleagues (PNAS, 2014) trained 12 healthy young men in a method combining breathing exercises, meditation, and cold exposure, then injected them with a bacterial endotoxin that reliably causes flu-like symptoms. The trained group showed a lower inflammatory response and fewer symptoms than controls.

That is a legitimate finding: it showed the inflammatory response can be voluntarily influenced, which was not the textbook view.

But note what it was. Twelve people. Healthy men. An artificial injected toxin, not an actual infection. And the intervention was a bundle, with the intense breathing the most likely driver, not the cold. Our breathwork safety guide covers why that breathing is not risk-free.

Reading this study as “cold plunges stop you getting sick” is several leaps beyond what it measured.

Does a sauna help you get sick less often?

Weak evidence, pointing gently in a positive direction.

The most cited trial is small and old: Ernst and colleagues (1990) followed about 50 people over six months and found the sauna group reported fewer common colds, mostly in the later months. It is one small study and has not been convincingly replicated.

The stronger sauna data is observational and about serious outcomes rather than colds. The Finnish cohort work links frequent sauna use to lower rates of pneumonia and respiratory illness, but those studies cannot separate the sauna from the healthier, wealthier, more consistent lives of people who sauna often. Our sauna and heart guide explains that same limitation in more detail.

Plausible mechanisms exist, including a mild heat-stress response and better airway conditions. None of it amounts to proof.

Is there any way it makes things worse?

Yes, and this is the practical part.

Do not use a sauna or a cold plunge when you are actually ill with a fever. Your body is already running hot and working hard, and adding heat stress or a cold shock on top is unnecessary strain, with a real risk of dizziness, fainting, and dehydration.

The old advice about sweating out a cold is not supported and can leave you feeling worse. Wait until the fever is gone and you feel roughly normal, then resume.

If you have an immune condition or take immunosuppressive medication, treat any of this as a question for your doctor rather than a wellness decision. Our cold plunge safety guide covers who should be careful generally.

So is it worth doing?

Yes, but for the reasons that actually hold up.

Regular sauna use has the strongest evidence in this field for cardiovascular outcomes, and both heat and cold reliably deliver something people value: they feel better, sleep better, and enjoy the ritual. Those are good reasons.

Immunity is simply not the strongest card in the deck, and picking a habit because of the weakest claim attached to it is how people end up disappointed. Our how often guide covers sensible dosing.

The bottom line

The cold evidence shows fewer sick days called in, not fewer infections. The endotoxin study was twelve people and probably the breathing. The sauna cold data is one small old trial plus association. Something is there, and it is not what the marketing says.

Use heat and cold because they are good for your heart, your head, and your week. Treat any immunity benefit as an unproven bonus.

To find a sauna or cold plunge near you, start with our city wellness maps.