Some people swear by a sauna session. They claim that sitting and sweating in a hot room rejuvenates them.
But for such a popular trend, the scientific evidence to support that feeling is surprisingly scarce.
Researchers in Finland have now found evidence that a single sauna session can have an immediate impact on the immune system at a cellular level.
In a small experiment, just 30 minutes in a Finnish sauna – with hot yet dry heat – triggered a significant increase in circulating white blood cells.
These crucial immune cells help defend the body against pathogens, and they often increase after exercise.
Simply sitting in a sauna could stimulate a similar stress response, and it could have benefits for the immune system.
The results are published in Temperature.
"This may indicate that sauna bathing mobilizes additional white blood cells into the bloodstream from tissues, where they are then redeposited after the session," speculates exercise scientist Ilkka Heinonen from the University of Turku in Finland.
"This kind of periodic release of white blood cells into the bloodstream is beneficial, as once they leave their storage sites, they are better able to patrol the body and respond to pathogens."
In the experiment, 51 middle-aged men and women took part in a traditional Finnish sauna session.
This included sitting in a room at around 73° Celsius (163° Fahrenheit) for 30 minutes, with a cold shower halfway through.
Immediately after the session, a whole host of white blood cells appeared to be mobilized in the blood of participants compared to before the sauna session.
This included an increase in neutrophils, which are first responders, and lymphocytes, which help build immune memory.
There was also an increase in the combined count of monocytes, which engulf pathogens and debris, and eosinophils and basophils, which are both linked to inflammatory responses.
Levels of immune messenger proteins called cytokines, on the other hand, barely shifted, on average, although a few did increase with body temperature.
Thirty minutes after the sauna session, however, this apparent immune surge had mostly dropped off again.
Most of the white blood cell increases had disappeared, but some monocytes lingered.
Researchers only analyzed participants' bloodwork up to 30 minutes after the sauna session, so it is unclear how their immune cells might change over longer periods of time or how that might impact their risk of getting sick.
But some observational studies in the past have found that frequent sauna bathing is associated with a lower risk of some illnesses, like pneumonia.
What's more, other experiments have shown that hot baths can lower your blood pressure and significantly increase red blood cells that carry oxygen.
Bathing in heat seems to produce similar physiological responses to what occurs during exercise, and these cardiovascular outcomes are linked to improved immune function.
That said, vigorous exercise can also backfire, temporarily weakening the immune system, so there's reason to tread cautiously.
Extreme heat isn't something to mess with.
Despite growing global interest in hot and cold therapy, scientists still know remarkably little about how the body copes with extreme temperatures, and what that means for our short-term or long-term health.
The recent experiment in Finland is limited in what it can tell us.
Researchers did not measure specific immune cell subsets, like T cells or natural killer cells, which could have provided more insight into how a sauna actually impacts immune function.
In 2023, for instance, a study reported that Finnish sauna sessions altered T cells and natural killer cells, but only when the sessions were repeated.
What's more, the impact was different depending on whether the participants were trained sauna-goers or not.
In this experiment, repeated sauna exposure increased white blood cell counts in trained participants, but it reduced some T cells in untrained participants.
That study, however, only analyzed healthy young men, which limits how these results can be generalized to others.
For now, all we can say is that it looks increasingly like bathing in heat can trigger a temporary change to immune cells.
Beyond that, it's all guesswork.
Plus, it's good to remember that just like cold therapy, hot therapy might also come with risks…
The research has been published in Temperature.
This article was fact-checked by Peter Dockrill and edited by Peter Dockrill. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.