Fifty-one Finnish adults sat in a 73-degree Celsius sauna for 30 minutes, took a brief cold shower halfway through, and had blood drawn three times. Every type of white blood cell measured rose immediately afterward. Most of that increase was gone within half an hour.
That is the finding, and it is more modest than the headlines it has generated.
The study was published in the journal Temperature by a team led by Jari Laukkanen at the University of Eastern Finland. It appeared online on March 31 and was widely covered in April, then recirculated through science aggregators in late July, which is why it may look new this week.
One thing the study did not do is measure whether anyone got sick. There were no infection outcomes, no illness tracking, and no comparison of who fell ill afterward. A change in a blood marker is not the same as a change in whether you catch something.
What Was Measured
Participants were 27 women and 24 men, with a mean age of 50 years in both groups, most of whom had at least one cardiovascular risk factor but no active cardiovascular disease. They were not sauna novices, which matters in a country where the practice is routine.
Each underwent a single 30-minute session at 73 degrees Celsius with 10 to 20 percent relative humidity, with a short cold shower partway through and 500 milliliters of water to drink during the session and recovery.
Blood was drawn before the sauna, immediately after, and 30 minutes later. Researchers counted white blood cell subtypes and measured 37 cytokines, the signaling proteins immune cells use to coordinate. They also tracked how much each person's body temperature rose.
Measuring both cells and signaling molecules at the same time is the design feature that makes this study more informative than a simple cell count, and it is also what produced its most interesting result.
What Actually Happened
All circulating white blood cell types increased immediately after the session. Neutrophils and lymphocytes, the two subsets most associated with infection defense, were back to baseline within about 30 minutes. The combined category of monocytes, eosinophils, and basophils stayed elevated longer. The overall proportions of cell types were preserved.
The cytokine results went the other way. Of 37 measured, only two changed significantly, and both decreased. One additional marker showed a borderline increase. That is a very small signal, and it is the opposite of what "sauna activates the immune system" implies to most readers.
The paper's own title states the conclusion plainly: acute sauna exposure induces stronger immune cell responses than cytokine responses.
There was also a curious mismatch. Cytokine changes correlated with the extent to which a person's body temperature rose. White blood cell counts did not. Whatever drives the cell movement does not appear to be a simple heat dose.
Laukkanen described the likely mechanism as relocation rather than production. The results "may indicate that sauna bathing mobilizes additional white blood cells into the bloodstream from tissues," he said, with the cells later redeposited.
That distinction matters. The body did not make more immune cells. It temporarily moved existing ones from tissue reservoirs into circulation, then returned them. Exercise produces a broadly similar transient shift.
Why a Marker Change Is Not a Health Outcome
This is where most coverage of this study goes wrong, and where readers should be most careful.
Circulating white blood cell counts rise for many reasons unrelated to improved defense, including stress, dehydration, and simple postural changes. A 30-minute heat exposure followed by a cold shower involves several of those. The study cannot distinguish a meaningful immune priming effect from a nonspecific physiological response.
Even if the mobilization is genuinely useful, a half-hour window of elevated cells is a long way from fewer infections. Demonstrating that would require a trial that tracks illness in people who sauna regularly against people who do not, over a season, with infections counted. That study was not done here.
The 51 participants were also a specific group: middle-aged Finnish adults accustomed to sauna use, most carrying a cardiovascular risk factor. Whether a sauna-naive 25-year-old or an 80-year-old responds the same way is unknown.
There is a separate and stronger body of evidence worth distinguishing from this. Large Finnish cohort studies have associated frequent sauna bathing with lower rates of cardiovascular disease, stroke, dementia, pneumonia, and death from any cause. Those are observational findings, meaning they show association rather than causation and cannot rule out that people who sauna four times a week differ from those who do not in other ways. This new study was designed partly to explore a mechanism that might explain those associations. It does not confirm them.
What This Does and Does Not Support
If you already use a sauna and enjoy it, nothing here argues against continuing. If you do not, this study is not a reason to start, and it certainly does not support a specific protocol.
Be skeptical of anyone selling one. Sauna businesses and wellness sites have already converted this paper into session-length prescriptions and claims of an immune boost lasting long after you step out, which the data do not support. The elevation in the two most-cited cell types lasted roughly 30 minutes.
Standard sauna safety still applies and is not addressed by this study. Heat exposure causes fluid loss, so drink water. People with cardiovascular disease, low blood pressure, or a history of fainting should ask a clinician before regular use. Alcohol and sauna are a poor combination. Pregnant people should check with their obstetric provider. Anyone who feels lightheaded should get out.
The confirmed finding is that a single 30-minute sauna session temporarily increased circulating white blood cell counts in 51 middle-aged adults, with the main subsets returning to baseline within half an hour and with almost no cytokine response. This is most relevant to researchers investigating why sauna use tracks with better health outcomes in population studies. The most reasonable action for readers is none. The central uncertainty is whether this transient cell movement affects anyone's risk of getting sick, which no one has yet tested.
Frequently Asked Questions
What did the study find? That a single 30-minute sauna session at 73 degrees Celsius temporarily increased all measured types of circulating white blood cells in 51 middle-aged adults.
How long did the effect last? Neutrophils and lymphocytes returned to baseline within about 30 minutes. Monocytes, eosinophils, and basophils stayed elevated somewhat longer.
Does this mean sauna use prevents infections? No. The study did not measure illness, infections, or any health outcome. It measured blood markers at three time points.
What happened to the cytokines? Of 37 measured, only two changed significantly, and both went down. The paper's title notes that the immune cell response was stronger than the cytokine response.
Did the sauna create new immune cells? No. The researchers describe the change as mobilization, meaning existing cells moved from tissues into the bloodstream and were later redeposited.
Is this new research? It was published in Temperature in March 2026 and covered in April. It recirculated through science aggregators in late July, which is why it may appear new.
Who should be cautious about sauna use? People with cardiovascular disease, low blood pressure, or a history of fainting should consult a clinician. Avoid combining alcohol with sauna, stay hydrated, and pregnant people should check with their provider.
