One Sauna Session Raises White Blood Cell Counts, Finnish Study Finds

A peer-reviewed study published in the journal Temperature in March measured what one Finnish sauna session does to the immune system: a single 30-minute session at 163°F raised white blood cell counts significantly in both men and women, with some immune populations staying elevated for at least 30 minutes after participants left the heat.

The research was led by Ilkka Heinonen of the University of Turku, alongside cardiologist and Professor Jari Laukkanen of the University of Eastern Finland, whose longitudinal KIHD cohort studies established that frequent sauna use is associated with a 40% reduction in all-cause mortality for 4-7x weekly users. This study asks what happens inside the body during a single session, rather than whether regular sauna use correlates with better long-term health outcomes.

What they did

Fifty-one adults from Jyvaskyla, Finland participated. Twenty-seven women, 24 men, average age 50, average BMI just under 27. These were people with at least one cardiovascular risk factor (high blood pressure, smoking history, dyslipidemia, family history of heart disease), but no active heart disease. Roughly a third hadn’t been in a sauna in months. Another third used one weekly or less. The remaining third went 2-3 times per week.

The protocol: one 30-minute Finnish sauna session. The room was set to 176°F and held at a measured average of 163°F with 10-20% relative humidity. Temperature monitoring was continuous using internal sensors designed by Harvia. Participants sat alone in sex-specific sauna rooms wearing swimwear, drank water throughout, and had a brief shower after the first 15 minutes. Blood was drawn immediately before, immediately after, and 30 minutes after the session. The researchers tested for white blood cell counts across all major subtypes, plus 37 cytokines. Body temperature was measured from the ear. It started at around 97.5°F and rose to 101.1°F on average by the end of the session.

White blood cell counts rose right after the session

Total leukocyte count rose significantly right after the sauna and stayed slightly elevated in women at the 30-minute mark. Neutrophils and lymphocytes both spiked immediately post-sauna, then returned to baseline by the 30-minute blood draw. The MXD group, the combined count of monocytes, eosinophils, and basophils, went up immediately and stayed elevated for the full recovery window in men.

Proportions staying the same while total counts rise tells researchers this is a generalized immune mobilization, not a targeted stress response to a specific threat. Every white blood cell type increased roughly in parallel.

The researchers also ruled out hemoconcentration, the artifact where blood appears to have more cells simply because sweat loss has reduced plasma volume. All results were corrected for individual plasma volume changes, and on average, plasma volume did not shift significantly because participants drank water throughout. This was a real mobilization of immune cells into circulation.

Only two of 37 cytokines changed

Of 37 cytokines tested, only two changed significantly. Pentraxin-3 dropped, more notably in women. sCD30 also decreased. One other cytokine, MMP-2, trended upward but didn’t reach statistical significance. For the vast majority of immune signaling proteins measured, the sauna session did nothing detectable in absolute terms.

The researchers did identify 18 statistically significant correlations between the change in body temperature and changes in specific cytokines, concentrated immediately post-sauna and largely gone 30 minutes later. The bigger someone’s body temperature rose during the sauna, the more certain cytokines moved in predictable directions. Among the cytokines most strongly correlated with that temperature rise were interferons.

Interferons tracked the temperature rise

The cytokines most positively correlated with body temperature rise included IFN-alpha2, IFN-beta, and IFN-gamma, three core members of the interferon family. Interferons are the immune system’s primary antiviral signaling proteins. When a cell detects viral invasion, one of its first moves is to secrete interferons that alert neighboring cells to activate their own antiviral defenses. Interferons are also central to cancer immune surveillance and are the basis for multiple cancer and autoimmune therapies.

This study did not show that sauna fights viruses or treats cancer. The design cannot support those conclusions. What it found is that people whose body temperature climbed the most during a sauna session showed the strongest correlations in interferon-type cytokines. Whether that represents meaningful antiviral activation or just a fever-mimicking temperature response remains an open question. The researchers are careful about this distinction, describing these findings as associations that “suggest” a mechanism rather than proving one.

Still, this sits alongside prior population-level research showing that regular Finnish sauna use is associated with significantly lower rates of pneumonia and chronic obstructive pulmonary disease. The plausible immune mechanism behind those associations has never been clearly identified. These cytokine correlations, transient and correlational as they are, point in a direction worth taking seriously.

The observed associations between changes in body temperature and circulating cytokines suggest that sauna-induced heat stress, along with immune activation, may partly mediate the health benefits of Finnish sauna bathing.

What this study does not prove

Fifty-one people is a modest sample. The participants were middle-aged Finns with cardiovascular risk factors, so the results may not generalize broadly. The tympanic thermometer used to measure body temperature does not capture true core temperature, though the researchers defend it as practical for a study of this scale and design.

The study did not break out T-cell subsets, natural killer cells, or specific B-cell populations. Monocytes, eosinophils, and basophils were measured only as a combined MXD figure. The cytokines were measured in serum, so the researchers cannot say where the cells migrated or what the cytokine signals ultimately triggered in tissue. The 30-minute post-sauna window may also simply be too short to see the full picture of recovery and downstream immune activity.

There were no differences in immune response based on how often participants used saunas at home. Habitual users (2-3 times per week) showed essentially the same immune response as people who almost never go. If regular sauna were building a stronger or faster immune response through repeated thermal adaptation, you would expect to see some signal here. The study did not find one, at least not in the variables measured.

A possible immune pathway behind the long-term data

Researchers have spent years documenting that regular sauna use correlates with better health outcomes across multiple disease categories: cardiovascular, respiratory, neurological, metabolic. The KIHD data, the COPD and pneumonia associations, and the dementia and Alzheimer’s data are all strong. But the question of why has been only partially answered, mostly through cardiovascular pathways involving blood pressure, arterial stiffness, and cardiac remodeling.

Heinonen, Laukkanen and their colleagues are opening up the immune pathway as a mechanistic explanation. If every sauna session mobilizes white blood cells and triggers temperature-correlated cytokine responses, including interferon activity that resembles what the body deploys against pathogens, then the long-term data showing fewer infections and less respiratory disease among frequent sauna users starts to have a plausible biological story behind it.

That story is not proven. Laukkanen is expected to speak at the World Sauna Forum in Jyvaskyla this June, where this research is likely to be discussed alongside the broader KIHD findings.