A new study shows that hitting a sauna for 30 minutes causes a transient spike in the number of circulating white blood cells. The researchers suggest that this exercise-like effect might provide health benefits by improving immune surveillance [1].
How does a sauna do its trick?
Robust epidemiological data has associated sauna use with health benefits. Large prospective cohort studies – mostly from Finland, where sauna use is culturally ubiquitous – have linked regular sauna bathing to reduced risks of cardiovascular disease, stroke, dementia, pneumonia, and all-cause mortality. However, the biological mechanisms explaining these robust associations between heat exposure and health outcomes remain poorly understood.
One plausible candidate pathway is the immune system. This link was investigated in a new study from the University of Eastern Finland, published in the journal Temperature. The same group had previously shown in large population studies that regular sauna users have lower levels of C-reactive protein (CRP), a standard blood biomarker of systemic inflammation [2].
In middle-aged adults, the authors measured the counts of different white blood cell subtypes circulating in the blood, and the levels of 37 cytokines before, immediately after, and 30 minutes after a single Finnish sauna session. Cytokines are small signaling proteins that cells use to coordinate responses, regulate inflammation, and communicate with other cells. Measuring both cell counts and cytokines together, alongside body temperature changes, was designed to give a more complete picture of what happens to the immune system during heat stress.
The group consisted of 51 adults (27 women, 24 men, mean age 50, mostly with at least one cardiovascular risk factor but no active cardiovascular disease). The participants underwent a single 30-minute sauna session at 73°C with 10-20% relative humidity. Participants were allowed to drink half a liter of water during the session.
A spike in circulating WBCs
The first step was to address a critical potential confounder: plasma volume change. Sweating causes blood to become more concentrated, meaning that cell and protein counts per unit volume go up simply because there’s less water diluting them. The authors measured hemoglobin and hematocrit, which measure red blood cell proportions, to calculate individual plasma volume shifts, then mathematically corrected all cell and cytokine measurements accordingly. In practice, plasma volume did not change significantly on average, as participants drank enough water to compensate, but there were slight hemoglobin and hematocrit changes, which the authors corrected for nonetheless.
Ear (tympanic) temperature increased from 36.4°C to 38.4°C on average by the end of the session. This indicates a genuine, if moderate, heat stress, putting the body in the low fever territory. It is worth noting that tympanic temperature is an imperfect proxy for true core temperature, but rectal or esophageal measurements, which are the gold standards, are less suited for large cohort studies.
Total white blood cell (WBC) count rose significantly immediately after sauna use in both sexes. Neutrophils and lymphocytes (T cells and B cells) both increased immediately post-sauna but returned to baseline within 30 minutes. The MXD cell group, a combined metric of monocytes, eosinophils, and basophils, also rose immediately but remained elevated at the 30-minute mark, particularly in women.
Importantly, while total cell counts changed, the proportions of each cell type within the WBC pool did not. In other words, all subtypes went up together rather than one being preferentially recruited, suggesting a generalized, non-selective mobilization as opposed to the immune system targeting a specific threat. The authors argue that this reflects immune cell mobilization from tissue reservoirs into the bloodstream.
“This may indicate that sauna bathing mobilizes additional white blood cells into the bloodstream from tissues, where they are then redeposited after the session. 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,” said Ilkka Heinonen, an Academy Research Fellow at the University of Turku, and one of the authors.
Cytokines mostly unchanged
Despite the clear WBC mobilization, only two of the measured 37 cytokines changed significantly in response to the sauna session, demonstrating that cytokine signals unexpectedly did not coordinate that mobilization. So, the coordination probably happens via non-cytokine-related pathways. Alternatively, the cytokine changes might be too localized or too brief to be captured in blood at these timepoints; future studies with more participants might help to elucidate this particular point. However, there was a certain link between cytokine dynamics and temperature: participants who heated up more tended to show different cytokine trajectories than those who heated up less.
“Interestingly, the levels of several cytokines changed in relation to how much body temperature rose during sauna bathing,” said Professor Jari Laukkanen, who led the study at the University of Eastern Finland, another author. “No similar association was observed between white blood cell counts and changes in body temperature.”
While the WBC recruitment that the researchers observed might indeed be beneficial, this was not something that the study tested. It also does not provide any additional insights into the possible mechanisms. However, similar transient WBC mobilization happens during exercise [3], suggesting that the two types of stress might have something in common. Other research has framed repeated sauna use as a hormetic stressor (hormesis is a dose-response phenomenon in which low-dose exposure to a stressor stimulates adaptive and protective effects, while high doses are harmful) [4].
Literature
[1] Heinonen, I. H., Koivula, T., Hollmén, M., Immonen, J., Kunutsor, S. K., Jalkanen, S., & Laukkanen, J. A. (2026). Acute Finnish sauna heat exposure induces stronger immune cell than cytokine responses. Temperature, 1-14.
[2] Kunutsor, S. K., Laukkanen, T., & Laukkanen, J. A. (2018). Longitudinal associations of sauna bathing with inflammation and oxidative stress: the KIHD prospective cohort study. Annals of medicine, 50(5), 437-442.
[3] Sand, K. L., Flatebo, T., Andersen, M. B., & Maghazachi, A. A. (2013). Effects of exercise on leukocytosis and blood hemostasis in 800 healthy young females and males. World journal of experimental medicine, 3(1), 11.
[4] Hussain, J., & Cohen, M. (2018). Clinical effects of regular dry sauna bathing: a systematic review. Evidence‐Based Complementary and Alternative Medicine, 2018(1), 1857413.
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