New research reveals how a brief Finnish sauna session mobilizes immune defenses within minutes, offering fresh clues into how heat exposure may influence human health.
Study: Acute Finnish sauna heat exposure induces stronger immune cell than cytokine responses. Image credit: Mr. Tempter/Shutterstock.com
Heat exposure is harmful but can sometimes have beneficial effects. Finnish sauna bathing (FSB) is an example of controlled acute heat stress that may influence immune function by altering immune cells and cytokines. A 30-minute Finnish sauna session rapidly increases circulating immune cells while leaving most cytokine levels unchanged, according to a study published in the journal Temperature.
How Finnish sauna bathing became a global health habit
FSB involves exposing people to relatively dry air at 70–100 °C for 10-30 minutes per session. It is part of Finnish culture and is shown to offer multiple health benefits. Existing literature reports lower risks of cardiovascular disease, dementia, psychosis, and some respiratory conditions. FSB is also associated with fewer adverse outcomes in disadvantaged socioeconomic settings and with better physical fitness.
Acute heat stress may reshape immune and inflammatory responses
Prior research suggests that regular FSB is associated with lower levels of systemic inflammation. The current study examined how heat stress associated with FSB manifests as acute changes in white blood cells and cytokines up to 30 minutes after the sauna.
Single sauna session triggers rapid immune cell mobilisation
Participants were free of known cardiovascular disease (CVD) but had one or more risk factors, such as smoking, hypertension, or obesity. The study included 51 middle-aged adults who were regular sauna users. During the 30-minute sauna session, body temperature, measured at the ear, increased steadily, reflecting the heat stress exposure, while plasma volume remained unchanged on average.
Among women, hemoglobin and hematocrit levels were lower, while platelet counts were higher than in men. In both sexes, all these factors increased with FSB. White cells also rose in women, and remained slightly higher at 30 minutes post-sauna.
Among white cells, neutrophils and lymphocytes returned to baseline at 30 minutes, unlike the persistent elevation seen with the combined MXD cell category (monocytes, eosinophils, and basophils). Overall, white cell proportions were preserved.
Only two cytokines changed significantly after FSB, and both decreased, while one additional marker showed a borderline increase. Immediately after FSB exposure, body temperature was correlated with circulating cytokine levels, though not with white cell counts. These included several interferons and interleukins, which were positively associated with raised ear temperature. Conversely, two cytokines were inversely correlated with ear temperature. These changes were independent of the frequency of sauna use across the week.
These findings suggest that white cells are mobilized in association with FSB exposure, but there was no direct correlation with increased temperature. Significant associations were observed for only a few cytokines, whereas a larger number showed correlations with temperature change in the immediate post-sauna period. The associations between temperature change and cytokines were largely unchanged after adjustment for sex or body mass index.
The authors propose that physiological challenge caused by FSB-induced heat stress may underlie these changes. The increase in hemoglobin concentration and hematocrit suggests a possible loss of plasma volume in the hot, dry FSB conditions, although average plasma volume did not change, and results were adjusted for individual variation.
The increase in neutrophil and lymphocyte counts could be due to immune cell mobilization in response to heat stress. The generalized nature of the response is suggested by the overall preservation of the differential count.
Apparently, a large temperature change is not required for such mobilization, as no correlation was found between temperature change and white cell count. Conversely, the former was associated with cytokine changes in the immediate post-sauna period, suggesting a heat-stress response.
The authors report that to the best of their knowledge, few prior studies have explored both immune cells and cytokines in relation to FSB-induced heat stress. The sauna use pattern in this study closely resembled that in real life and in some earlier research.
A few other studies have shown changes in some interleukins using different sauna protocols. This highlights the need for further research to distinguish sauna-induced responses in habitual sauna users (who are likely heat-adapted) from those in individuals who seldom or never use it.
Limited immune profiling leaves key cell responses unclear
The authors measured three main white cell categories (neutrophils, lymphocytes, and the combined MXD group), but did not assess monocytes, eosinophils, and basophils separately. A more detailed lymphocyte assessment could have better assessed the heat-stress response to the FSB.
The physiological effects of cytokines correlated with temperature change following FSB remain unknown. The origin of the possibly mobilized white cells is unclear. The possibility of low plasma volume remains relevant, but was not factored into the analysis. Moreover, the study measured only the acute physiological response rather than chronic resting conditions.
Future directions should include research on the relevance of intermittent heat and cold stress exposure and whether these acute responses translate into long-term health effects.
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Journal reference:
Heinonen, I. H. A., 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. DOI: https://doi.org/10.1080/23328940.2026.2645467. https://www.tandfonline.com/doi/full/10.1080/23328940.2026.2645467#abstract
