A single Finnish sauna session temporarily increased circulating white blood cells.
Sauna studies often focus on long-term health patterns, but the immediate changes that occur in the body during heat exposure are less clear. In a new study, researchers tested how one traditional Finnish sauna session affected immune-cell counts, inflammatory signaling molecules, and body-temperature change.
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The study included 51 middle-aged adults in Finland who had at least one cardiovascular risk factor (e.g., high blood pressure or diabetes) but no known cardiovascular disease. Each participant completed a 30-minute dry sauna session at about 73°C with 10 to 20% relative humidity, including a short 2-minute shower after the first 15 minutes. Blood samples were collected about 2 hours before sauna, immediately after sauna, and 30 minutes later to measure immune cells, which help the body detect and respond to threats, and cytokines, signaling proteins that help immune cells communicate.
- Body temperature rose from 36.4°C before the sauna to 38.4°C immediately afterward.
- Immune cells increased right after the sauna. In men, counts rose from 6.1 to 6.7 billion cells per liter of blood (E9/L), then returned near baseline after 30 minutes. In women, they rose from 6.2 to 6.7 E9/L and stayed slightly elevated 30 minutes later.
- Several immune-cell groups showed a similar short-term rise. Lymphocyte and neutrophil levels increased right after the sauna, then moved back toward baseline, while a mixed group of immune cells (monocytes, eosinophils, and basophils) stayed modestly elevated after 30 minutes.
- The relative proportions of white blood cell types did not clearly change. This suggests a general increase in circulating immune cells rather than a major shift in the balance of cell types.
- Out of 37 cytokines measured, only two changed: sCD30/TNFRSF8 and pentraxin-3, which are involved in immune-cell activation and early inflammatory responses.
- Body-temperature change correlated with changes in several cytokines, but not with total leukocytes, neutrophils, lymphocytes, or the mixed group of immune cells. In an additional exploratory analyses, usual sauna habits did also not appear to change the response.
The findings suggest that acute sauna heat exposure can mobilize immune cells into the bloodstream without broadly increasing inflammatory signaling molecules. Because the researchers accounted for small shifts in blood concentration (plasma volume), the increase was unlikely to be explained by fluid loss alone. In practical terms, this may reflect a temporary increase in immune surveillance, with more cells available to circulate through the body and potentially encounter signals of stress or damage.
The study only examined an acute physiological response and had no control group. It did not directly test the effects on immune function or infection risk. Nevertheless, the results suggest that sauna heat can cause rapid, temporary immune changes that may help explain some of the health benefits linked to regular sauna use. In this clip, I outline my sauna routine, including temperature, duration, and hydration.