A single Finnish sauna session temporarily increased circulating immune cells. Digest
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 the immune system.
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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 (163.4°F) 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 the sauna, immediately after, and 30 minutes later to measure white blood cells, which are immune cells in the blood that help the body respond to threats, and cytokines, which are signaling proteins that help coordinate immune responses.
- Average body temperature rose from 36.4°C (97.5℉) before the sauna to 38.4°C (101.1℉) immediately afterward.
- In men, circulating white blood cells rose from 6.1 to 6.7 billion cells per liter of blood (E9/L) right after the sauna, 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.
- The relative proportions of white blood cell types did not clearly change, suggesting a general rise in circulating white blood cells rather than a shift in cell balance.
- Out of 37 cytokines measured, only two showed changes in circulating levels: 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 white blood cells or their main subtypes. In additional exploratory analyses, usual sauna habits did not appear to change the response.
The findings suggest that acute sauna heat exposure can mobilize white blood 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, had no control group, and did not directly test the effects on immune function or infection risk. Nevertheless, the results suggest that sauna bathing 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.