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.
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 immune-cell function or infection risk. The rise in circulating cells therefore should not be interpreted as proof that one sauna session improves resistance to illness.
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The acute response is nevertheless relevant to a broader pattern. In a prospective study that followed 1,935 Finnish men for a median of 25.6 years, two to three sauna sessions per week were associated with a 27 percent lower risk of the combined endpoint of chronic obstructive pulmonary disease, asthma, or pneumonia, compared with no more than one weekly session. Four or more sessions per week were associated with a 41 percent lower risk. For pneumonia specifically, the corresponding estimates were 28 and 37 percent lower.[ref doi='10.1007/s10654-017-0311-6']
A small controlled trial provides limited evidence for another respiratory outcome. Twenty-five volunteers who used a sauna regularly experienced fewer common colds than 25 controls over six months. The difference emerged primarily during the final three months, when cold incidence was roughly half as high in the sauna group. Sauna did not significantly change the duration or severity of colds that occurred.[ref doi='10.3109/07853899009148930']
Repeated sauna use has also been associated with lower resting levels of high-sensitivity C-reactive protein, fibrinogen, and circulating leukocytes.[ref doi='10.1080/07853890.2018.1489143'] One possible model is that each heat exposure briefly mobilizes immune cells, while adaptation to repeated exposure supports a less inflammatory resting environment. The available studies have not directly demonstrated that sequence, however, and the long-term respiratory findings remain observational.
Taken together, the evidence suggests that sauna can produce rapid, temporary immune changes and that frequent users may experience fewer respiratory illnesses. It does not show that sauna prevents or treats influenza, pneumonia, or an active common cold. In this clip, I outline my sauna routine, including temperature, duration, and hydration.