Study: Single Sauna Session Increases Circulating Immune Cells

A recent study published in the journal Temperature observed an increase in circulating white blood cell counts following a single Finnish sauna session. The research found minimal average changes in cytokine levels, though several cytokines showed changes correlated with increased body temperature.

The researchers note the study examines only acute physiological responses and cannot determine long-term health effects.

Study Design and Participants

The research was conducted by scientists from the University of Eastern Finland and the University of Turku. It involved 51 participants with an average age of 50 years. All participants were regular sauna users and had at least one cardiovascular disease risk factor, but no known cardiovascular disease.

Participants underwent a single 30-minute Finnish sauna bathing (FSB) session with air temperatures between 70–100°C. The session included a brief cooling period.

Key Physiological and Immune Findings

Body Temperature: Core body temperature, measured at the ear, increased steadily during the sauna session.

White Blood Cells: Circulating white blood cell counts increased during the session.

Counts of neutrophils and lymphocytes returned to baseline levels 30 minutes after the session concluded.
The combined category of monocytes, eosinophils, and basophils (MXD cells) showed a persistent elevation at the 30-minute post-session measurement.
No direct correlation was found between the change in body temperature and the change in white blood cell counts.

Cytokines: On average, sauna bathing had minimal impact on circulating cytokine levels. Only two cytokines showed a significant decrease post-exposure, with one additional marker showing a borderline increase.

However, the levels of several cytokines, including specific interferons and interleukins, were positively associated with the increase in ear temperature. Two other cytokines were inversely correlated with temperature change.

Other Measures: Hemoglobin and hematocrit levels increased in participants during the session. Platelet counts were also higher during FSB and were observed to be higher in women than in men. Average plasma volume did not change significantly.

Researcher Interpretation

Researchers suggested the increase in white blood cells may indicate that heat exposure mobilizes cells from tissues into the bloodstream, with subsequent redeposition after the session.

They note this periodic release, which also occurs during physical exercise, could allow for more effective immune surveillance.

Professor Jari Laukkanen, who led the study, stated that while white blood cell counts were not correlated with temperature change, the levels of several cytokines were.

Research Context and Limitations

The authors placed the study within existing literature that reports associations between regular sauna use and lower risks of cardiovascular disease, dementia, and some respiratory conditions, as well as lower levels of systemic inflammation. They noted few prior studies have explored both immune cells and cytokines in relation to FSB-induced heat stress.

The study outlined several limitations:

It measured only three main white blood cell categories and did not assess monocytes, eosinophils, and basophils separately.
No detailed lymphocyte assessment was conducted.
The physiological effects of the cytokines correlated with temperature change remain unknown.
The origin of the mobilized white cells is unclear.
The research measured only acute physiological responses to a single session rather than chronic resting conditions.

Conclusions and Future Research Directions

The researchers concluded that a single Finnish sauna session acutely increases circulating immune cell counts with minimal changes in cytokines, though cytokine levels correlate with body temperature rise.

They emphasized that the study's focus on a single session prevents definitive conclusions about longer-term health benefits. Future research directions highlighted include:

Distinguishing sauna-induced responses in habitual users from those in non-users.
Examining the relevance of combined heat and cold stress exposure.
Determining whether acute physiological responses translate into long-term health effects.

The study was published in the scientific journal Temperature.