Tracing Sauna’s Mechanism of Action for Health

By Allison Proffitt 

Researchers in Finland have sought to quantify how the practice of Finnish sauna bathing (FSB) contributes to reduced morbidity and mortality. They found that sauna bathing releases white blood cells into the bloodstream that play a key role in the body’s defense against various pathogens and diseases. The study was published last week in the journal Temperature (DOI: 10.1080/23328940.2026.2645467).   

Heat stress poses a significant physiological challenge to the human body; however, regular sauna bathing, as a form of controlled heat exposure, has been linked to health benefits. The study aimed to explore potential mechanisms underlying these benefits, the authors wrote.  

The researchers studied 51 adults (27 women; 24 men) with a mean age of 50 years who were exposed to a 30-minute session of acute FSB at a temperature of 73°C with one brief cool-down shower halfway through. Venous blood samples were collected at baseline, immediately after and 30 minutes after the FSB and analyzed for immune cells and 37 cytokines. The participants were allowed to drink water throughout. 

The researchers found that the sauna session increased the number of all circulating white blood cells. “Total white blood cell (WBC) count rose significantly and remained slightly elevated 30 minutes post-sauna in women. Neutrophil and lymphocyte counts increased immediately after the FSB but returned to baseline after 30 minutes, whereas MXD cells (monocytes, eosinophils, basophils) remained elevated,” the authors wrote.  

Ilkka Heinonen, an Academy Research Fellow at the University of Turku and first author on the paper, considered the implications. “This may indicate that sauna bathing mobilizes additional white blood cells into the bloodstream from tissues, where they are then redeposited after the session. This kind of periodic release of white blood cells into the bloodstream is beneficial, as once they leave their storage sites, they are better able to patrol the body and respond to pathogens,” she said in a statement.  

In addition to immune cells, the researchers also measured the levels of numerous cytokines, which act as mediators in immune defense. On average, sauna bathing had little effect on circulating cytokine levels. 

“Interestingly, however, the levels of several cytokines changed in relation to how much body temperature rose during sauna bathing. No similar association was observed between white blood cell counts and changes in body temperature,” says Professor Jari Laukkanen, who led the study at the University of Eastern Finland. 

The study, based on one sauna session, has limitations. The authors posit that regular sauna users may have different results from those who used it seldom as regular users are likely more heat-adapted, but no definite conclusions can be drawn about the development of longer-term health effects from one session. But it’s clear, the authors write, that a 30-minute session of acute Finnish sauna bathing induces immune cell mobilization. “The association between changes in body temperature and circulating cytokine levels suggests that heat stress and immune activation may partly, but certainly not completely, mediate the beneficial health effects of sauna bathing,” they assert.