Spending 30 minutes in a sauna triggers a rapid immune response by raising protective white blood cells, University of Turku researchers found.
The findings, published in the scientific journal Temperature, reveal that exposure to high ambient heat prompts the body to mobilize defensive cells into the bloodstream within minutes.
Researchers recruited 51 healthy volunteers, comprising middle-aged men and women, to observe how intense thermal exposure alters circulating blood components.
The volunteers spent half an hour inside a traditional Finnish sauna heated to approximately 73 degrees. The protocol included a scheduled pause at the halfway mark, during which participants took a cold shower before completing the second half of the heat session.
Blood test results
Blood samples collected immediately after the sauna visit showed a significant rise in total leukocytes, the specialized cells responsible for defending the body against infections.
Laboratory tests confirmed that the surge was driven largely by higher counts of neutrophils and lymphocytes, alongside increases in several other varieties of immune cells.
In human biology, leukocytes form the primary cellular defense against disease. Neutrophils act as first responders that quickly engulf and destroy bacteria, while lymphocytes, including T cells and B cells, target viral infections and coordinate long-term antibody responses.
According to the study authors, high heat appears to stimulate a temporary migration of white blood cells from bodily tissues directly into the bloodstream. The researchers suggested this shift could help immune cells travel more actively around the body to detect potential threats.
Temporary immune activation
The observed immune boost was brief. Just 30 minutes after completing the sauna session, the vast majority of measured blood markers had returned to their baseline levels.
The researchers also found that levels of immune signaling proteins remained virtually unchanged throughout the experiment, showing that the high temperature did not cause an inflammatory surge.
These signaling proteins, known as cytokines, act as chemical messengers that coordinate systemic inflammation during illness. Because their concentrations did not rise, the heat appeared to redistribute existing cells rather than trigger a full-scale immune alarm.
The scientists noted that the biological impact of heat exposure closely resembles the body's natural response to physical exercise, where increased circulation temporarily washes immune cells off vessel walls and into the bloodstream.
Sauna health effects
The research team cautioned that it remains unknown whether this short-term mobilization of white blood cells provides any meaningful protection against catching colds or fighting off infections.
Saunas are deeply rooted in Finnish culture, where wood-lined hot rooms are commonly used for relaxation and physical recovery. Thermal heat causes blood vessels to dilate and elevates heart rates in a manner similar to moderate cardiovascular training.
The results build on prior findings regarding the health effects of passive heating. Earlier studies established that spending 30 minutes in a hot bath can lower resting blood pressure and enhance vascular blood flow throughout the body.
The University of Turku, located in southwestern Finland, is one of the nation's leading research institutions. The journal Temperature focuses on peer-reviewed investigations into thermal biology, environmental medicine, and how extreme temperatures affect human physiology.
