A single session in a Finnish sauna briefly raises the number of white blood cells, but the study does not show that this boosts immunity or protects against illness. Neutrophils and lymphocytes go back to their starting levels within 30 minutes of the session ending.

A team from the University of Eastern Finland and the University of Turku, led by Prof Jari Laukkanen, carried out the work. It was published in 2026 in the journal Temperature. White blood cells are part of the body's defences. Neutrophils are the first to reach a site of inflammation or infection, while lymphocytes take part in the more targeted response.

73C sauna with a cold shower in the middle

The trial involved 51 adults, 27 women and 24 men, with an average age of about 50. Each spent 30 minutes in a dry Finnish sauna at an average of 73C and 10-20% humidity. Halfway through, they cooled off with a short cold shower. Blood was taken before the session, right after it and 30 minutes later.

According to a description of the study, most of the participants already used a sauna regularly. None had known heart or blood vessel disease, but each had at least one risk factor, such as high blood pressure or being overweight.

Body temperature, measured in the ear, rose from 36.4C to 38.4C. All the types of white blood cell that were measured went up. A mixed group of the remaining cells stayed raised a little longer than the neutrophils and lymphocytes.

Authors suggest cells came from tissue

The authors did not measure where the extra cells came from, and this is their own interpretation. Had the body made new ones, they would not have dropped back to their starting level so fast. The researchers therefore suggest that the sauna pushes cells out of their "stores" in the tissues and into the blood, where they later return. Ilkka Heinonen of the University of Turku believes such periodic release is useful, because cells that leave their stores patrol the body better. A similar short rise is seen after physical exercise.

Cytokines, the signalling proteins of the immune system, reacted less. On average, the sauna barely changed their levels. Only two of the 37 cytokines measured changed significantly, and one showed a borderline change. Laukkanen says the levels of several cytokines varied with how far body temperature rose. No such link was found for white blood cells. The authors speak of a "two-speed" response, in which cells move quickly while cytokines respond less and more selectively.

Just one session and 30 minutes of follow-up

The study looked only at the immediate effects of one session. The authors say firm conclusions about long-term health cannot be drawn from this. The results show neither a lasting boost to immunity nor a lower risk of illness, and the changes in the cells are short-lived.

The cold shower was part of the routine, so the study measured the effect of the sauna together with the cooling, not of the heat alone. There was no comparison group that skipped the sauna, and blood was tracked for only 30 minutes after the session. Monocytes, eosinophils and basophils were not separated out, and finer groups, such as natural killer cells or T cells, were not examined.

In observational studies in Finland, sauna use has been linked to fewer deaths from heart and blood vessel disease, but this trial does not prove that. Such studies can also be affected by lifestyle.

The study does not show whether regular sessions leave a lasting mark on the immune system, and the team has not announced a follow-up.