A Finnish Sauna Session Changed Immune Cells Without Triggering Broad Inflammation
Apr 15 2026
Edited and approved by Stephen C. Rose, Ph.D.
Why This Study Matters
A 30-minute Finnish sauna session pushed immune cells into circulation far more clearly than it changed cytokines, according to a new study of 51 middle-aged adults with cardiovascular risk factors [1]. White blood cell counts rose right after the heat exposure. Most cytokines, by contrast, barely moved. The result suggests that one standard sauna session produces a brisk cellular response, but not a broad inflammatory surge.
That matters because sauna bathing has been tied to better long-term health outcomes in Finnish cohort studies, including lower risks of fatal cardiovascular events and dementia [2,3]. Those associations are intriguing, but they do not explain themselves. Researchers still need to identify the short-term physiologic changes that might help connect repeated heat exposure with those long-term patterns. The new paper focuses on one plausible pathway: immune signaling.
How the Study Was Done
The study was designed as a close look at what happens immediately after sauna exposure.
The research team studied 51 adults from Finland. This included 27 women and 24 men. They were 50 years old on average [1]. Study participants were not known to have heart or blood vessel disease, but each had at least one risk factor, like high blood pressure or obesity. Most were already sauna users, which makes sense in Finland, though it also means the findings may not translate perfectly to people who rarely use saunas.
That participant profile is worth noting for another reason. This was not a study of elite athletes, very old adults, or patients with active heart disease. It was a typical population of at-risk individuals, making the results even more compelling to the sorts of people targeted by purveyors of preventative advice. However, it still left the question of how other groups might respond unaddressed.
The process was true to life and not overly complicated: participants had one 30-minute session in a dry Finnish sauna, followed by a brief shower after 15 minutes. The mean sauna temperature was 73 degrees Celsius, and the humidity was around 10 to 20 percent. Water was allowed during the session and during the 30-minute recovery period. Blood samples were collected before sauna exposure, immediately after, and again 30 minutes later. The researchers measured total white blood cell counts, several major white blood cell subtypes, and a panel of 37 cytokines. They also tracked tympanic temperature and adjusted blood measures for individual shifts in plasma volume, so the results would not simply reflect dehydration [1].
That methods setup matters because it was tailored to capture immediate effects, not delayed ones. The three blood draws made it possible to distinguish what changed right after the sauna from what persisted into early recovery. At the same time, the design could not address whether different immune signals might appear hours later, or whether repeated sauna use would produce the same pattern over weeks or months.
What Changed After the Sauna
The heat load was clear. Mean ear temperature rose from 36.4 degrees Celsius before sauna exposure to 38.4 degrees Celsius immediately after [1]. At the same time, average plasma volume did not change substantially. That point is important because it supports the authors' argument that the rise in immune cells reflected actual mobilization into circulation rather than a concentration effect caused by fluid loss.
The strongest result involved white blood cells. Total white blood cell counts increased significantly after sauna exposure. Immediately following the session, neutrophils and lymphocytes rose, but returned to normal within 30 minutes. A mixed segment including other immune cells stayed elevated a while longer [1]. Importantly, the overall proportions of the major white blood cell groups did not shift much. The effect looked broad rather than highly selective.
That pattern is consistent with what researchers see after other short, controlled stressors. Immune cells can move into circulation quickly when the body is challenged, whether by exercise, temperature, or other physiologic demands. In that context, the new sauna data read less like evidence of harmful inflammation and more like evidence of temporary immune-cell mobilization.
The cytokine findings were much quieter. Of the 37 cytokines measured, only two changed significantly over the sauna session. Levels of soluble CD30/TNFRSF8 and pentraxin-3 fell, while MMP-2 showed only a borderline increase [1]. That is a notably restrained signal compared with the clear white blood cell response. In practical terms, the study did not show a broad cytokine surge following one sauna session.
One of the more interesting details came from the temperature analysis. Changes in white blood cell counts did not correlate strongly with how much body temperature rose. Cytokine shifts, however, showed multiple associations with temperature change, particularly immediately after sauna exposure. The researchers reported 18 significant associations between temperature change and circulating cytokines, including several interferons and interleukins [1]. That does not prove that the temperature rise directly caused those cytokine changes. It does suggest that cytokine behavior may be tied more closely to the heat response itself than the white blood cell response was.
How to Interpret It
Taken together, the results suggest a two-speed picture. Immune cells responded quickly and visibly. Cytokines responded more selectively. One plausible explanation is that white blood cells are mobilized rapidly into circulation during acute heat stress, while cytokine effects are smaller, more delayed, more tissue-specific, or simply harder to capture in peripheral blood over a short time frame.
That interpretation fits reasonably well with earlier sauna research. A 2020 crossover study found that sauna bathing changed some inflammatory markers, including interleukin-6 and interleukin-1 receptor antagonist, but did not produce a sweeping inflammatory response [4]. A 2018 study in non-naive sauna users also reported short-term changes in cardiovascular biomarkers after sauna exposure without broad evidence of systemic inflammatory disruption [5]. The new paper extends that literature by showing that immune-cell mobilization may be one of the clearest immediate signals.
What the Study Cannot Show
However, the study also has limits, and they matter. It was modest in size, it did not include a non-sauna control condition, and it followed participants for only 30 minutes after the session. The researchers did not separate monocytes, eosinophils, and basophils, nor did they examine more detailed immune-cell subsets, such as natural killer cells or specific T-cell populations [1]. Tympanic temperature was also used as the temperature measure, which is useful in practice but not equivalent to direct core temperature measurement.
Most importantly, this study cannot demonstrate that the short-term immune changes induced by a single sauna session account for the long-term health associations observed in observational research [2,3]. That is still a larger and more difficult question. Observational studies can show that frequent sauna users tend to do better over time. They cannot fully separate sauna exposure from the rest of a person's lifestyle, health habits, or social context.
The Bottom Line
Even so, the new study gives the sauna literature something valuable: a clearer short-term biologic signal. A standard Finnish sauna session appears to mobilize immune cells in the blood more consistently than it alters circulating cytokines. That does not make sauna an immune therapy, and it does not prove a longevity effect. But it does move the field a step beyond broad epidemiologic association and closer to a workable physiologic explanation for why repeated heat exposure might matter.
References
1. Heinonen, I.H.A.; Koivula, T.; Hollmen, M.; Immonen, J.; Kunutsor, S.K.; Jalkanen, S.; Laukkanen, J.A. Acute Finnish sauna heat exposure induces stronger immune cell than cytokine responses. Temperature 2026.
2. Laukkanen, T.; Khan, H.; Zaccardi, F.; Laukkanen, J.A.Association between sauna bathing and fatal cardiovascular and all-cause mortality events. JAMA Intern. Med. 2015, 175, 542-548.
3. Laukkanen, T.; Kunutsor, S.; Kauhanen, J.; Laukkanen, J.A. Sauna bathing is inversely associated with dementia and Alzheimer's disease in middle-aged Finnish men. Age Ageing 2017, 46, 245-249.
4. Behzadi, P.; Gravel, H.; Neagoe, P.E.; Juneau, M.; Nigam, A. Impact of Finnish sauna bathing on circulating markers of inflammation in healthy middle-aged and older adults: A crossover study.Complement. Ther. Med. 2020, 52, 102486.
5. Kunutsor, S.K.; Hakkinen, A.; Zaccardi, F.; Laukkanen, T.; Kurl, S.; Jauhiainen, M.; Laukkanen, J.A. Short-term effects of Finnish sauna bathing on blood-based markers of cardiovascular function in non-naive sauna users. Heart Vessels 2018, 33, 1515-1524.
