A new Finnish study has upended a common assumption about saunas: heat is only half the story. Researchers at the University of Jyväskylä and the Finnish Institute of High-Performance Sport (KIHU) found that air humidity is an independent driver of the body's response to a session, on a par with temperature itself. The study was published in the peer-reviewed journal Temperature in July 2026 and detailed in an August 2026 write-up.
Fifty healthy adults sat through four 10-minute sauna sessions in cabins set to about 75°C, using electric, wood-burning, gas and hydrogen heaters. The work was carried out in collaboration with Finnish sauna manufacturer Harvia.
The gap between a dry cabin and a steamy one turned out to be dramatic. Without water on the hot stones, relative humidity in the room sat at just 6 percent. Once volunteers threw water on the stones, the traditional Finnish practice known as löyly, humidity climbed to 59 percent.
That difference translated into real cardiovascular strain. Average heart rate rose from 79 to 121 beats per minute over ten minutes, comparable to moderate-intensity exercise. Core body temperature climbed from 37.4 to 37.8°C.
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Why steam changes the equation
When the air holds more moisture, sweat cannot evaporate off the skin as easily. Evaporation is the body's main cooling mechanism, so blocking it forces the heart and circulatory system to work harder to keep core temperature in check. The extra work is where much of the sauna's cardiovascular training effect appears to come from.
The corresponding author is Iida Laatikainen-Raussi of the University of Jyväskylä's Faculty of Sport and Health Sciences. She and her colleagues concluded that, while temperature remains the single most important factor, humidity has a clear, measurable effect on heart rate and body temperature that had not been isolated in earlier sauna research.
What the long-term evidence shows
The finding matters because most saunas outside Finland run dry, including many gym and hotel cabins. The wider body of Finnish sauna research is built on the löyly tradition and points to substantial benefits from regular use.
In the Kuopio Ischemic Heart Disease study, a long-running cohort of 2,315 middle-aged Finnish men, those who took a sauna 4 to 7 times a week had a 77 percent lower adjusted risk of dying from cardiovascular disease than once-a-week bathers. Similar heart-protective patterns have since been observed in women.
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Follow-up work in the same cohort showed a 46 percent lower risk of developing hypertension over 22 years among the most frequent bathers. And in a 20-year analysis, men in the 4-to-7-times-a-week group had a 66 percent lower risk of dementia and a 65 percent lower risk of Alzheimer's disease than once-weekly users.
These are observational associations, not proof of cause and effect, and Finnish participants typically grow up with sauna use from childhood. Anyone with heart disease, low blood pressure or who is pregnant should speak with a doctor before starting regular sauna sessions.
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.