Both air temperature and humidity independently shape how the body responds during Finnish sauna sessions

A controlled study with 50 healthy adults measured heart rate, core temperature, skin temperature, and perceived effort across four consecutive sauna sessions while systematically tracking both air temperature and relative humidity. Researchers found that each environmental variable was independently linked to greater cardiovascular and thermal strain, even after accounting for sex, body composition, and fitness level.

What the study found

Researchers recruited 50 recreationally active adults (32 men, 18 women; average age roughly 35) to complete four 10-minute sauna exposures separated by 30-minute rest periods. Participants were permitted to pour water on the heated stones in the traditional Finnish manner, allowing relative humidity to vary naturally alongside air temperature. Environmental conditions were tracked at multiple points throughout each session.

Heart rate climbed substantially during sauna exposure — on average about 42 beats per minute above baseline — while core temperature rose more modestly, by roughly 0.4°C. Statistical modeling showed that both air temperature and relative humidity were independently associated with increases in heart rate and core temperature. In practical terms, higher humidity appeared to add physiological load on top of whatever the air temperature alone would produce.

These associations held up after the researchers adjusted for individual characteristics including sex, physical activity level, body fat percentage, and body surface area, suggesting the environmental effects were not simply a reflection of who was in the room. Perceptual discomfort tracked most closely with air temperature, though humidity contributed a smaller but detectable share.

One candidate mechanism — the difference between dew point and skin temperature, used as a proxy for how readily moisture condenses on the skin — showed only weak links to heart rate and no meaningful relationship with core temperature. Repeating the sauna session across four rounds did not substantially change heart rate responses, and core temperature differences between sessions were minor.

Limits worth knowing

The study enrolled a relatively homogeneous group of healthy, recreationally active adults, so findings may not extend to sedentary individuals, older adults, or those with cardiovascular or metabolic conditions. Each session lasted only 10 minutes, and the overall protocol spanned a single visit, leaving open questions about how repeated sauna use over days or weeks might alter these responses. The study did not measure sweat rate directly or assess hormonal, inflammatory, or longer-term cardiovascular markers. No funding disclosures were identified in the available information. Because participants controlled the löyly themselves, humidity levels were not experimentally fixed, which adds ecological validity but limits precise dose-response interpretation.

About this entry. The Heartwood Review summarizes published, peer-reviewed research in its own words and links to the original source. It is not medical advice; consult a physician before beginning or changing any thermal or recovery practice. Findings summarized here describe the study cited, not any product sold by Teak & Marble Atelier. Corrections.