Humidity Has Been Overlooked in Sauna Research. A New Study Shows Why That Matters.
A new study measures, for the first time, both the sauna environment and the human body simultaneously — and finds that humidity is an independent force
A new scientific study from the University of Jyväskylä and KIHU — the Finnish Institute of High Performance Sport — has done something that no sauna study has done before: it measured what was happening inside the sauna room and inside the human body at the same time, with the same precision, and linked the two together.
That combination sounds straightforward, but it is rarer than you might expect. Most previous sauna research has either measured physiological responses in broadly described environments, or studied the environment without detailed human data. This study brought both together: Harvia provided the environmental measurement expertise and infrastructure, capturing temperature and humidity at six points inside each sauna throughout every session. KIHU and the University of Jyväskylä provided the human measurement expertise — swallowed core temperature capsules, skin probes, continuous heart rate monitoring, and the statistical methods to make sense of it all.
The result is the most complete picture yet of what actually happens to the body when you sit in a Finnish sauna and throw löyly.
The headline finding: humidity is not just a background condition. It independently drives the physiological response — on top of, and separately from, temperature.
How the study was set up
Fifty healthy adults — 32 men and 18 women, average age 35 — each completed four separate 10-minute sauna sessions, with 30-minute rest breaks in between. The four saunas used different heater types: electric, gas, hydrogen, and wood-burning. All were set to approximately 75 °C, but participants were free to throw löyly as much or as little as they liked after the first two minutes of each session.
The environmental measurement setup went well beyond what sauna studies typically use. Each sauna had six sensors — measuring both temperature and humidity at two heights and three horizontal positions inside the room. This captured the real conditions at both head level and sitting level simultaneously, not just the thermostat setting or a single measurement point at the top of the bench.
On the human side, each participant swallowed a small capsule the day before that measured core body temperature from the inside every 30 seconds. Skin temperature was tracked continuously at the forehead, chest, and foot. Heart rate was monitored throughout. Body weight was measured before and after each session to estimate sweat loss. After every session, participants also rated how hot they felt and how comfortable they were.
Crucially, the researchers could match every environmental reading to the physiological data from that exact moment. That direct linkage between what the sauna was doing and what the body was doing is what makes this study different.
What the environment actually looked like
The first striking finding involves the gap between the thermostat and reality. Across all sessions, the average measured air temperature was around 60 °C — not the 75 °C the saunas were set to. At sitting height it was closer to 51 °C; at head height around 69 °C. This is not a flaw in the study — it reflects the real thermal stratification inside a sauna cabin, which single-point measurement at the top simply cannot capture.
Humidity ranged even more widely. Average relative humidity across all sessions was around 41%, but individual sessions varied from near-dry to steam-heavy, depending on how much löyly participants threw. At sitting height, average humidity was 54%. This variation — invisible in studies that report only thermostat settings and call conditions "dry" — is exactly what the study was designed to capture and use.
What Happened to Heart Rate
Heart rate rose substantially during every session. On average, participants' heart rate increased by 41.6 beats per minute — starting around 79 bpm at rest and reaching around 121 bpm during the session. That is a cardiovascular response comparable to moderate-intensity exercise, from simply sitting still in the heat.
The key result: both temperature and humidity independently predicted how much heart rate rose, even after accounting for differences between individuals in sex, fitness, body fat percentage, and body size. Temperature was the stronger driver, but humidity was clearly measurable and statistically robust.
Each additional °C of air temperature: +1.4 bpm
Each additional percentage point of relative humidity: +0.37 bpm
Average heart rate rise across all sessions: +41.6 bpm
In practical terms: two saunas at the same temperature, one dry and one with generous löyly, will produce meaningfully different heart rate responses. A 40 percentage-point difference in relative humidity — entirely plausible between a dry session and one with active steam — would add roughly 15 extra beats per minute on top of the temperature effect alone.
What Happened to Core Temperature
Core temperature rose more modestly than heart rate — an average of 0.4 °C per 10-minute session, from about 37.4 °C to 37.8 °C. This reflects the body working to regulate its internal temperature even as the skin surface is being intensely heated.
Again, both temperature and humidity were independent contributors. Each additional degree of air temperature was associated with a 0.032 °C greater rise in core temperature; each additional percentage point of humidity with a 0.008 °C greater rise. Fitter participants and those with lower body fat tended to show smaller core temperature increases — likely because their cardiovascular systems regulate heat more efficiently.
What People Felt
How hot participants perceived the sauna to be was primarily driven by air temperature — as expected. Humidity also had a detectable independent effect on thermal sensation, though a smaller one. A more humid sauna feels hotter, even at the same temperature reading.
Comfort improved across repeated sessions — participants reported feeling more at ease in later rounds even when the objective conditions were similar. This is consistent with what experienced sauna bathers know: tolerance to heat builds during a session, and those who use sauna regularly develop a capacity to feel comfortable in conditions that might feel intense to a newcomer.
The Condensation Question
The study also tested whether condensation — the phenomenon where water vapor deposits heat directly onto the skin when the dew point rises above skin temperature — had a measurable independent effect. The results were nuanced: there was a weak association with heart rate in some statistical models, but it was not consistent and disappeared when individual differences were accounted for. There was no significant association with core temperature.
This does not mean condensation is irrelevant. Löyly throws change both the moisture content and the thermal environment of the sauna at the same time, making it statistically difficult to isolate condensation as a separate variable. The researchers note this explicitly and call for more targeted future studies. Understanding condensation remains an open and important research question — one that Harvia Labs is actively pursuing with its own measurement programme.
Why This Study Matters Beyond the Numbers
The study makes two contributions that will matter to anyone interested in sauna science.
The first is the methodology itself. By combining precise multi-point environmental measurement with detailed human biometric data — and doing so in real sauna conditions where participants could throw löyly freely — this study demonstrates what rigorous sauna research can look like. Many previous studies have described their conditions as "dry" and measured temperature at one point. This study shows what gets missed when you do that. The researchers note that their measured temperatures were considerably lower than thermostat settings — a discrepancy also seen in other studies — and that humidity levels were higher than typically reported, precisely because löyly use was captured in real time.
The second contribution is the result itself: humidity matters, independently, and the magnitude is large enough to be practically relevant. This means that characterising sauna conditions accurately — measuring humidity alongside temperature, ideally at multiple points — is not optional if you want to understand what the sauna is doing to the body. And it means that people optimising sauna use for health or recovery need to think about steam, not just heat.
The study was conducted by Iida Laatikainen-Raussi, Tom Mikkola, Johanna K. Ihalainen, and Essi Ahokas from the University of Jyväskylä and KIHU. Research infrastructure was provided by Harvia; partial funding by Toyota Motor Corporation.
Temperature and humidity independently influence thermoregulatory responses during Finnish sauna bathing
A new scientific study from the University of Jyväskylä and KIHU — the Finnish Institute of High Performance Sport — has done something that no sauna study has done before: it measured what was happening inside the sauna room and inside the human body at the same time, with the same precision, and linked the two together.
Read the full study
MyHarvia Smart Sauna Sensor
If this article has made you curious about what is actually happening inside your own sauna, the MyHarvia Smart Sauna Sensor is a good place to start. Install the wireless sensor in any sauna — including wood-burning — and the free MyHarvia app tracks temperature and humidity in real time, session by session. The humidity graph gives you a tangible feel for how löyly changes the sauna environment — you can see the difference between a dry session and one with generous steam. It won't give you the precision of a research lab, but it does make the invisible visible, and it builds a picture of your sauna habits over time.
Learn more