Steam, Not Just Heat, Ramps Up Heart Rate In The Sauna

Why That Sauna Steam Might Be Working Your Heart Harder Than the Heat

Stepping into a Finnish sauna feels like a full-body reset, but a new study finds that the steam curling off the stones matters in its own right, not just the heat. That humidity, invisible but unmistakable on the skin, has been overlooked as a factor pushing the body’s physical response harder than most sauna-goers, or researchers, have appreciated.

A study published in the journal Temperature found that both air temperature and humidity independently correspond to physical stress on the body during sauna sessions. “In addition to temperature, the relative humidity of sauna air affects the thermoregulatory responses,” says Iida Laatikainen-Raussi of the University of Jyväskylä’s Faculty of Sport and Health Sciences, the study’s lead author. “While temperature is still the most significant factor, steam also clearly impacts heart rate and core temperature.”

Finnish sauna culture has a long tradition of throwing water onto heated stones, a practice called löyly, to generate short bursts of steam that many past studies didn’t measure. Every ladleful sent humidity spiking and fading throughout a session, and this research finally tracked that fluctuation across multiple points inside real saunas during actual use, a notable strength compared to prior work that often measured conditions at only one spot.

Researchers Tracked Real Sauna Conditions, Not Just Thermostat Settings

Researchers at the University of Jyväskylä recruited 69 healthy adults, ultimately analyzing data from 50 participants, 32 males and 18 females, with an average age of about 35. Most were regular sauna users, the kind of people who’d already worked up a tolerance for the heat.

Each person sat through four separate 10-minute sauna sessions in one day, with 30-minute breaks in between to cool off. Small groups of two or three shared a sauna, and each of the four saunas used a different heater. For the first two minutes of every session, nobody could splash water on the stones. After that, participants poured freely, the way most people actually do, sending humidity rising and falling for the rest of the sitting.

To see what was happening inside each body, participants swallowed a temperature-sensing capsule the day before, wore skin sensors on the forehead, chest, and foot, and had heart rate tracked continuously through a chest strap. Body weight was checked before and after each session to estimate sweat lost. Six temperature sensors and six humidity sensors at two heights and three spots around each sauna caught swings a lone sensor by the door would’ve missed.

Heart Rate Jumped 41 Beats Per Minute During Sauna Sessions

Inside the saunas, average air temperature across all sessions was about 60 degrees Celsius, around 140 degrees Fahrenheit, and average humidity sat near 41%. At sitting height, humidity averaged over 53%, while sensors near head height recorded closer to 28%, a gap that shows how different conditions can be around a person’s lower body and head during the same session.

Heart rate climbed sharply, jumping from about 79 beats per minute before a session to nearly 121 beats per minute at its peak. Core body temperature rose from an average of 37.4 degrees Celsius to a peak average of 37.8 degrees Celsius during the session or the recovery that followed.

Every additional degree Celsius of sauna temperature added about 1.4 beats per minute to that heart-rate jump, and every additional percentage point of humidity added about 0.37 beats per minute more. The pattern held after researchers factored in sex, fitness, body fat, and body size.

Humidity Blunts the Body’s Ability to Cool Itself Through Sweat

One possible explanation is that humid air makes sweat less effective at carrying heat away from the skin, since evaporation slows once the surrounding air is already loaded with moisture. “This increases thermal strain and forces the cardiovascular system to work harder to maintain an adequate body temperature,” says Laatikainen-Raussi. Researchers also checked whether the potential for steam to condense on the skin played a role, using a calculated stand-in rather than a direct measurement, and found that link to be weak and inconsistent, unlike the clearer connection to humidity overall.

Participants also rated how hot and how comfortable they felt after each session. Comfort ratings improved across later sessions, though the study couldn’t pin down why. Average body mass declined by about 0.10 kilograms per session, a rough proxy for sweat loss, though the researchers noted error sources including moisture absorbed by swimwear and water drunk during breaks.

Past Sauna Studies Rarely Measured Humidity Directly

One of the study’s notable conclusions addresses methodological gaps in the research literature. Many past sauna studies described conditions simply as “dry,” without directly measuring humidity at all. “Future sauna studies and practices should pay more attention to humidity and consider and measure sauna conditions more thoroughly,” says Laatikainen-Raussi. That gap matters for anyone comparing studies, since saunas set to the same temperature but with different amounts of water thrown on the stones are fundamentally different exposures.

For the average person settling onto a wooden bench with a ladle of water, this research offers something worth knowing. Every splash of steam sent up isn’t just tradition, it’s measurable physical strain, and science is only now catching up to it.

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Disclaimer: This article is based on peer-reviewed research but is not a substitute for medical advice. Anyone with heart conditions or other health concerns should talk to a doctor before starting or changing a sauna routine.

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