Finnish Sauna Study Shows Steam Boosts Heart Rate and Core Temperature

A study conducted by the University of Jyväskylä and the Finnish Institute of High Performance Sport (KIHU), with collaboration from Harvia and partial funding from Toyota Motor Corporation, examined how air humidity affects physiological strain during traditional Finnish sauna bathing. Fifty healthy adults completed four 10‑minute sauna sessions while researchers measured heart rate, core body temperature and perceived heat.

The results showed that both higher temperature and increased relative humidity independently raised heart rate and core temperature. Average heart rate rose from about 79 beats per minute to 121 beats per minute, and core temperature increased from 37.4 °C to 37.8 °C. Higher humidity hampers sweat evaporation, forcing the cardiovascular system to work harder to maintain body temperature.

Lead author Iida Laatikainen‑Raussi highlighted that while temperature remains the dominant factor, steam generated by water on sauna stones (löyly) plays a clear and measurable role in the body's thermoregulatory response.