“Infrared heat penetrates 3 to 4 cm into your body.”

The research found this

The one direct test put temperature probes into muscle. Ten adults sat for 45 minutes in a commercial far-infrared cabin while probes read muscle temperature at three depths and a swallowed pill tracked core temperature.1Source 1 · Clinical trialMuscle temperature increases during a single far infrared sauna session without changes in intestinal temperatureReed EL, Uzoekwe CC, Atencio JK, et al.. Journal of Applied Physiology, 2025.Read the paper ↗PubMed ↗ The thermostat started at 45°C and was then set to 65°C so the emitters stayed on for 93% of the session.

Warming fell off with depth. Muscle rose 3.0°C at 1.4 cm below the skin and 1.1°C at 3.4 cm.1Source 1 · Clinical trialMuscle temperature increases during a single far infrared sauna session without changes in intestinal temperatureReed EL, Uzoekwe CC, Atencio JK, et al.. Journal of Applied Physiology, 2025.Read the paper ↗PubMed ↗ The authors defined effective thermal penetration as the depth where heating had dropped by 63%, and they put it at 2.4 cm. The pill showed no change in core temperature, while mean skin temperature rose 6.2°C.

Infrared radiation is absorbed near the surface. Germany's Federal Office for Radiation Protection says near infrared (IR-A, 780–1,400 nm) reaches up to about 5 mm into skin, and the longer IR-B and IR-C bands are absorbed mostly in the epidermis.2Source 2 · RegulatorAnwendungen von Infrarot-Strahlung in Medizin und Wellness [Applications of infrared radiation in medicine and wellness]Bundesamt für Strahlenschutz. German Federal Office for Radiation Protection (BfS), 2026.Read the paper ↗ Far-infrared sauna heaters run at about 300–400°C and emit wavelengths from 3 µm upward.3Source 3 · ReviewFar infrared radiation (FIR): Its biological effects and medical applicationsVatansever F, Hamblin MR. Photonics & Lasers in Medicine, 2012.Read the paper ↗PubMed ↗

Traditional saunas warm muscle as well. In one study, 40 minutes at 80°C raised muscle temperature by 1.95°C and esophageal temperature by 1.5°C.4Source 4 · Clinical trialThe vascular response to acute sauna heating is similar in young and middle-aged adultsLeach OK, Strong K, Mack GW, et al.. Journal of Applied Physiology, 2024.Read the paper ↗PubMed ↗

The figure is older than the research. A Health Mate ad promised an inch and a half in 1998, and a US patent filed in 2002 said the same.5Source 5 · Brand pageHealth Mate Sauna home pageHealth Mate. healthmatesauna.com (Wayback Machine capture), 1998.Read the page ↗6Source 6 · Web pageInfrared sauna (US Patent 7,120,353 B2, filed 2002)Schaeffer BC, Schaeffer J, Schaeffer W. Google Patents, 2006.Read the page ↗ A 2015 study of infrared sauna after training in 10 men later gave 3–4 cm as background without measuring it.7Source 7 · Crossover trialEffects of far-infrared sauna bathing on recovery from strength and endurance training sessions in menMero A, Tornberg J, Mäntykoski M, et al.. SpringerPlus, 2015.Read the paper ↗PubMed ↗