Thermal radiation in a sauna

Thermal radiation in a sauna is electromagnetic energy exchanged among the heater, stones, room surfaces and bathers as a consequence of their temperatures. Unlike convection, it does not require bulk air movement. It occurs in every sauna: conventional heaters, stones and warmed linings radiate, while the emitters and other surfaces of an infrared cabin do so as well.

Radiation is a two-way exchange because all the bodies involved emit simultaneously; the net transfer depends on their absolute temperatures, surface properties and geometry. It is only one component of Heat transfer in a sauna, alongside convection, conduction, evaporation and condensation. The labels conventional and infrared therefore describe equipment and operating arrangements, not mutually exclusive heat-transfer modes.

Physical basis and net exchange

All bodies above absolute zero emit electromagnetic radiation. At ordinary sauna surface temperatures, most of the emitted energy lies in the infrared region. For an ideal blackbody, total radiant emission varies with the fourth power of absolute temperature, expressed in kelvin rather than degrees Celsius. Real surfaces differ from an ideal emitter, and their spectral behaviour is commonly represented by emissivity.

A surface can absorb, reflect and emit radiant energy. Emissivity can vary with wavelength, temperature and surface state, so a value established for one material and finish is not automatically applicable to every sauna board, stone, glazed area or metal part. A cool wall and a hot heater both emit radiation, but their combined balance produces net heat transfer from the warmer radiant environment towards the cooler one.

Geometry enters through view or configuration factors: the fraction of radiation leaving one surface that reaches another depends on their areas, orientations, separation and obstructions. A heater guard, bench, stone bed or another person can block part of a direct line of sight. Because the sources are extended surfaces within an enclosure, the inverse-square relationship for an ideal point source is not a general room rule. Moving away may reduce exposure, but the changing visible area, angle and shielding also matter; the evidence does not define one universally safe distance.

The radiant field in the room

A Sauna heater transfers energy to its casing, elements or combustion surfaces and to the stones. These components radiate towards visible surfaces and occupants while also heating air. The walls, ceiling, floor and benches absorb part of the incoming energy, warm, and contribute their own emission. The resulting field is room-wide rather than a one-way beam from the heater.

The share assigned to a particular path depends on the system boundary and apparatus. A 1994 study combining airflow measurements with computational modelling estimated that radiation represented about 75 per cent of stove output in its test configuration. That denominator was stove output, not the fraction absorbed by a bather or stored in the body. It is not transferable to other heaters or rooms.

A different apparatus gave a contrasting pathway. In a 2001 experiment and calculation for one ceiling-panel room operated at 50 °C, the authors found that heat reached their human representation mainly through air convection and radiation from warmed walls, rather than directly from the overhead panel. This demonstrates a possible indirect radiant route; it does not establish the balance in other infrared cabins. Results based on different rooms, denominators and models cannot be averaged into a universal radiation-to-convection ratio.

The field also changes with time. Components with greater thermal mass store and release more energy for a given temperature change, and the heat capacity of the stones influences their response. Air and solid surfaces need not warm or cool at the same rate. Consequently, two rooms with the same thermometer reading can have different radiant fields during warm-up or after control cycling; no fixed adjustment can convert air temperature into radiant temperature.

Exchange with bathers

Mean radiant temperature represents a non-uniform radiant field as an equivalent uniform surrounding temperature for a defined position and body representation. It is not a simple arithmetic average of surface thermometer readings and need not equal the temperature of any one wall. In four controlled comparisons, a 2017 method study found differences of 1–2 °C among a local radiant-intensity calculation, a classical surface-and-view-factor calculation and experimental results. The values describe those method comparisons, not an offset that can be added to sauna measurements.

Position and orientation alter which surfaces a person sees. One side of a seated bather may face a hot heater while another faces cooler glazing or timber, producing radiant asymmetry even when a wall thermometer is unchanged. Guards and benches can reduce a direct view without removing radiation from the warmed enclosure.

Radiation alone cannot determine perceived heat or physiological strain. At the body it acts with convection, contact conduction and sweat evaporation. Vapour pressure, skin wetness, posture, clothing, exposure time and local airflow also influence the combined outcome. ICNIRP similarly states that whole-body heat stress must be evaluated with air movement, temperature, humidity and the heat source considered together. Mean radiant temperature is therefore a physical descriptor, not a personal safety rating or a complete “feels-like” scale.

Conventional and infrared systems

ICNIRP defines infrared radiation as wavelengths from 780 nanometres to 1 millimetre. It divides the region into IR-A from 780 nanometres to 1.4 micrometres, IR-B from 1.4 to 3 micrometres and IR-C from 3 micrometres to 1 millimetre; IR-C is also called far infrared. These names classify wavelengths, not heater quality or medical action. ICNIRP reports penetration of several millimetres for IR-A, less than 1 millimetre for IR-B and absorption of IR-C in the uppermost dead-cell layer, the stratum corneum. Claims that far infrared is uniquely “deep penetrating” are inconsistent with that distinction.

An Infrared sauna is identified by its emitter arrangement and intended operation rather than by the mere existence of radiation. Panels warm occupants, surfaces and air; bathers also exchange heat convectively, evaporate sweat and conduct heat at contact points. Conversely, a conventional stone heater and the room it warms emit infrared. The system label alone cannot quantify net radiant gain.

In a 2026 study, twelve healthy adults entered a single 45-minute far-infrared exposure at a measured 65 °C. Core and skin temperatures, heart rate, thermal sensation and discomfort rose, and two participants could not complete the protocol. The one-arm study measured total cabin exposure with all transfer processes active; it neither isolates a wavelength effect nor provides a conventional-sauna comparison. It therefore establishes no health benefit or therapeutic superiority. Clinical evidence belongs under Sauna and health.

Change during löyly

Löyly is primarily a transient phase-change and transport event. Some water applied to the hot stones evaporates, and vapour-rich air moves into the room; condensation, sorption, convection and evaporation at surfaces can change rapidly. A four-page 1996 model treated these coupled processes. It does not show that the immediate sensation is a burst of radiation or provide a universal modal share.

Surface temperatures can change during the event and thereby affect later emission. In a modified, sealed spruce-lined room, one-, two- and three-litre water applications were followed by an approximately 2.5 °C rise in the measured ceiling-surface temperature. The authors reported an operative-temperature response of about 0.5 °C or less. The partly foil-covered heater, sealed door and duct, unstable thermostat and very large water doses make these apparatus-specific observations rather than normal-use values. They also do not isolate radiation as the cause of löyly intensity.

Air is not perfectly transparent at every infrared wavelength. Water vapour and carbon dioxide absorb and emit radiation in particular bands. A 2023 field and modelling study measured mean radiant temperature below air temperature in one sauna and found that including gaseous absorption improved model agreement. This one geometry does not mean that humidity simply blocks or amplifies infrared. Spectrum, gas concentration, path length and the temperatures and geometry of surrounding surfaces all affect the result.

Measurement and interpretation

ISO 7726:2025 covers minimum instrument characteristics and methods for measuring physical quantities that characterise a thermal environment. Its public catalogue description supplies neither a sauna-specific procedure nor a basis for certifying a consumer device.

A globe thermometer is influenced by both radiation and convection. Deriving mean radiant temperature from it requires an appropriate calculation and information about local air conditions and the instrument. A radiometer instead responds over stated spectral and angular ranges; its reading is not automatically the radiant heat absorbed by a whole body. Measurement position, orientation, field of view, spectral response, operating phase and uncertainty therefore belong with the result.

Thermal imaging detects radiation from viewed surfaces within the camera's spectral band and applies assumptions to estimate apparent surface temperature. It does not directly measure air temperature, mean radiant temperature or heat flow into a bather. Emissivity, reflected radiation, viewing angle and obstruction can influence interpretation; the Nore experiment, for example, specified its spectral range, chosen emissivity, reflected-temperature correction, distance and uncertainty. Detailed camera practice belongs to its specialist article, while comparative study design belongs under Sauna research.

Excessive local infrared exposure can cause thermal injury to skin or eyes, but the risk depends on wavelength, intensity, exposed area and duration, as well as the wider thermal environment. No universal safe heater distance, radiation percentage, therapeutic band or comfort threshold follows from radiation alone. Equipment-specific guarding and exposure requirements require their own assessment.