Heat, infrared and the body's response

How heat and infrared reach the body, and how the body responds.

Terms (20)

Near-infrared (NIR)Near-infrared (NIR) is the band of infrared light closest to visible red, roughly 780 to 1,400 nanometres. It passes several millimetres into tissue rather than being absorbed at the surface, which is why it is used as a light modality rather than as a heat source.
Far infrared (FIR)Far infrared (FIR) is long-wavelength infrared radiation, in sauna use roughly 3 to 100 micrometres. Water absorbs it so strongly that almost all of its energy is deposited in the outermost fraction of a millimetre of skin, warming you directly rather than heating the air.
Micron (µm) and the 12 micron far infrared ratingA micron (µm), properly a micrometre, is a thousandth of a millimetre and a thousand nanometres. So our "12 micron far infrared" and the Red Light sauna's "far-infrared: 12,000 nm" specification are the same number written in two different units.
Radiant heatRadiant heat is thermal energy travelling as electromagnetic radiation. Every object above absolute zero emits it, and it needs no air or contact to cross a gap. In a sauna it is the warmth you feel directly from a hot surface or an infrared emitter.
Convective versus radiant heatingConvective heating warms you through hot air or steam touching your skin. Radiant heating warms you through infrared radiation crossing the gap directly. A Finnish sauna is mostly convective, a far infrared sauna mostly radiant, and a steam sauna adds condensation on top of both.
Wet heat versus dry saunaA dry sauna heats air to roughly 70 to 100 °C at low humidity, so sweat evaporates and cools you. A steam sauna runs far cooler but close to saturated, so sweat cannot evaporate. They are two different ways of taking heat.
HumidityHumidity is how much water vapour the air holds. Relative humidity expresses it as a percentage of the maximum that air can hold at that temperature. It matters in a sauna because sweat only cools you by evaporating, and it cannot evaporate into air that is already saturated.
Core body temperatureCore body temperature is the temperature of the deep tissues and organs, as opposed to the skin and limbs. It is normally held close to 37 °C, varying about half a degree across the day. External heating raises it; the body then works to bring it back.
HyperthermiaHyperthermia is body temperature above the normal regulated range because heat gain outpaces heat loss. Mild, deliberate hyperthermia is what any sauna produces. Uncontrolled hyperthermia is a medical emergency: heat exhaustion, and above roughly 40 °C core temperature, heat stroke.
Heat shock responseThe heat shock response is a cell's emergency reaction to heat and other stresses. It switches on heat shock proteins, which act as chaperones — holding damaged proteins in shape until they refold or are safely broken down. It is a conserved, well-characterised repair mechanism.
ThermogenesisThermogenesis is the body producing its own heat through metabolism: basal metabolism, shivering, brown fat and the cost of digesting food. A sauna does the opposite — it heats you from outside, so the body suppresses heat production and switches to shedding heat instead.
Sudorific effectSudorific means causing sweating; diaphoretic is its synonym. Both are genuine English words, but they are dated, and current physiology says sweating, diaphoresis or sudomotor function. "Sudorific effect" in sauna material simply means the session makes you sweat.
VasodilationVasodilation is the widening of blood vessels as the smooth muscle in their walls relaxes, increasing blood flow through them. Heat causes it in the skin so the body can shed heat. Blood pressure often falls as a result, which is why people can feel light-headed on standing up.
TRPV1TRPV1, transient receptor potential vanilloid 1, is an ion channel on sensory nerve endings that opens above roughly 43 °C and also responds to capsaicin, the pungent compound in chilli, and to acid. It is why heat past a certain point is felt as burning pain.
Inflammatory cytokinesInflammatory cytokines are small signalling proteins — TNF-α, interleukin-1 beta and interleukin-6 among them — that cells use to start, sustain and switch off inflammation. They operate as a balanced system, not a single dial.
Capillary development / angiogenesisAngiogenesis is the growth of new blood vessels, usually capillaries sprouting from existing ones under signals such as vascular endothelial growth factor. It is how a tissue increases its own blood supply over weeks, as it does with exercise training.
Tissue oxygenationTissue oxygenation is how much oxygen actually reaches the cells of a tissue. It depends on three things: how much oxygen the arterial blood carries, how fast that blood flows, and how far the oxygen must diffuse. It is not the same as blood oxygen saturation.
Red blood cell flexibility (rheology)Rheology is the study of how fluids flow. Red blood cells are about 8 micrometres across and must fold to pass through capillaries half that width, so their flexibility — with haematocrit and plasma viscosity — sets how easily blood moves through the smallest vessels.
DetoxificationIn physiology, detoxification is the work of the liver and kidneys: chemically altering substances so they can be excreted in bile and urine.
Session settingsSession settings are the operational recipe for a sauna session: preheat, session length, sauna temperature, ozone level, and how often sessions are repeated. They are an operating procedure rather than a clinical one, and the full version ships with each sauna.