Heat, infrared and the body's response
Hyperthermia
Hyperthermia 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.
Also called heat stress, overheating
Hyperthermia is not fever. In fever the body deliberately resets its own target temperature upward in response to infection, and the regulating system is working normally toward a higher set point. In hyperthermia the set point is unchanged and the body is simply losing the argument with its environment. Drugs that lower fever do not treat hyperthermia; removing the heat load does.
The spectrum. Mild heat strain causes flushing, sweating, a rising heart rate and a falling blood pressure. Heat exhaustion adds weakness, nausea, headache, dizziness and clammy skin. Heat stroke — classically above about 40 °C core temperature, with confusion or collapse — is life-threatening and needs emergency care, not a rest.
In a sauna context. Every sauna session is a deliberate, bounded hyperthermic exposure. That is the point of it, and it is also the reason for the exclusions: check who should not use an ozone sauna before a session. Alcohol, dehydration, several classes of prescription medication and anything that impairs sweating all raise the risk at a given sauna temperature. Our exclusions are not a complete medical contraindication set — a condition’s absence from them is not clearance.
Practically. A client saying they are too hot is reporting a physiological limit, not a preference. Reduce the temperature or end the session.
Why it matters here
Every sauna session is a deliberate, bounded heat exposure, which is why the exclusions exist. If you run sessions for other people, recognising heat exhaustion early is the most useful thing on this page.