Heat, infrared and the body's response
Humidity
Humidity 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.
Also called relative humidity, RH, moisture content
Warm air can hold more water vapour than cold air, which is why relative humidity is always tied to a temperature. At 100 per cent relative humidity the air is saturated and no further evaporation can occur; extra vapour condenses out, including onto skin.
The physiological consequence. Sweating cools you only at the moment sweat evaporates. In a near-saturated sauna, sweat forms and runs off without evaporating, so you lose fluid at a high rate while getting comparatively little cooling in return. Sweat volume is therefore a poor guide to how hard a session is working, and fluid loss is real even though cooling is not. This is the physical reason for the hydration guidance around sessions: drink before, and drink afterwards.
The skin consequence. High humidity hydrates the stratum corneum, the skin’s outer barrier layer. The steam research on our studies page links that hydration to increased permeability, which is the proposed basis for transdermal ozone delivery.
What is controlled in the sauna. Integrated steam generation and precision temperature control are the engineering specifications, and temperature is the setting an operator adjusts. Humidity in a steam sauna sits near saturation by the nature of the machine, so the practical lever on how demanding a session feels is the temperature and the length of the session rather than a humidity figure.
Why it matters here
Humidity, more than the temperature on the display, is what makes a session feel hard and what drives fluid loss. It is the reason drinking water around a session matters.