Anaes · Measurement & monitoring physics

Humidity and heat

Also known as Humidity · Absolute humidity · Relative humidity · Dew point · Latent heat of vaporisation · Heat and moisture exchanger · Inadvertent perioperative hypothermia

Humidity and heat are the physics behind two questions the anaesthetist answers every day — is the gas I am delivering kind to the airway, and is my patient's temperature safe? The framework rests on six exam-critical ideas. First, HUMIDITY is described three ways: ABSOLUTE humidity (the mass of water vapour per unit volume of gas, in mg/L), RELATIVE humidity (the percentage of water vapour actually present relative to the maximum the gas can hold at that temperature), and the DEW POINT (the temperature at which the gas becomes fully saturated and water begins to condense). Second, HEAT and TEMPERATURE are not synonyms: heat is the THERMAL ENERGY content of a body, measured in joules, while temperature is the DEGREE of hotness on a scale, measured in degrees. Third, heat is transferred by four mechanisms — CONDUCTION (direct molecular contact), CONVECTION (movement of a warmed fluid or gas), RADIATION (infrared transfer between surfaces, accounting for about 60 per cent of perioperative loss), and EVAPORATION (from the surgical wound and the respiratory tract). Fourth, the LATENT HEAT OF VAPORISATION of water is about 2.4 MJ/L — the energy needed to convert liquid water to vapour — which is why evaporation is such a powerful coolant and why a dry gas draws heat and moisture from the airway. Fifth, medical gases from a cylinder or machine are essentially dry and cold, so they must be HUMIDIFIED to prevent desiccation of the mucosa, ciliary dysfunction, retained secretions and heat loss; the two methods are the PASSIVE heat and moisture exchanger (HME — a hygroscopic membrane that retains exhaled heat and water, single-use, low resistance) and the ACTIVE heated water-bath humidifier (more effective, used for long-term ventilation). Sixth, under anaesthesia the core temperature falls by 1 to 1.5 degrees C in the first hour from REDISTRIBUTION hypothermia (vasodilatation moves core heat to the periphery), then falls more slowly by ongoing loss; prevention rests on forced-air warming, fluid and blood warming, radiant warming and an adequate ambient temperature, because hypothermia causes wound infection, coagulopathy and increased blood loss, shivering with cardiac stress, and prolonged drug action. Built on the hygrometric-properties-of-passive-humidifiers study (Lellouche 2026), the ventilatory-failure-from-retained-HMEF report (Zou 2026), the circuit-humidification-during-mechanical-ventilation review (Tsai 2026), the intraoperative-thermal-injury-from-warming-device report (Yeh 2026), the airway-warming-devices systematic review (Lee 2026), the blood-warming-device-performance study (Williams 2026), the risk-factors-for-postoperative-hypothermia review (Ji 2026), the double-wall-warmer randomised trial (Kabra 2026), and NICE CG65 on hypothermia prevention in adults having surgery (2008, updated 2016).