Abstract: An air humidifier for a surgical breathing machine has an evaporating chamber containing a temperature regulated low thermal mass heater. A water drip feed unit drips water onto a sheath of porous material of the heater at a predetermined rate. The water is substantially completely and instantaneously evaporated in the manner of a flash boiler to create an air temperature and relative humidity within predetermined limits. The heater temperature and water supply rate have manual adjustment means coupled together so that the predetermined limits are not exceeded. The evaporating chamber is tubular and of similar cross section to interconnecting conduits so as not unduly to interfere with pressure waves from the breathing machine.