The humidification of anaesthetic gases: its importance and control.

THiS REV,EW studies the humidi ty output of anaesthet ic sys tems in common use and the steps taken to improve them. The amount of water vapour which an a tmosphere can hold varies with temperature . The warmer the ambient gas mixture, the more water vapour it can contain. There is a max imum amount of water vapour which the a tmosphere can maintain at each tempera ture when it becomes saturated, S a t u r a t e d h u m i d i t y can be expressed as: ( l ) 100 per cent moisture at a given temperature , (2) the amount of water weight for volume held by the gas (rag HxO/I o r g HxO/ma). or (3) the partial pressure exer ted by the water vapour in mil l imeters of mercury (tot'r), or S.I. units as ki lo-Pascals (one kPa = 7.5 torr). R e l a t i v e h u m i d i t y is the pet" cent saturat ion of the a tmosphere at a given temperature . It can be expressed as such or in absolute values (amount of water in gas as weight for volume or as pressure of water vapour). Water vapour enters the a tmosphere by emerging from a water supply or by sublimation of ice. The amount of vapour which is formed depends on the temperature of the water because vapour p(essure rises with temperature . As vaporizat ion proceeds, the water cools to provide the energy required for t ransformation into yapour. The emiss ion of vapour is reduced as the source of water cools unless heat is avai lable to provide the energy for t ransformation (latent heat of vaporizat ion). If a gas saturated with water vapour is cooled e i ther by a drop in ambient temperature or by passage through a condenser , it will rain out the amount of water vapour it held at the original t empera ture less the amount it can now hold at