Relative humidity in incubators.

In a previous paper Hey and Maurice (1968) have reported the effect of changes of relative humidity on the heat production and heat loss in normal naked babies in a specially constructed metabolic chamber under controlled environmental conditions. In order to evaluate the clinical relevance of these observations we have studied the control of relative humidity that could be achieved in three commercial incubators. A means of assessing the operative environmental temperature in such incubators has already been published (Hey and Mount, 1967). The amount of water that can be contained as vapour in air depends on the temperature of the air (Fig. 1 heavy line, 100% relative humidity). By definition the absolute humidity is the amount of water present as vapour in a given weight or volume of air, and the relative humidity (RH) is the amount of water vapour actually present, expressed as a percentage of the maximum amount that could be present at that temperature. The air in hospital wards is usually about 50% RH (Fig. 1 point A); if this air is warmed from about 20 to 300 C. without the addition of extra water vapour, its absolute humidity will remain constant but its relative humidity will fall to 25% (Fig. 1 point B). For this reason warm-air incubators contain humidifying devices. Air can contain more water than indicated by the heavy 100% RH line in Fig. 1 at any given temperature only as liquid droplets, and these droplets of mist will only remain in suspension for a limited time, depending on their size. Avery, Galina, and Nachman (1967) have recently reviewed the use of mist therapy in paediatrics. This specialized aspect of humidity control was not covered in the present study.