What is optimum humidity?

The relevant aspects of the normal physiology of the airway mucosa and the customary ways of describing humidity are briefly reviewed. The need for surrogate markers of optimum humidity is identified, and two such markers are proposed. The relationship between the humidity of the inspired gas and mucosal function is examined using a model that is then validated. Optimum humidity is achieved when the inspired gas is at body core temperature and 100% relative humidity.