This paper introduces current knowledge on the effects of humidity on physiological and psychological responses. Reviewing this topic is quite challenging, for two reasons. The first is the fact that humidity is not an independent variable, but a dependent one. Since humidity is strongly associated with temperature, the effect of humidity per se may not be clearly defined. The second reason is that the physiological and psychological impacts of humidity have brought attention to various research fields related to human comfort and health, and knowledge has thus been accumulated from various disciplines. To arrange vast amounts of information, therefore, limitations on the scope are required before introduction. This paper investigated four different research fields: thermal physiology; exercise physiology; hygiene; and dermatology. Selected papers were then introduced chronologically to confirm current knowledge in each research field.

From the perspective of exercise physiology, the magnitude of heat stress during exercise in hot and humid environments appears to be clearly estimable from the current knowledge. From the perspective of hygiene, inspired gas conditioned to core temperature and fully saturated with water would provide the optimal condition for mucosal health and function. In terms of dermatology, properties of the stratum corneum appear to have been fully clarified, and water diffusion through the skin can be properly estimated from current knowledge. With thermal physiology and engineering related to indoor environments, current knowledge may be sufficient to provide optimal thermal environments in consideration of effect of humidity. However, discrepancies between physiological and psychological responses and large individual differences in psychological responses to variations in relative humidity have not yet been resolved, due probably to unknown mechanisms in the perception of humidity without specific receptors within the skin. Such issues may require further studies in all research fields related to human comfort and health.