The harmful effects of moisture trapped in parts of a hearing aid are well known. Removal of this moisture is an important topic for users and dispensers alike. Water vapour in the air space is not usually a problem.
Problems usually begin when moisture condenses inside the hearing aid.
Simply put, the air around us holds a finite amount of moisture. Warm air holds more moisture and cold air holds less. Relative humidity is a comparison of how much water is actually in the air to how much the air could hold if it were saturated. In fact, the lower the temperature drops, the more condensate is formed. This is the same condensation effect seen on the inside of un-insulated windows in winter, or on a bathroom mirror after a hot shower.