What few openings they have are small, and there is rarely a clear path through which air can move freely. Moisture removal in such an environment is very difficult, making it a very important topic for both patients and dispensing professionals. Water vapor in the air space is not usually a problem.
Problems usually start when moisture condenses.
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 uninsulated windows in winter, or on a bathroom mirror after a hot shower.