The SPE consists of a proton-conductive solid polymer electrolyte and porous electrodes with a catalytic layer composed of noble metal particles. When a voltage is applied to the porous electrode attached to the membrane, the moisture on the anode side (dehumidifying side) dissociates into hydrogen ions (H+) and oxygen. The hydrogen ions migrate through membrane to be discharged on the cathode (moisture discharging) side where they react with oxygen in the air, resulting in water molecules (vapor), being discharged. Oxygen is released from the dehumidifying side and if a large amount of water is introduced to an airtight enclosure then oxygen can build up inside the enclosure.