The resistance-based switching humidity property of crosslinked acrylic acid-acrylamide copolymers was measured with an LCZ meter, and their ionic mobilities, which varied with humidity, were measured by the polar-inversion method. Their conduction mechanism was discussed, and it was found that the ionization effect of some binding ions against a critical humidity takes place in switchable humidity-sensitive materials. The effect is that some binding ions in materials turn into free ions when a given ambient humidity, called a critical humidity, is reached. The avalanching-conduct electricity then occurs in materials.