This is what D'Addaio says about this product:
The D’Addario Humidifier Pro protects your guitar even in the driest conditions. Using a specialized hydrophilic sponge that holds 12X its dry weight in water (much more than ordinary sponge), the Humidifier Pro allows for longer periods between re-fills and increases protection in drier environments. Guarding against cracks, warped necks, and shrinkage is now easier and less work preserving your instruments ultimate playability. Suspended by the strings, the humidifier releases moisture evenly and slowly inside your guitar, yet never touches the guitar’s body. The humidifiers stopper allows you to check the sponge status without removal of the unit and provides a tight seal when in place. Maintaining the humidifier is as easy as moistening the sponge when it becomes dry.