The invention provides a humidity control system (HCS) to control indoor humidity within a specified range so as to reduce indoor house-dust mite allergen levels. The invention also provides a hydrogel absorber/desorber for reversibly absorbing water vapor from air releasing water vapor to air. The invention employs a reversible hydrogel desiccant as the active element for absorption and desorption of water, thus rendering the HCS more effective and energy-efficient than traditional humidification/dehumidification systems. The HCS of the invention treats indoor air by drawing fresh air in from outside and adjusting the humidity of the air-stream. In certain embodiments, fresh air is drawn in from the outside and mixed with recirculated air. The HCS uses a controller to introduce fresh air into the interior space and to control air circulation within it. Relative humidity is monitored remotely throughout the interior space using wired or wireless sensors.