Your Walgreens humidifier unit uses the 809997 water panel filter below. This is what captures impurities and minerals in the water, preventing lime scale build-up in the system. This will ensure that the water will not become stale before being dispersed into the room. This filter pad has an approximate size of 6" x 9" x 1-3/8" and should be changed every 2 months. This filter is also compatible with the 6610 model.