Ordinary tap water is full of lime, calcium and a host of other minerals. When this water is vaporized, "white dust" can form - a kind of residue buildup that can develop on your walls, floors and furniture. The Vornado Mineral Cartridge is specifically designed for use in your Vornado Ultrasonic Humidifier to help filter out the minerals that cause this dust to form. The cartridge contains a specially designed blend of water softening minerals, reducing lime and calcium buildup. Completely Odorless, this filter scrubs your water clean, extending the life of your humidifier. Receive FREE SHIPPING on future mineral cartridge orders when you set up a product subscription. Click the "Subscription Wizard" button above to set a customized schedule to receive your cartridge - conveniently billed automatically.This Mineral Cartridge, part number MD1-0018, works with models Ultra1, Ultra2 & Ultra3.