Several of Vaportek's products require cartridges. The Optimum Cartridge is compatible with the Optimum 4000, Restorator, and Restorator DUO. Cartridges are filled with an essential oil membrane. The oils "sweat" through the tiny membrane pores and are dispersed into the air as a dry vapor with the assistance of air movers or natural air currents. The Classic Neutral Cartridge is designed to tackle tough odors while imparting a neutral scent. In addition to Classic Neutral, this cartridge is available in Berry, Forest, Lemon, Orchard, Potpourri, and Winter. When ordering, please specify fragrance selection in the drop down menu.