Abstract: The present invention provides improved apparatus, systems, and methods of delivering a breathable fluid stream carrying an atomized or vaporized functional liquid. In particular, a fluid-permeable exchangeable cartridge is provided for containing the functional liquid to be introduced into a fluid stream comprising a breathable base fluid flowing through the cartridge. Preferably, the breathable base fluid includes a solvation enhancer such as atomized or vaporized propylene glycol or glycerin. In another aspect, a throttling valve system is provided for adjusting the ratio of flow from a breathable base fluid source through the cartridge or around the cartridge via a bypass channel, to vary the concentration of the functional liquid component mixed with the breathable base fluid.