Ozone is a form of OXYGEN in which three atoms combine to form a molecule. It is a blue gas with a pungent odor, noticeable when the gas is formed by an electrical discharge. Ozone is a powerful oxidizing agent and an effective antiseptic and bleaching agent; in high concentrations it is severely irritating. Used as an alternative sanitizer, usually requires use of a residual halogen for most effective use. Units use ultra-violet light to create the OZONE, in air (as it passes over the lamps) that is inducted into the water system. Still, most popular with hard covered spas and hot tubs, some swimming pool builders are incorporating OZONE dispensers into the design of the pool. On existing pools, ozone must be inducted (usually by a venturi type fitting placed in-line) at the equipment pad, after all other equipment, (it is corrosive, you know) and does most of the sanitizing while still in the return lines (after all, it does bubble up to the surface pretty quick)! It can be used more effectively by pointing the return fittings downward and running the pump longer or more often (i.e. twice daily).