There were a number of studies done at Standard University, the University of Maryland, the University of Florida, Gainesville Municipal and Florida motel pools. All of these studies showed the sanitizer to be an effective at killing bacteria based on laboratory test results. Yet the sanitizer is not used today. What was it?
Iodine. Iodine is a halogen. Halogens make up a column on the periodic table of chemical elements. All of the elements in a column have similar properties. Some of the halogens are: fluorine, chlorine, bromine and iodine. Chlorine and bromine are sanitizers heavily used for pools and spas and are toxic. In fact, chlorine was used as war gas in World War I. The concentrations in the pool will not kill you but they do account for red eye, skin irritations, bleached hair, bleached bathing suits, and other potential health problems. Iodine is actually an essential nutrient and causes none of the reactions that Chlorine does and in fact I was able to bath in my hot tub with iodine for hours without even wrinkled fingers.
Since many people are deficient in iodine you can potentially get health benefits from its use.
Iodine is more expensive per pound than chlorine, but since it is a solid at room temperature instead of a gas, it lasts longer. The chlorine you add to most pools are compounds. The compounds break down and release chlorine as gas. Because of this iodine actually lasts longer and may be cheaper to use. The studies in the 70s came to this conclusion but pricing has changed considerably since then.
Iodine is commonly introduced into water as potassium iodide which is readily soluble. Something must be used to break it down into potassium and iodine. Ozone, which is extensively used in pools in Europe and hot tubs in the US is very effective at breaking down potassium iodide and oxidizing organic matter in pools and hot tubs. However, ozone is not very stable so most health departments want at least a low level of residual sanitizer. Also Iodine does not handle organic matter but ozone does. They make a great combination. Adding, a residual sanitizer also means that the ozonator does not have to be run 24 hours a day. Iodine makes a wonderful residual sanitizer. I have been using it with ozone in my hot tub for about a year with no health problems or algae problems either even though iodine is not supposed to be an algaecide. In fact, I left my hot tube at 106 degrees for several month with the ozone off, and the aeration off just circulating the water through the filter and heat exchanger. I thought I was going to have a green mess but the water stayed clear! I highly recommend it. I also had some calcium bentonite clay, a Nature2 Mineral Cartridge and epsom salts in the hot tub.
Iodine is an approved sanitizer for food establishments. “Iodine Concentration: 12.5 to 25 ppm
Iodine compounds or iodophors are fast-acting and effective against all bacteria. They are relatively nontoxic, non-irritating to skin, and stable. Iodophor solutions may stain porous surfaces and some plastics.” Southern Nevada Health District. However in the concentrations mentioned the water will turn brown and could stain things but these concentrations are for rapid sanitizing equipment in a food establishment not swimming pools.
“With iodine as metallic iodine, 2.0 ppm is necessary to provide a result equivalent to the 0.6 ppm of available chlorine control with both E. coli and S. faecalis.” Comparison of Chlorine, Bromine, and Iodine as Disinfectants for Swimming Pool Water
T. A. Koski, L. S. Stuart, and L. F. Ortenzio
Based on the above about 3 ppm iodine should be enough when used with ozone.