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Natural Occurrence

With a boiling point of -111°C and a pungent odour at higher concentrations, (1) ozone (O3) exists as a colourless gas at room temperature. It occurs naturally in the upper atmosphere, where ultraviolet (UV) radiation from the sun or electrical discharges such as lightning split oxygen (O2) molecules into highly reactive oxygen "radicals". These radicals combine with other oxygen molecules to give ozone, as depicted below. The cumulative result is the so-called "Ozone Layer".

Ozone in Pools and Spas

Just as in nature, ozone is produced by two processes for pool and spa applications. The first uses high intensity UV lamps to photochemically generate ozone from oxygen. Many residential pools in the USA use these UV ozonators.(2) In the second, "coronial discharge", ozone is formed by pumping oxygen through a high voltage electrical field between a pair of electrodes. Coronial discharge is the method of choice for pools and spas in Australia and New Zealand.


Ozone is an extremely effective sanitiser and oxidant, killing bacteria and destroying organic waste faster than either chlorine or bromine. Unfortunately, its use in pools and spas is restricted because of its very short lifespan in water - a matter of minutes at best, and mere seconds under certain conditions. This is primarily due to its highly reactive nature; it oxidises many organics in water almost instantaneously, subsequently returning to the atmosphere as oxygen. As such, it is impossible to maintain an ozone residual in a pool or spa unless the ozonator is run 24 hours a day, 7 days a week. To offset this shortcoming, ozone should always be used with a measurable residual of either chlorine or bromine.


For commercial pools and spas in Australia and New Zealand, this is a mandatory requirement set down by the health authorities. On the positive side, the combination of ozone and chlorine or bromine is highly efficacious against bacteria and, when used with the latter, ozone regenerates spent bromine (bromide), thus reducing overall bromine consumption. As ozone is a strong oxidant, there is usually no need for shock dosing with either chlorine or a non-halogen compound such as potassium monopersulfate (eg. BioGuard OxySheen). Ozone is compatible with all common balancers, algaecides and, as noted elsewhere,6 clarifiers and flocculants.


Apart from the short-lived nature of the compound, there are other drawbacks involved with the use of ozone in pools and spas. These include persistent increases in pH due to the formation of hydroxide as a by-product of ozone generation, and the ability of ozone to break down rubber components, such as gaskets and O-rings. With a low TLV of 0.1 ppm, (3) there are serious health concerns regarding the toxicity of ozone. As the compound has limited solubility in water (490 cm3 per litre of water at 0°C), (4) any excess will evaporate into the atmosphere immediately above the pool or spa. This excess can be inhaled and give rise to health problems ranging from headaches and dizziness through to asphyxiation via congestion of the lungs. (1) As it is possible to detect ozone by its odour at concentrations as low as 0.01 ppm, the usual rule-of-thumb is "If you can smell the ozone, the level is too high".

Ozone and Water Testing

The concentration of ozone in pool and spa water can be determined using DPD No. 1 or OTO, (5) but this requires a photometer or colour comparator specially calibrated for the purpose. The short lifespan of ozone in water necessitates that the sample MUST be taken and analysed on site using a securely capped vial or cuvette. For this reason, water samples brought in by consumers operating pools or spas on a combination of ozone and chlorine or bromine will test for chlorine or bromine ONLY, as the ozone will have dissipated completely. For this reason, too, ozone has no effect on any other tests performed routinely on pool or spa water samples, nor does it contribute to Total Dissolved Solids (TDS). As pointed out above, the use of ozone often leads to a significant increase in pH; values of 7.8 or above are not uncommon. As this increase is solely due to hydroxide, and not ozone (which is pH neutral anyway), testing samples that have been stored for some time will still return a pH value indicative of the pool or spa.



1. G.D. Muir, "Hazards in the Chemical Laboratory", 2nd edition, Chemical Society, London, 1977, p. 339.

2. National Spa and Pool Institute, "Basic Pool and Spa Technology", 2nd edition, NSPI, Alexandria VA, 1992, pp. 312-315.

3. The Threshold Limit Value (TLV) is the time-weighted average concentration of the work atmosphere for a normal 8-hour day and a 40-hour week to which nearly all workers can be repeatedly exposed day after day without adverse effect.

4. R.C. Weast, "CRC Handbook of Chemistry and Physics", 57th edition, CRC Press, Cleveland OH, 1976, p. B-138.

5. Ortho-toluidine (OTO) is no longer advised for testing pool and spa water in Australia and New Zealand due to concerns over its toxicity and carcinogenic properties.

6. "Compatibility of Water Clarifiers with Ozone", Bio-Lab Australia Technical Information Bulletin BG-059.


The above information is supplied by Bio-Lab and represents its best interpretation of available technical information at the time of preparation. The sole purpose is to supply factual information to Bio-Lab customers. It is not to be taken out of context nor used as support for any other claim not made herein.