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Tuesday, July 3, 2012

Disinfectant Chemistry Report Card #5 – Ozone – Do good things come in 3’s?

I have always been lead to believe that bad things come in “3’s”.  Could it be that ozone is the exception to the rule?

Ozone is derived from the Greek word ozein, which means to smell.  Ozone, or trioxygen, is a molecule, consisting of three oxygen atoms.  In essence ozone is nothing more than oxygen (O2), with an extra oxygen atom, formed by an electric high charge. In nature ozone is produced by some chemical reactions. The most familiar example is of course the ozone layer, where ozone is produced from the sun’s ultra-violet (UV) rays. But ozone is also produced at thunderstorms and waterfalls. The extreme high voltages attended with thunderstorms produce ozone from oxygen. The special “fresh, clean, spring rain” smell is a result from nature-produced ozone. For commercial applications, ozone is mostly prepared using corona discharge method.  In layman’s terms, an electrical discharge accompanied by ionization of the surrounding gas and gives off a pretty bluish colour (check out YouTube if you want to see a demonstration).

Ozone is mainly used in water and wastewater treatment for drinking water disinfection, color reduction, algae control, odor and taste removal, etc. Other applications include disinfecting aquarium recycling water, treating cooling water, food processing (food preservatives) and sanitation (CIP), pulp bleaching, etc. Newer technologies are also available for medical device reprocessing and environmental surface disinfection.  As an oxidizing agent, ozone can degrade organic matter, however at the very low concentrations used for disinfection; it is not expected to have any cleaning activity, given that current ozone technologies do not contain detergents which facilitate the cleaning process.

Ozone is a powerful oxidant and has many industrial and consumer applications related to oxidization. This same high oxidizing potential, however, causes ozone to damage mucus and respiratory tissues in animals, and also tissues in plants, above concentrations of about 100 parts per billion. This makes ozone a potent respiratory hazard and pollutant near ground level. Death or severe and permanent lung injury may result from low concentrations and short-term exposure. Even extremely low ozone concentrations can enhance airway reactivity to other inhaled chemicals and therefore leads to inflammatory response in respiratory tissue.  Exposure to 0.25-0.75ppm has been shown to result in dyspnea, dry throat, cough, shortness of breath, tightness of the chest, wheezing, nausea, and headache. Long-term occupational exposure to even very low Ozone concentrations can lead to lung congestion, irritation of nose and throat, and chest constriction in exposed workers. 

Once generated, ozone decays very rapidly, with a significantly shorter half-life in water than gas. At ambient temperatures in drinking water, ozone half-life is about 12 to 20 min.  Ozone decay in water depends on several factors such as Temperature (the higher, the less the stability), pH of solution (ozone is less stable in alkaline pH, the higher the pH, the lower the stability), dissolved solid concentration (organic matter decomposes ozone and as such, ozone is more stable in distilled water than tap water). 

Ozone gas has been shown to attack any polymer possessing olefinic or double bonds within its chain structure, such as natural rubber, nitrile rubber, and styrene-butadiene rubber. Products made using these polymers are especially susceptible to attack, which causes cracks to grow longer and deeper with time; the rate of crack growth depends on the load carried by the product and the concentration of ozone.

Here’s how we would score ozone on the key decision making criteria:
  • Speed of Disinfection – B to C
    • Contact times range from minutes to hours depending on the concentration and method of application (gas versus liquid)

  • Spectrum of Kill – A to C
    • Similar to the speed of disinfection, performance in this criteria is tied to the in-use concentration and method of application

  • Cleaning Effectiveness – D
    • As an oxidizer, Ozone will oxidize organic materials allowing them to be more easily removed from the surface, however, lacks surfactants which are instrumental in facilitating the cleaning process
  • Safety Profile – B to C
    • This is  another parameter largely affected by concentration and length of exposure
    • Ozone is a toxic gas that must be monitored in the workplace when used to disinfect equipment
    • Exposure to ozone at 0.1 – 1.0 ppm can result in headaches, dry throat, irritation to the respiratory system and eyes while at 1.0 – 100 ppm exposure can cause asthma-like symptoms, tiredness and loss of appetite

  • Environmental Profile – A
    • Ozone degrades into oxygen making it an environmentally preferable disinfectant chemistry
  • Cost Effectiveness – B to C
    • High initial investment for ozone generating equipment
    • Long term investment requires routine maintenance and replace of parts

**For more in-depth scientific information about Ozone and other disinfectant chemistries, stay tuned to www.infectionpreventionresource.com.

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