Chlorine dioxide (ClO2) is a yellow-green gas with an odor similar to chlorine with excellent distribution, penetration and sterilization abilities due to its gaseous nature. Although chlorine dioxide has chlorine in its name, its properties are very different, much like carbon dioxide is different than elemental carbon. Chlorine dioxide has been recognized as a disinfectant since the early 1900s and has been approved by the US Environmental Protection Agency (EPA) and the US Food and Drug Administration (FDA) for many applications. It has been demonstrated effective as a broad spectrum, anti-inflammatory, bactericidal, fungicidal, and virucidal agent, as well as a deodorizer, and also able to inactivate beta-lactams and destroy both pinworms and their eggs.
As can be seen in the chart above, the size of a chlorine dioxide gas molecule is 0.124 nm, much smaller than microorganisms and viruses, allowing the gas to easily penetrate into any areas where these microorganisms might be concealed.
Although chlorine dioxide has "chlorine" in its name, its chemistry is radically different from that of chlorine. When reacting with other substances, it is weaker and more selective, allowing it to be a more efficient and effective sterilizer. For example, it does not react with ammonia or most organic compounds. Chlorine dioxide oxidizes products rather than chlorinating them, so unlike chlorine, chlorine dioxide will not produce environmentally undesirable organic compounds containing chlorine. Chlorine dioxide is also a visible yellow-green gas allowing it to be measured in real-time with photometric devices.
Chlorine dioxide (ClO2) acts as an oxidizing agent and reacts with several cellular constituents, including the cell membrane of microbes. By "stealing" electrons from them (oxidation), it breaks their molecular bonds, resulting in the death of the organism by the breakup of the cell. By altering the proteins involved in the structure of microorganisms, their enzymatic function is broken and causes very rapid bacterial kills. This oxidative attack on many proteins simultaneously is behind the potency of chlorine dioxide and also prevents microorganisms from mutating to a resistant form. Because of the selective reactivity of chlorine dioxide, its antimicrobial action is retained longer in the presence of organic matter than most other decontaminating agents.
The difference between spore and bacterial inactivation is the same as the difference between sterilization and disinfection. For a chemical agent to be classified as a sterilant, it must be demonstrated to be effective at inactivating spores. Spores are among the hardest organisms to kill and for this reason sterilizing agents are considered the most rigorous decontaminating agents and offer complete kill of all antimicrobial life. Disinfection, on the other hand, does not require the complete inactivation of spores or all microbial life and is normally validated against a few vegetative bacteria species. For this reason, disinfecting agents are less rigorous decontaminating agents and are not as effective as sterilizing agents.
"Bacterial endospores are one of the most persistent forms of microbial life and typically require aggressive inactivation procedures. Vegetative bacteria are generally much more easily inactivated than are bacterial endospores. This is primarily because the sensitive areas of bacteria are easily contacted by chemosterilizing agents. The spore, however, has a more complex structure than the vegetative bacterial cell. Its sensitive material is contained within a core and that core is surrounded by a cortex and spore coats. These coats tend to act as a permeability barrier to the entry of chlorine dioxide and other compounds" (Knapp, 2000).
Chlorine dioxide's special properties make it an ideal choice to meet the challenges of today's environmentally concerned world and is an environmentally preferred alternative to elemental chlorine. When chlorine reacts with organic matter, undesirable pollutants such as dioxins and bio-accumulative toxic substances are produced. Thus, the EPA supports the replacement of chlorine with chlorine dioxide because it eliminates the production of these pollutants. It is a perfect replacement for chlorine, providing all of chlorine's benefits without any of its weaknesses and detriments. Most importantly, chlorine dioxide does not chlorinate organic material, eliminating the formation of trihalomethanes (THMs), haloacetic acids (HAAs) and other chlorinated organic compounds. This is particularly important in the primary use for chlorine dioxide, which is water disinfection. Other properties of chlorine dioxide make it more effective than chlorine, requiring a lower dose and resulting in a lower environmental impact.
Chlorine dioxide is widely used as an antimicrobial and as an oxidizing agent in drinking water, poultry process water, swimming pools, and mouthwash preparations. It is used to sanitize fruit and vegetables and also equipment for food and beverage processing and widely used in life science research laboratories. It is also employed in the health care industry to decontaminate rooms, passthroughs, isolators and also as a sterilant for product and component sterilization. It is also extensively used to bleach, deodorize, and detoxify a wide variety of materials, including cellulose, paper-pulp, flour, leather, fats and oils, and textiles. Approximately 4 to 5 million pounds are used daily.
Unlike many decontaminating agents, chlorine dioxide has the unique ability to retain its sterilization capacity in water. In order to maximize process reproducibility it is best to avoid pools or puddles of water. However, if small amounts of water are present the efficacy of chlorine dioxide is not affected. The reason that small amounts of water will not impact sterilization efficacy is that chlorine dioxide is readily soluble in water. The partition coefficient (CClO2(H2O)/CClO2(air)) of chlorine dioxide at 22°C and 101 kPa is about 38 (Masschelein). And provided that the quantity of water is small the gas concentration in the water reaches equilibrium quickly.