March 2004

If chemical compounds can be thought of as actors in the theater of public health, chlorine dioxide (ClO2) is one of the busiest performers, playing supporting roles in such imagined productions as:

  • "Food Safety for a Small Planet"
  • "Good-bye, Potato Blight"
  • "Preserving the Pages of History"

ClO2 is a disinfectant compound capable of destroying a wide variety of disease-causing bacteria, viruses, molds and algae (known as pathogens). Like sodium hypochlorite, calcium hypochlorite and chlorine gas, ClO2 is a chlorine-based drinking water and wastewater disinfectant. A gas at room temperature, ClO2 dissolves easily in water. The resulting solution may be applied at various strengths to poultry, red meat, seafood and the outer surfaces of fruits and vegetables to destroy pathogens. Archivists have even applied a dilute ClO2 solution to valuable documents to destroy harmful mold. And speaking of paper, the forest products industry substituted ClO2 for Cl2 as a bleaching agent when dioxin pollutants were found in bleached pulp mill wastewater in the 1980s. This role substitution virtually eliminated dioxin in the wastewaster. As a relatively safe and environmentally friendly actor, ClO2 is in high demand for many important disinfection roles.

Major Supporting Role: Fighting Foodborne Sickness

Electron micrograph of
a Listeria bacterium
in tissue (CDC Public
Health Image Library)
Listeria monocytogenes is a rare but hardy bacterium that can live on fruit and vegetable skins. According to Dr. Richard Linton, director of Purdue University's Center for Food Safety, "Just 10 to 100 cells of Listeria on a piece of food can cause illness, and it's possible for 1,000 to 10,000 cells to be on a piece of fruit."1 Dr. Linton and his fellow researchers tested chlorine dioxide on Listeria-infected apple skins over a range of strengths and periods of time. They studied Listeria because it is considered one of the worst of the foodborne pathogens, but there are many other more common microbes that can cause people to become ill. Dr. Linton's group found that ClO2 gas is 1,000 times more effective than any other method tried so far for eliminating foodborne pathogens.

Major Supporting Role: Preserving Documents

Mold is a major enemy of the printed page—it infects books and documents—especially old ones—that have been subjected to humid air. Mold can destroy printed materials and present a health hazard to those who are allergic to it. Although the best way to control mold is by controlling temperature and humidity within the library, many mold infestations can be traced to electrical power failures that temporarily suspend indoor climate control. At the University of Oklahoma, librarians found that by fogging (spraying) library stacks and floors with a very dilute solution of ClO2 (and controlling humidity and temperature), they have been able to prevent a mold outbreak for at least five years2.

Generating Chlorine Dioxide Gas

ClO2 gas can be generated on-site-when and where it is needed-by several methods. One method consists of reacting the salt sodium chlorite (NaClO2) with an acid (a compound containing hydrogen ions, H+, such as HCl--hydrochloric acid) according to the reaction:
5 NaClO2 + 4 HCl -> 4 ClO2 + 2 H2O + 5 Cl- + 5 Na+

As the ClO2 gas destroys pathogens by disrupting cell membranes, it largely breaks down to form salty water (note the Na+ and Cl- ions on the product side of the reaction), making it an environmentally friendly disinfectant.

An electron micrograph of spores from the Sterne strain of Bacillus anthracis bacteria (CDC Public Health Image Library, Photo Credit: Janice Carr)

Starring Lead Role: Destroying a Bioterror Agent

ClO2 played the role of hero in 2001 when it was used to destroy deadly spores of the bacterium Bacillus anthracis in the U.S. Hart Senate Office Building in Washington, D.C. (Many people call the bacterium "anthrax," but in fact, anthrax is the name of the disease caused by Bacillus anthracis, the bacterium.) In a cowardly act of terrorism, the deadly spores were enclosed in letters and delivered through the U.S. mail to senators in the nation's capital, a media company in Florida and newsrooms in New York. In all, the mailings caused five deaths and 22 cases of illness.

With microscopic airborne spores spread through the ventilation system and over three floors of the Hart Building, the challenge became one of destroying the spores without damaging the building. Field tests carried out in Maryland determined that fumigation with ClO2 (filling the air with the gas) would be effective under specific air temperature, humidity and circulation conditions.

Guided by the field test results, decontamination of the Hart Senate Office Building was successfully achieved as ClO2 gas circulated throughout the building, destroying the Bacillus anthracis spores. Soon afterward, senators and their staffs reoccupied their offices, ending a disturbing and disruptive period.

Encore! Encore! A Continuing Role for a Popular Disinfectant

Producing and delivering safe, nutritious food in today's global marketplace is challenging. Modern consumers demand a huge variety of fresh fruits and vegetables year-round, so paths from the farm to the fork have lengthened, increasing opportunities for contamination, and creating a role for safe, effective disinfectants. Whether it is used to destroy harmful microbes on foods, document-degrading mold, or agents of bioterrorism, ClO2 will remain a busy actor in the theater of public health.

Follow-Up Activities

  1. Write an essay comparing the risks associated with applying ClO2 to food surfaces to the risks of eating fruits and vegetables that are untreated for pathogens.

  2. The fungus Phytophthora infestans (Late Blight) was responsible for the devastating Irish Potato Famine of the 1840s. Today, ClO2 is used to control this fungus. How would history have been different if ClO2 had been known to Irish farmers of the 1840's?

  3. Although there are many harmful microbes to guard against in nature, it is also true that humans cannot live without certain microbes. Construct a "Gallery of Essential Microbes," (include drawings and written descriptions) that play important roles in processes like human digestion, bread-making, and composting. [Hint: Some excellent Internet resources are: The U.S. Centers for Disease Control and Prevention's Public Health Image Library at: and the American Society for Microbiology's Microbe World at:]


1"Chlorine dioxide gas kills dangerous biological contaminants," September 12, 2002. Purdue News, [On-line].

2Weaver-Meyers, P.L., Stolt, W.A. and Kowaleski, B. (2000). Controlling mold on library materials with chlorine dioxide: An eight-year case study. Abbey Newsletter, vol. 24.

For a list of previous "Chlorine Compound of the Month" features, click here.


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