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New UNC experiments show very weak chlorine solutions can kill noroviruses

December 20, 2005

CHAPEL HILL - Chlorine solutions much weaker than previously believed can still be used to kill more than 99 percent of noroviruses, the chief cause of outbreaks of gastrointestinal illness around the world, a new University of North Carolina at Chapel Hill study concludes.

Researchers presented their findings over the weekend at the 2005 International Conference on Antimicrobial Agents and Chemotherapy, which ends today (Dec. 19) in Washington, D.C. They discovered for the first time that dilute solutions of hypochlorous acid, or free chlorine, as low as 200 - or even 20 - milligrams per liter will completely inactivate noroviruses on surfaces such as stainless steel and ceramic tile.

Dr. Mark D. Sobsey, professor of environmental sciences and engineering at the UNC School of Public Health, and postdoctoral fellow Dr. Geunwoo Park conducted the research. They also found that the dilute chemical worked quickly - in five minutes or less.

"This is good news since noroviruses are the leading cause of viral gastroenteritis," said Sobsey, director of the school's Environmental Health Microbiology Laboratories. "They have caused countless outbreaks of gastroenteritis in health-care facilities, schools, food establishments, hotels and resorts and on cruise ships."

Decontamination of affected facilities can prove difficult since the viruses persist on environmental surfaces and are resistant to some widely used sanitizers, he said. And they are highly infectious even at low doses.

In their studies, the scientists dried a group II norovirus - the predominant form circulating in the USA - and a widely used indicator virus, bacteriophage MS2 infecting E. coli, on stainless steel and ceramic surfaces, Sobsey said. After treating those surfaces with a 200 milligrams per liter solution of hypochlorous acid for one minute, they tested them to learn how much virus remained. The viruses dropped 99.99 percent.

"Even a lower concentration of 20 milligrams per liter of hypochlorous acid reduced the viruses by 99.9 percent in five minutes," he said. "Our results show that environmental surfaces can be readily decontaminated of noroviruses with dilute hypochlorous acid, which is the active ingredient of household bleaches like Chlorox."

In practical terms, that means that household bleach can be diluted by a factor of 1,000 and still work, Sobsey said. In all likelihood, it would even disinfect noroviruses on hard surfaces if diluted by a factor of 10,000. At full strength, the bleach contains 50,000 to 60,000 milligrams per liter of chlorine.

"Previously, many people have diluted household bleach to one part in 10, which is still a strongly corrosive solution," he said. "They often are reluctant to use at that strength since it can mar surfaces and make them look bad."

Regularly disinfecting bathrooms, kitchens and other areas prone to harbor viruses and bacteria is important because outbreaks of gastrointestinal illness, which include nausea, vomiting and diarrhea, can kill people, especially if they are old or dehydrated, Sobsey said. While most otherwise healthy patients recover after a few days of discomfort even without treatment, noroviruses can continue to sicken others until facilities have been thoroughly sanitized with products proven to be effective.

In the research, Sobsey and Park generated hypochlorous acid from a dilute solution of salt using an electrolytic generator made by Sterilox Technologies. That company, which supported the research, manufactures on-site generators of room-temperature, high-level hypochlorous acid disinfectant for use in biomedical, food production, water treatment and other applications.

University of North Carolina at Chapel Hill