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updated 5/6/2011 6:42:45 PM ET 2011-05-06T22:42:45

Soap may not always be as clean as it seems, suggests a new study, which found that every soap dispenser at an elementary school in Ohio was contaminated with bacteria that are known to cause illnesses. When kids washed with the soap, microbe levels on their hands soared.

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It's not yet clear how much of a health risk people face from microbe-laden soap, and public health experts continue to urge regular hand washing as one of the most important things that people can do to stay healthy. But the findings suggest that administrators of schools, gyms and other public places might want to re-examine the kinds of soap dispensers they use and the methods they use to clean them.

"This is kind of counterintuitive because soap is supposed to clean you," said Carrie Zapka, a microbiologist at Gojo Industries, a company in Akron, Ohio, that makes skin health and hygiene products, including soaps and soap dispensers. But the new study showed, she said, "that soap can be a source of bacteria that can spread beyond the person washing his hands."

"There's no need to panic," she added. "I want people to wash their hands because hand washing is proven to be effective at preventing sickness. We don't know what the true risk level is in the community."

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The idea that bacteria can grow in or near soap is not new. For years, hospitals have mandated the use of dispensers with replaceable bags or cartridges that contain their own nozzle and are sealed so they can't be contaminated. But plenty of community settings contain dispensers that are repeatedly refilled with pourable liquid soap, and these kinds of dispensers are rarely, if ever, cleaned.

Several years ago, some of Zapka's colleagues came to her with soap samples they had taken from places like gym showers, where they saw fuzzy molds growing. Her tests on those samples found high levels of contamination with a variety of bacteria. More recently, a large, nationwide study found contamination with infectious bacteria in one out of four soap dispensers in public bathrooms.

To begin to address the extent of the problem and the public health risks involved, Zapka and colleagues started with a laboratory experiment. First, they intentionally loaded samples of soap with two species of bacteria that are most commonly found in contaminated soap. Both have caused infectious outbreaks in healthcare settings.

The researchers tested two types of hand washing techniques: the rigorous, minute-long, nail-scraping, full-rinsing version that doctors practice, and the 10-second version that just about everyone else does. They also tested a range of contamination levels. At the low end, concentrations hovered below 1,000 bacteria per milliliter-sized squirt of soap, which is generally the level considered acceptable. At the high end, some samples contained as many as 10 million bacteria per mL, which was the level of the most contaminated soap ever identified.

When people washed with more highly contaminated the soap, the researchers reported in the journal Applied and Environmental Microbiology, more bacteria stuck to their hands after they were done washing. And when there were more bacteria on people's hands, more bacteria were transferred to the surfaces they touched.

The new measurements, Zapka said, should provide a baseline for future studies that will translate numbers of bacteria into overall health risks. In the meantime, the researchers went to a nearby elementary school that had a total of 14 open-style refillable soap dispensers. The school's dispensers were never cleaned or sanitized. All were contaminated with potentially harmful bacteria.

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Just as in the lab experiment, kids and staff members who scrubbed with the contaminated soap ended up with 26 times more bacteria on their hands after they washed. But there was an easy solution. The school replaced its dispensers with sealed versions. A year later, they were still contamination-free.

The kinds of bacteria that contaminate soap, particularly in public bathrooms, can cause health problems like rashes, urinary tract infections and eye infections, though the risk is greatest for the very young, the very old and people with compromised immune systems.

For everyone else, it's still best to wash your hands regularly and often, said Kellogg Schwab, an environmental microbiologist at the Johns Hopkins Bloomberg School of Public Health in Baltimore, who advocated using regular everyday soap over antimicrobial versions and alcohol-based hand sanitizers.

The bacteria that live on us already outnumber the cells in our body, he said, and they aren't necessarily a problem if they sit on our skin. Most microbes have to get inside our bodies to cause illnesses.

"This warrants further research and is worth further discussion and evaluation," Schwab said. "But hand washing for 20 seconds with warm water soap has been documented to be by far one of the single most important things we can do for our health."

© 2012 Discovery Channel

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