Written by Ross Grant
Updated on July 19, 2002
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FRIDAY, July 19 (HealthDay) -- Ultraviolet lamps, which are increasingly being used for home and public water purification, are not all effective in eliminating Escherichia coli bacteria from drinking water.
That's the disturbing finding of a new study from Canadian researchers, who discovered that commonly used low-pressure UV lamps can't prevent the bacteria from regenerating their DNA after exposure. However, medium-pressure lamps work better, according to the scientists, from the University of Waterloo in Ontario, whose study appears in the July issue of Applied and Environmental Microbiology.
"Bacteria have been around for millions of years, and they have been exposed to the sun for millions of years, so it's not surprising that they would develop a mechanism to repair themselves after UV exposure," says Peter M. Huck, a water treatment engineering professor who participated in the study.
The research becomes significant as more cities and towns are moving toward UV water treatment. Seattle is building such a system, and Phoenix, Los Angeles, New York City, Atlanta, San Francisco, Detroit, Salt Lake City and Portland, Ore., are testing the technology.
UV radiation has been used extensively in Europe for 50 years to kill bacteria in water and food, but it has only become viable in North America over the last decade. Its most recognized use is eliminating Cryptosporidium from water, a pathogen that contaminated Milwaukee's drinking water nine years ago, killing more than 100 people and making 400,000 more sick.
The technology, as it becomes affordable, is also being included in more and more home water treatment systems, researchers say. Its popularity is expected to increase as federal authorities require smaller towns to treat Cryptosporidium and protect against bioterrorism.
"I think 20 years from now you're going to see this technology on a widespread basis," says Albert Ilges, a project supervisor for the American Water Works Association Research Foundation in Denver.
"As people become more and more concerned about the byproducts of chlorination, UV is being considered as an option," adds Jim Sliwa of the American Society of Microbiology.
The Waterloo study, sponsored by the Natural Sciences and Engineering Research Council of Canada, comes at a critical time as cities, towns and homeowners are picking between two competing UV technologies: low- and medium-pressure lamps.
The researchers targeted E. coli because it's commonly used as an indicator of disinfection. They also used small doses of radiation to simulate home-treatment systems or a larger system that is clogged. At much higher doses, low-pressure lamps are better able to eliminate E. coli, they say.
However, at the lower doses, the study found bacteria could bounce back after exposure to a low-pressure lamp. With a medium-pressure lamp, the bacteria could not repair their DNA, making it impossible for them to reproduce. The higher pressure lamps attack bacteria from a spectrum of wavelengths, while weaker ones emit a single wavelength.
"It's sort of like the difference between a light bulb and a fluorescent glow lamp," explains lead researcher Janis L. Zimmer.
However, Ilges, who has supervised a number of UV treatment installations, argues the evidence isn't clear. UV is generally used along with other purifiers, and the choice between low- and medium-pressure lamps often rests on other considerations, such as water flow, available space and energy use, he says.
"There's not definitive, widely accepted evidence either way," he says. "My perspective is that one shoe doesn't fit all. For certain applications, one would choose one technology over the other."
Moreover, he argues that E. coli is less of a threat to drinking water than the other toxins that might travel with it, such as Cryptosporidium, which can't regenerate its DNA after UV exposure.
"Most of the strains of E. coli are not pathogenic to humans. Traditionally, it has been used to check for other contaminants," Ilges says.
According to the Centers for Disease Control and Preventions, while there are hundreds of strains of E. coli, there has been relatively few U.S. health outbreaks as a result of water contamination. The four known harmful strains most generally spread through food, such as uncooked hamburger, infected lettuce and sprouts, and unpasteurized milk and juice.
What To Do
To learn about ultraviolet light, go to the International Ultraviolet Association.
To find out how your local water system rates with the Environmental Protection Agency, go here.
For more on E. coli, check out the Centers for Disease Control and Prevention.
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