(American Society for Microbiology) -- Researchers from the University of Waterloo in Ontario, Canada have determined that some types of ultra violet light are not effective in killing harmful bacteria in drinking water. Their results appear in the July 2002 issue of Applied and Environmental Microbiology.

In the study the researchers tested the ability of Escherichia coli to repair DNA damage caused by varying levels of UV radiation from low and medium pressure lamps. They found that while E.coli was able to fully repair damage done to its DNA after being subjected to low-pressure UV rays, they remain unclear as to the effects of medium-pressure rays.

"The results of this study show that polychromatic medium-pressure UV radiation may offer an advantage over monochromatic low-pressure UV radiation in lower-dose water treatment applications," claim the researchers. "It is recommended that further studies be carried out with medium-pressure UV to determine which wavelengths cause additional damage and where the damage is induced."