- The American Dental Association most recent (1999) public statement about dental water contamination.

Although there is not a documented case of disease transmission involving dental-unit water lines, the concern is that contamination may occur. This could happen when bacteria, originating in community drinking water, enter the dental unit, colonize and reproduce inside the water-line tubing. The accumulation of these bacteria is termed "biofilm."

More at http://www.ada.org/public/media/newsrel/9907/nr-02.html