As tainted water flows through pipes, the spores adhere to the inner surfaces, which would further complicate efforts to decontaminate the water before it could reach the tap.

Spore-forming microbes, such as Bacillus anthracis, the cause of anthrax, are different from typical disease-causing microbes that might be in drinking water system, researcher Dr. Jon Calomiris told Reuters Health. "Because typical drinking water (microbes) are not spore formers, they are much more sensitive to killing by heating or disinfection than spores are."

In fact, he noted, attempts to decontaminate water containing anthrax spores by heating could create an aerosol, increasing the risk that the spores would be inhaled and cause disease.

Calomiris from the Air Force Research Laboratory at Aberdeen Proving Ground in Edgewood, Maryland found that anthrax spores survived when suspended in water with a chlorine level of 1 mg/L, a concentration typically found in tap water. In order to kill virtually all spores in 1 hour, an undrinkable concentration was required.

He constructed a model pipe system using segments of copper, chlorinated polyvinyl chloride and iron pipe to see what would happen when municipal chlorinated water contaminated with spores flowed through.

Results showed that after 6 hours, about 20 percent of spores adhered to the copper and plastic pipes, and about 80 percent to iron pipe. When copper pipe was coated with a biofilm, the rate of spore attachment increased fourfold.

In the event of a deliberate contamination of a municipal drinking water system by anthrax spores, Calomiris said, "I would first recommend that the supply of water to the public be stopped" and "that the concentration of disinfectant and the duration of exposure to the disinfectant be increased, thereby increasing the amount of kill that would occur."

However, he also noted that further research is needed to determine exactly what measures would be required to rid the entire system of viable spores.