U.S. health officials say they have tested about 300 postal facilities and other buildings for anthrax spores since the bacterial scare emerged through the mail in early October. They say cleanup continues at many of the buildings found to be contaminated. Experts point out that decontaminating biologically infected structures is hampered by a lack of knowledge, and that the compounds in use are often worse than the contaminant.

A U.S. Senate office building remains closed while authorities argue over the best way to eliminate anthrax spores mailed to the chamber's majority Democratic Party leader, Tom Daschle. They have abandoned plans to pump chlorine dioxide gas into it to attack the spores, fearing the fumes would not work in disinfecting a building.

This illustrates the confusion over decontamination methods. Experts told the House of Representatives Science Committee that not enough is known about the techniques' effectiveness and safety.

Part of the problem is that disinfecting buildings was not a high priority before October, according to Johns Hopkins University environmental health scientist Lynn Goldman.

"There would have been no real incentive for the private sector to engage in research in this area," she says, "and given that there is no research going on within the government, here we are in a situation where nobody has really given this much thought."

There are two broad categories of compounds for decontaminating biological agents reactive gases, such as chlorine dioxide, and liquid solutions and foams, such as chlorine and formaldehyde.

But University of Washington physician Charles Baker notes that because many of the chemicals are so potent, anthrax spores might be the lesser poison.

"One of the biggest problems with anthrax spores is that the spores are much tougher than people are, and many of the materials to decontaminate them are very toxic and probably would be worse than the minimal levels of contamination these people would have," he explains.

The experts agree, however, that some toxicity must be acceptable in order for decontaminants to work successfully.

Another perplexing question is how much should anthrax contamination be reduced in a building? Most of what scientists know about anthrax infection is based on studies done in the 1960s on monkeys, but how many spores are necessary to infect people is still a medical mystery.

Therefore, Dr. Baker says, no one knows what a safe level of residual contamination is.

"In addition, people are not all the same. People with immune problems, people that are pregnant, people that have underlying respiratory conditions may be more prone to infection with this. So setting a standard may be more difficult than we think," he says.

But Dr. Baker says determining that standard is important because, while many compounds can sterilize surfaces, a three dimensional building with hidden surfaces and ventilation ducts is impossible to completely decontaminate.

"We will not remove every spore, and we need to define for ourselves what is an acceptable or safe level to reduce contamination to and how we will determine that," says Dr. Baker.

The experts say the government must develop the standards to assess the biological risks and the degree to which they should be eliminated. But so far, they say the government's effort has lacked focus.

Johns Hopkins' Dr. Goldman blames this on a decline in the U.S. public health infrastructure.

"There is an enormous fragmentation in our public health system, that our public health service doesn't actually exist as a real entity anymore," he says. "So you do have the expertise scattered throughout the government, but right now there isn't anybody who is clearly in charge."

U.S. health authorities have responded to such criticism by reorganizing for the bioterrorist threat. President Bush's Health Secretary, Tommy Thompson, recently established an Office of Preparedness within his department to coordinate its various public health functions and work with other agencies that also have public health roles.

But as Dr. Goldman notes, it is too early to know how much authority this new office will have inside and outside the health department. Also unclear is whether establishing biological decontamination protocols will be a priority.