powder incident, where the agent is visible; standard methods exist for collecting white powder for analysis and forensic use. If, however, there is only trace evidence in a room, a random, probability-based method—sampling X number of places in the room in a random fashion, to provide a 95 percent chance or greater of picking up material—is appropriate. In general, the larger the room, the greater the number of samples required.

After decontamination, a combined judgmental and random sampling would be appropriate to ensure that the area is free of contamination. The U.S. Environmental Protection Agency (EPA) uses composite sampling. They collect samples, put them in the same tube, and analyze the contents. They simply want to know if there is contamination present, regardless of its location. This approach could apply to forensic sampling to reduce labor and cost. But it will reduce the ability to reconstruct the event with that/those sample(s).

In the event of a wide-area release, for example, if someone releases B. anthracis over a city, there is a limited time to remediate the area. According to statements made at several exercises, substantial delays in remediating the affected area would reduce the likelihood that people would return. Leaders will want to be able to tell the public when or whether it is safe for them to return, and that decision will also be based on sampling. However, the public could lose confidence in their leadership if any residual infections occur after they move back in.

Morse was asked to comment, in terms of validating collection methods, how much confidence he has that there are no spores when he gets a zero response from sampling a room, and how he establishes probability or statistical confidence limits. Morse responded that a negative result only means that there were no viable spores detected, but one does not know if there are spores below the limit of detection; current sampling methods will detect less than 1 spore per cm2 on a surface. The CDC feels comfortable saying that a negative response would be equal to less than 1 spore per cm2. Although the analytical method may be able to detect a target at very low levels, sampling may or may not be efficient. Therefore, lack of detection may be related to collection method, sampling efficiency, and limit of detection of the analytical assay.

In the United States, the CDC has a network of analytical laboratories, the Laboratory Response Network, comprising 160 laboratories throughout the country. Similarly, the Food Emergency Response Network has protocols for analyzing food for the presence of threat agents, the Animal Health Network has them for agricultural animals, the EPA has an Emergency Response Laboratory Network, and there is a plant network. All use basically the same methods. The labs can process environmental samples using the exact same method; the results in one lab are equivalent to those in another lab. This is important because in the case of a wide-



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