Exposure reconstruction and risk characterization are important to epidemiological investigation and to decontamination. In the case of the letters tainted with B. anthracis spores, it was important to understand that exposure can be associated with the passage of powder-containing letters through the mail and to validate the model using empirical outcome data (CDC, 2001). Other research priorities include analysis of reaerosolization of settled spores and identification of risk for disease among secondarily exposed individuals; follow-up surveillance in those potentially exposed; the effects of long-term, low-level exposures; quantification of background contamination by potential agents in urban and rural environments; and identification of the occurrence of sporadic cases of zoonotic diseases that are considered possible threats. It also is necessary to decide how much sampling and decontamination will be done at satellite locations to which agents could have been transported.

Specific knowledge about the harmful biological agent used in an attack is important for emergency response, and it is essential for proper cleanup. Unlike the spores of B. anthracis, Yersinia pestis cells are sensitive to extremes in environmental conditions and therefore should not pose the same long-term hazard to the general population after a release (Inglesby et al., 2000). Naturally occurring Y. pestis is unlikely to remain viable for more than a few days after release, so its detection and identification can be troublesome. Recovery of viable organisms is unlikely unless samples are obtained and tested immediately after a release. Culturing the organism takes several days so PCR identification would be most timely, despite the fact that PCR cannot answer questions about viability. Like Y. pestis, variola major is sensitive to environmental conditions and in its natural form would not persist in droplets for long outside a human host.

For the case of biological agents, there also is the possibility of weaponization—engineering of the organism to improve its stability or other properties. In general, the weaponization begins with the growth of the agent (lag, log, and stationary phases each have unique properties mixed in with the culture media), then fermentation; centrifuging and separation; drying; milling for respirable particle size; additives to prevent aggregation and clumping, neutralize electrical charge, and increase survival in air; and microencapsulation for stability and viability. Each phase leaves physical and chemical clues that can help investigators to distinguish the agent substance from a normal background presence. Expertly prepared weapons are likely to be more resistant to natural attenuation and may be more resistant to decontamination.