“Threat type” refers to different categories of methods and agents that a perpetrator could use in an attack—biological, chemical, and radiological agents and explosives in combination with a dissemination means. “Threat agent” refers to the specific biological or chemical agent used in an attack, such as B. anthracis or sarin. Within the two threat types that are considered in this report, biological and chemical, many threat agents could be used against buildings and occupants. Different threat agents can have different physical properties—they could be solid particulates, liquid, or vapor. There are advantages in choosing different threat agents or physical properties depending on the configuration, location, and condition of the specific target building and the emergency preparedness of the building and its occupants. Target selection depends on building design; security in place; design of the HVAC system; personnel schedules; availability of agents; and technical capability of the perpetrator(s). Agent selection considerations might include availability, ease of handling, volume required, and the intent and skill of the perpetrator. Agent-disseminating devices can be simple or complicated; volatile chemicals or very fine prepared powders can be disseminated more easily than nonvolatile liquids. Taking all of the variables into account, it is clear that no building can be completely protected from a sophisticated, well-trained, determined aggressor. In buildings that are at high risk of a terrorist attack, the goal of building protection is to reduce vulnerabilities to and consequences of external or internal delivery of threat agents into buildings. Enhanced physical and operational security is as important as sensors and mechanical response systems in reducing the likelihood and impact of an attack.
Biological threat agents include bacteria (vegetative and spores), viruses, and products of such organisms as toxins. They can be delivered as nonvolatile particulates or suspended liquids. The most likely mode is the delivery of aerosolized particles in the size range of 0.3 to 10 µm. Generally, particles larger than 10 µm are less likely to reach the lungs, but they could be lodged in the mucosa of the nasal passage or the pharynx or simply settle out of the airstream and contaminate the area. Biological threat agents include disease-causing microorganisms and protein or low–molecular weight toxins. Biological threat agents are often mixed with inert materials that enhance dispersal and improve stability (see NRC, 2005a, Table 2-1). Once detected, microorganisms and toxins can be identified definitively by various means of analysis.