. "4 Metrics and System Evaluation." Protecting Building Occupants and Operations from Biological and Chemical Airborne Threats: A Framework for Decision Making. Washington, DC: The National Academies Press, 2007.
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Protecting Building Occupants and Operations from Biological and Chemical Airborne Threats: A Framework for Decision Making
tion offered by a system can be assessed by comparing protection metrics in a building with or without (or before and after deployment of) a protection system. According to some prior and existing building protection studies, the protection of people could outweigh the protection of contents. Some commonly used protection metrics seen in prior and existing building protection programs include fraction of building exposed (FBE), fraction of occupants exposed (FOE), and lives saved.
The Immune Building Program of the Defense Advanced Research Projects Agency (DARPA) used FBE as a primary metric. FBE is defined as the fraction of the building (by volume) in which occupant exposures would exceed a prescribed level or guideline, typically evaluated as a function of the mass of agent released.
For chemical warfare agents and most toxic industrial chemicals, the exposure criterion is the acute exposure guideline level (AEGL) (NRC, 2000, 2002, 2003b, 2004, 2006) or other similar estimate of acute toxic potency. For biological agents, the exposure criterion is the infectious dose, that is, the number of organisms believed to be necessary to overwhelm host defense mechanisms and establish an infection.1 FOE measures the fraction of occupants that are exposed to a threat agent. For this metric to be useful, the amount of exposure at a given time or duration and the type of exposure (for example, skin, inhalation, or ingestion) need to be specified. FOE then can be derived in a similar manner, provided information is given about the location of the occupants and their exposure levels. FOE is not commonly used as a primary quantitative metric, but it is inherent in some of the chemical and biological protection architectures that have been developed and deployed. (See Chapter 5 for specific demonstrations that use FOE and FBE metrics.)
FBE relies on an experimental measurement or analysis of the transport and dispersal of agents or toxic materials within the interior spaces as a result of releases either outdoors or within the building. For example, DARPA’s Immune Building Program uses multizone contaminant transport modeling as a principal means of estimating the concentration and exposure profiles within the interior spaces. Several such models are available; the Immune Building Program adopted the use of the CONTAM2 multizone code (NIST, 2006a; Walton and Dols, 2006). Tracer experiments conducted as part of the Immune Building test bed program have shown that the modeling approach can provide results comparable to actual measurements. As is the case with most models, accuracy is highly dependent on an adequate understanding of the input parameters and inherent model limitations.
FBE and FOE have several limitations. They do not take into account the
Infectious dose might not be the optimal exposure criterion to use because it is highly dependent upon the methods used for agent preparation and the strain of agent used.
CONTAM is a multizone airflow and contaminant transport analysis software.