Sensor Systems for Biological Agent Attacks Protecting Buildings and Military Bases (2005) / Chapter Skim
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10 Design Considerations for Detect-to-Warn Defensive Architectures
10 Design Considerations for Detect-to-Warn Defensive Architectures
Pages 155-177
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From page 155... ...
An incremental deployment strategy for detect-to-wam systems might exploit current defensive opportunities while detection technologies mature sufficiently to provide protection in a wider range of attack scenarios. This chapter examines architectural concepts for protection of two important target classes against the release of biological aerosols: buildings and extended military installations.
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From page 156... ...
Some release scenarios create potential detection sites at which agent concentration is much higher than background biological aerosol levels. For example, to attack a large facility, a rapid interior release in one of the rooms of that facility will result in very high local agent concentrations immediately following the release.
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From page 157... ...
Nommal transport delays, dilution in the HVAC system, and isolation of major air handling zones limit the rate of concentration buildup at points away from the release area. Actrve air handling responses to improve these natural mitigating factors include shutdown or redirection of airflows toward the release
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From page 158... ...
· Interzonal transport, including exchange across zone boundaries that spreads agent to adjacent air handling zones. Specific times associated with each of these steps will depend on the operational status of the air handlers at the time of the attack as well as the layout, ductwork, and physical barriers associated with the facility and the HVAC system.
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From page 159... ...
An example that postulates a release inside a multizone office building is included in the following section to illustrate defensive concepts and the approximate time lines and agent concentrations resulting from a nominal attack scenario. While this example does not represent any particular building, the parameters used in the simplified model are representative of a modem office building designed with attention to airflow control and vulnerability reduction.
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From page 160... ...
This value is not atypical of the rate that can be achieved in modem office buildings with well-designed HVAC systems.s For this example, the only fresh air input is through the air handler inlets with no infiltration or | AH1 l | AH2 l l Release Room A1 FIGURE 10.2 AirRows in the multizone office building. A2 See, for example, the case study in Sohan et al, 2002 See note 4 above AS
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From page 161... ...
The agent concentrations in the release room and in the return plenum of the A1 air handling unit (as shown in Figures 10.3 and 10.4) are expected to be well above biological aerosol backgrounds, particularly for facilities that employ good filtration and other design features to reduce background levels.
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From page 162... ...
This condition might Great more favorable conditions for detection of an attack by sensors in the air handling unit but might also reduce the effectiveness of airflow controls in isolating other rooms or other air handling zones to reduce agent speed. Estimates for the probability of infection of occupants in the various zones for the baseline scenario with no HVAC response and HVAC shutdown are shown in Figure 10.5.
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From page 163... ...
, early shutdown appears to provide flhe most important contributions to reduction in probability of infection. It is important to note that the larger attack sizes postulated in the 10x and 1 Box scenarios will result in very high biological aerosol loadings in the release room and in the air handler in zone A1.
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From page 164... ...
Other changes that increase interzonal isolation, such as separation of ductwork in adjacent zones or physical barriers to block flow from adjacent zones can be retrofitted into existing facilities, although costs may be pnahibibvely high. Filters and Other Background Reduction Measures High-quality filters can have a significant impact on the concentration of biological agents that are transported through the air handling systems.
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From page 165... ...
Reducing monalityfrom tersest releases of chemical and biological agents: 1 Filtnation for ventilation systems in commercial buildings Lawrence Berkeley National I aboratory, Report LBNL-44srjo The committee notes that these programs would likely have considerable overlap with efforts to protect the occupants of buildings from clouds of radioactive material.
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From page 166... ...
Concentrations at the inlet duct may be comparable to those resulting from interior releases if the air handlers are operating in a mode that recirculates a significant fraction of the uncontaminated interior air. For trickle releases at the air handler inlet, the detection and defense problems are expected to be comparable to similar slow releases at other interior locations.
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From page 167... ...
Compared to distributed detection assets placed near the release, somewhat greater time delays will be experienced by the centralized detectors, associated with the flow of return air to the air handler and a slower rate of change in agent concentration buildup. One disadvantage is that the greater number of detectors in a distributed system could result in more false alarms.
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From page 168... ...
The ability to achieve rapid and sensitive identification of low agent concentrations is important for defense against low-level releases and other sophisticated attacks. In the most demanding of these scenarios, bioaerosol detectors may be of limited value, so initial alarms would need to be generated by continuously nunning idenbficabon detectors.
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From page 169... ...
Some of the facility defensive measures discussed in the previous section might apply here as well. For exposed personnel outdoors and away from the potential sheltering capabilities of nearby facilities, there are few low-regret response options in the absence of personal protective gear.
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From page 170... ...
The agent concentrations at ground level as the cloud passes over the defended perimeter are low relative to those noted in the interior facility release. Current bioaerosol detectors using thresholds low enough to detect this attack would likely experience relatively E 5 o high false alarm rates.
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From page 171... ...
The building has a filter efficiency of 98 percent and an air handler that produces 4 air changes per hour with 90 percent recirculation. (This is equivalent to 0.4 fresh air changes per hour and 3.6 recirculated air changes per hour.)
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From page 172... ...
However, the most effective approach would be to naduce the air infiltration into the building. This infiltnabon comes from a variety of sources including doors, windows, cracks, and other openings and is driven by a variety of means, including pnassuna-driven flows from the air handlers, external winds, and thermal effects.
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From page 173... ...
The philosophy underlying this strategy is similar to that recommended for facility protection. Available bioaerosol detectors deployed as close as possible to the release point can trigger identification detectors and might initiate other response actions for very large attacks that increase bioaerosol levels well above background.
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From page 174... ...
Phase 2 Priorities SENSOR SYSTEMS FOR BIOLOGICAL AGENTATTACKS 1 a, _ 10 1 ~ A 10~ , 103 1nJ A Outside '~1 \Conr entrabon | ~ Inside Conrentrabon, I /~i r_i i1 ~ t5 20 25 30 35 40— Inside Concentration, Infiltration = 0.1 ACfhr l Time Since Retease (minutes)
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From page 175... ...
Such unambiguous detection can occur when agent concentration well above background levels is created in the area of the release. Recommendation 10-1: Nonspecific, bioaerosol detectors should be deployed in a diverse set of facilities to understand the perfommance of these detectors in interior spaces.
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From page 176... ...
The task is made more difficult, however, by a more demanding detection environment and by the absence of effective low-regret response options, particularly for exposed personnel. Several specific findings and recommendations that apply to this scenario follow: Finding 10-7: For outdoors releases, agent concentrations relative to backgrounds are often much lower than in facility scenarios.
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From page 177... ...
With current probit curve models of infection, very low concentrations of agent are required to achieve such a low level of infectivity. Recommendation 10-11 : The validity of widely used infectivity models should be reviewed before applying those models to generate defense system requirements.
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