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A Framework for Assessing the Health Hazard Posed by Bioaerosols 5 Recommendations A broad test and evaluation (T&E) program supports the development and deployment of biological warfare agent (BWA) detectors. From the development of testing requirements for deployment of detectors in the field, the end goal of the T&E program is to protect military personnel from BWAs and to minimize the health impact of exposure. In its current form, the T&E program relies on Agent-Containing Particles per Liter of Air (ACPLA) as the standard unit of measure of concentration for biological aerosols. This unit, while providing some mechanism for the comparison of candidate detector capabilities, does not adequately allow for conversion between various measurements or provide substantial information about the health hazard posed by detected bioaerosols. By considering the characteristics of biological aerosols that most impact the overall health risk associated with exposure, a new, robust standard framework has been developed. This new framework will have practical application for both candidate detector evaluation and a broader impact on the overall detector T&E program. Recommendation A: A unit of hazard should be adopted as part of the evaluative framework. The committee recommends the unit be a particle size (Dae) dependent measure of biologically active units per liter of air (BAULADae). In the context of the framework described above, a unit has been developed that describes the physical characteristics of an aerosol with the greatest impact on health outcome. A primary standard unit of biological aerosol measurement that quantifies biological activity per unit volume of air and incorporates prediction different regions of deposition sites in humans is recommended. This measure is the size-resolved number of biologically active units per liter of air (BAULADae). The BAULADae unit takes into account the key information needed to estimate health hazard. Biologically active units (BAU), though measured and calculated differently for each threat agent, allow for a common measure of hazard between agents by linking the amount of active agent detected to biological activity normalized to a population activity end point (e.g., LD50, ID50). Incorporation of particle size into the unit accounts for the different activity levels demonstrated for particles that deposit in different regions of the respiratory tract or human body in general. When considered in the context of Recommendation A, this unit will provide baseline information about the specific hazard posed by any particular agent. From a detector development standpoint, this unit will help direct the tailoring of detectors and requirements for specific biological agents. For example, a detector should be more sensitive to virulent agents that require less concentration to cause disease or death than for less virulent agents or those against which the at-risk population is routinely immunized. The committee further proposes BAPLADae (size-resolved number of biologically active particles per liter of air) as a secondary standard unit of bioaerosol measurement to quantify the size distribution of particles in an aerosol cloud that contain biologically active material. This secondary unit of measure is recommended to provide a unit for historical comparisons with
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A Framework for Assessing the Health Hazard Posed by Bioaerosols ACPLA measurements, and also in anticipation of the possible discovery that the number of sites where aerosol particles deposit biologically active units in the respiratory tract may need to be known to predict health risk for some agents. BAPLADae would be measured in a manner similar to current methods for ACPLA, with the exception of employing a defined challenge aerosol particle size distribution and normalizing biological activity for each agent. Recommendation B: In support of an overall Department of Defense DOD detector evaluation philosophy that relates health risks and aerosol exposure, the committee recommends using the relationship presented in Figure 3.1 to provide a framework for relating health hazards and aerosol exposure and to identify the information provided by different types of detectors. This relationship, described in greater detail in Chapter 3, provides a framework for evaluation of health risk presented by exposure to aerosolized biological warfare agents. Note that, though the physical and biological characteristics of a bioaerosol are the features evaluated by a detector, the data provided by that detector cannot be appropriately assessed without consideration of the physiological effect of exposure. In addition to those specific considerations, other factors contributing to predicting health risk can be inserted into the equation to aid in the development of appropriate requirements and response protocols. Recommendation C: Standardize testing procedures for evaluating detectors. C.1 Aerosol challenges need to be well characterized, including Dae, BAULADae, BAPLADae, and material rendered inactive during the process of aerosol dissemination and transport. C.2 Challenge biodetectors with aerosols of defined size distributions. At least three challenge aerosols with different median aerodynamic diameters (Dae) should be used in chamber or component testing of detectors. These should be chosen to represent deposition in the three regions (extrathoracic, tracheobronchial, and pulmonary) of the respiratory tract. C.3 The standardized unit of hazard, BAULADae, and the broader evaluative framework for health hazard should be adopted as DOD-wide standards, including use in T&E and procurement. The method for implementing the unit and framework should be documented, externally peer reviewed, and published. Revisions and updates should follow similar vetting processes so that calibration, referee instruments, and testing reagents are standardized and variation is identified. To take full advantage of the benefits provided by the new unit of hazard and the health risk evaluation framework, the T&E protocol for detector evaluation must be standardized. A full review of the testing methodology is beyond the scope of this committee. However, some issues that were brought to light during the course of this study are noted here. This list should not be considered comprehensive, but is indicative of the potential value an in-depth review could provide. For example, as BAULADae is linked to biological activity, it will be necessary to prepare test samples consistently to ensure that the amount of active agent presented to the detector does not vary across tests. It is important to understand the effect that sample preparation, aerosolization, and cloud transport have on the aerosol particles. Generated biological aerosols will change with relative humidity and temperature. These changes must be addressed to have a consistent, comparative, and scientifically rigorous test and evaluation
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A Framework for Assessing the Health Hazard Posed by Bioaerosols program. The effect of desiccation of the biological material with propagation will change the aerodynamic properties of the particles as well as the biological properties and must be taken into account. The referee instruments will need to be carefully calibrated and chosen to enable characterization of particle size and the proportion of active agent in the aerosol. Effective use of BAULADae and the broader risk framework will require coordination between the various areas of the DOD test and evaluation community, as it is critically linked with the health outcome resulting from exposure. This will impact the development of detector requirements and will require input from, and coordination with, the biological testing laboratories. Recommendation D: Survey the literature to better understand the transport and inhalation of particles to improve selection of appropriate Dae (median aerodynamic diameter) challenge aerosols for test and evaluation. Balance this knowledge with intelligence of possible threats. Specifically examine whether to include testing of particles smaller than 1µm and larger than 10µm. The site of deposition of a particle with active agent in the respiratory tract has a significant impact on the final health outcome of exposure. As the body of research on the transport and inhalation of particles continues to grow, it should be reflected in the selection of particle size distributions that best represent the potential health threat in the field. Recommendation E: Maintain ability to learn from unanticipated events by archiving data. E.1. Archive parameters, methods, measurements, and test conditions during T&E of detectors. E.2. Consider archiving raw data collected by deployed systems to guide future development of detectors through performance evaluation. Once a detector has been deployed in the field, it can still provide important information to aid in the development of future detector technologies and refinement of the application of BAULADae. By archiving raw data received by deployed detectors, it is possible to compare data from true or false positives with the data received in the test chamber. In addition, it is likely that the parameters used to calculate BAULADae will change over time for specific agents as more clinical data about the physiological effects of exposure are acquired. A data archive of test parameters, methods, measurements, and test conditions will make it possible to compare datasets acquired at different times and places. Recommendation F.1: For priority DOD scenarios, an evaluation of uncertainty in the science, measurements, and situational awareness should be conducted so that resources can be invested in reducing the largest uncertainties that impact decision making. The data matrix required to make BAULADae maximally useful is quite large and acquiring all pertinent data about the health effects of size-resolved particles for every strain and variant of agent is not feasible. As a result it will be necessary to periodically review and prioritize the allocation of resources so that they may be invested in reducing the largest uncertainties that impact decision making.
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A Framework for Assessing the Health Hazard Posed by Bioaerosols Recommendation F.2: An applied science program should be designed and executed to obtain information that will both improve the accuracy of the terms of the equation and help evaluate threats and their inherent uncertainties. This will require information about different biothreat agents, instrumentation types, and scenarios important to DOD. The information will be of value to biodetector testing and many other aspects of the overall military biodefense program. An applied science program should be created to act on the priorities as they become apparent. For example, as intelligence is gathered, it may become apparent that one type of agent is a greater threat than another. In this case, a program to investigate in detail the effect of particle size on health outcome for that strain or agent type may be in order. An effective applied science program would enable a quick response to emerging threats and any new data.