possible. Referee equipment may also include instruments to measure overall aerosol particle count, sample fluorescence, and other characteristics of the aerosol. Variation in both the methods of characterization and the units used to quantify the amount of airborne agent material has created a barrier to comparison and cross-correlation of quantitative test results.

To address these issues, the DOD Joint CBD Program seeks a standard unit of measure that can be used for biological material independent of the state of the material (aerosol or aerosol resuspended or captured in liquid) and independent of agent type (i.e., bacteria, viruses, or toxins). The department asked the National Academies to conduct this study addressing the use of measurement in the testing of aerosol detector, whether ACPLA is the most appropriate measure and what alternatives exist. If one unit of measurement is not obtainable, the DOD seeks to establish a relationship for comparison of different units.2


The charge was to determine whether it is possible to develop a single unit of measure for airborne biological agents that may include bacterial vegetative cells, bacterial spores, virions, and biological toxins. The ultimate purpose of this unit of measure is to aid in the evaluation of sensors that are designed to warn and provide a measure of protection against the risk of exposure. Modes of action and the nature of the health threat vary widely among the different possible agents, and may even vary with the mode of exposure to a particular agent. However, the different biological agents share a common feature, and that is the hazard they pose to exposed humans. This feature is also shared by pathogens associated with naturally-occurring epidemics such as SARS, avian influenza, or toxins associated with red tide. This report focuses on biological warfare agents, but the concepts presented are equally applicable to a broad range of airborne pathogens, biological toxins, and chemical agent aerosols.

The potential harm of a BWA attack depends on such factors as the type of agent, activity or viability of the agent, quantity released, and method and circumstances of dispersal. As an illustration, the received dosage of a biological agent, such as anthrax, from which 50 percent of the exposed population dies (lethal dosage 50 percent, or LD50) can be obtained from multiple (e.g., internet) sources as a number between 5000-20,000 spores. What is not generally discussed in this type of reference is the dependence of the LD50 quantity on the specific strain of anthrax. For example, the Aum Shinrikyo cult conducted several unsuccessful biological warfare agent (BWA) attacks in Tokyo in the early 1990s. Only one of these attacks, in 1993, was even recognized at the time. The lack of harm and the failure to detect the attack were due in large part to the selection of a vaccine strain of B. anthracis as the agent. If the group had selected a virulent strain to distribute, the outcome could have been different. Thus, LD50 is a critical characteristic of a BWA, and one can quickly appreciate that in order to gauge the effectiveness of an attack, a detection system should not only measure the number of B. anthracis spores present but also provide a measure of both viability and virulence of the agent. If a unit of measure does not take biological activity of the agent into account, it will have little use as a measure of the hazard a released agent presents.

Despite the many sources of potential variability in BWA attacks, it is useful to consider an idealized detector to help define appropriate units of detector performance. If consensus can be reached on an appropriate method of quantification for BWA aerosol sensors, the new method


See Appendix B for the full statement of task.

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