organism or molecule and those that might contain hundreds or thousands of units of the bioagent. Finally, the health effects of particles containing bioagents can be dramatically affected by the size of the particle—small particles can be drawn deeply into the lungs where they are more harmful, while large particles may only reach the nasal passages, or not be inhaled at all. ACPLA makes no particle size distinction.
The problem with a unit of measure that does not include such parameters can be illustrated by imagining the response to two different threats of a detector measuring ACPLA: in one scenario, an average liter of air contains 100 particles, each carrying one avirulent bacterium. The detector would give a reading of 100 ACPLA. In the other scenario, an average liter of air contains 1 particle carrying 100 extremely virulent, live bacteria. The detector would need to be sensitive down to the level of 1 ACPLA to detect this attack. If the detector were set to sound an alarm at, say, 10 ACPLA, the detector would alert military personnel to the first attack—which is, in fact, harmless—and fail to alert to the second attack—which might be extremely dangerous. Even if the detector were sensitive enough to detect 1 ACPLA, it would still sound an alarm in both situations, where only one warrants taking precautionary measures.
The challenges described above are just some of those associated with the measurement of biological species. The complexity of the bioagent threat is such that there is only one relevant characteristic shared by all agents of interest: the capacity to interact with the human body and potentially cause harm. Thus, a unit of measure that considers the health hazard posed by a given concentration of aerosolized biological agent in air would allow comparison across all agent types against a characteristic that would have real utility both for test and evaluation of detectors and in the field.
ACPLA is easily understandable and measurable; thus, it is straightforwardly incorporated into a system of detector requirements and evaluation, but it fares poorly in the more complicated task of providing a tool to measure the actual hazard posed by a biological attack or a system’s ability to detect that hazard. ACPLA focuses attention on a generic characteristic (quantity of agent containing particles) that cannot be related, even relatively, to health hazard. Instead, it would be more useful to adopt a framework of measurement that makes it possible to evaluate relative hazard by including agent identity and activity, particle size and infectious dose. The new measurement framework would be more complicated than ACPLA. Not all of the information needed to compare the health hazard of different agents is readily available. Even with imperfect knowledge, however, the new framework could be implemented with current technology. More importantly, implementing the new framework would serve to focus future development efforts on detectors that measure parameters relevant to health risk in both military and civilian settings.
At the request of PD TESS, the National Research Council was asked to evaluate current units of measure for biological aerosols and, if necessary, determine a standard unit of measure that can be used for biological material independent of the state of the material (aerosol or aerosol resuspended in liquid) and independent of agent type (bacteria, viruses, or toxins).
The committee addressed the following questions:
Is there a single unit of measure that is appropriate for use in the evaluation of aerosol detectors?