extended targets such as military bases, but more work is needed to better understand just how such systems would be used (i.e., the concept of operations). Provided that robust concepts of operations can be formulated, the development of a hybrid infrared-ultraviolet (IR/UV) standoff biodetection capability should be expedited.

In addition to these nonspecific detection techniques, the Department of Defense has also recently begun to investigate advanced standoff techniques—ranging from ultraviolet resonant Raman scattering to passive infrared detection—in the hope of providing either longer ranges or more specific identification. These are still in the early research stage. More convincing laboratory data to support modeling projection of detection ranges and ability to discriminate against expected backgrounds is needed before considering any acceleration of these efforts.

Spectroscopic Point Detectors

Spectroscopic point detectors typically measure some properties of the suspended aerosol at the detector itself rather than at some standoff distance. Some of the simplest spectroscopic sensors use particle counting with size discrimination to detect a sudden increase in aerosol concentration in the sizes of interest. Although this information is accurate and rapid, normal fluctuations in particles within the 1- to 30-micrometer diameter range can result in an unacceptable level of false alarms. Also, because an aerosolized biological agent can produce morbidity and mortality in exposed personnel even when its concentration is very low (even lower than that of nonpathogenic microbes), nonspecific spectroscopic point detectors cannot protect personnel against such low-level attacks.

A more capable spectroscopic point detector uses an ultraviolet laser to excite the tryptophan, reduced nicotinamide adenine dinucleotide (NADH), and flavin fluorescences that are characteristic of biological materials and uses the time rate of change of the signal to differentiate a rapid biowarfare agent release from the more gradual fluctuations of the natural background. The false alarm rates of these bioaerosol detectors are a function of the detection threshold and the ambient bioaerosol background. When operated outdoors and at modest sensitivities (tens of ACPLA) they currently exhibit false alarms rates of between one and tens per day. Their false alarm rates in filtered indoor environments and for the 102 to 104 ACPLA levels discussed above may be dramatically lower. This would depend on the size and nature of fluctuations in the level of nonpathogenic microbes as a result of human or other activity in the indoor environment. Research on the inclusion and exploitation of additional spectroscopic signature information should reduce this false alarm rate even further. Therefore, the committee considers bioaerosol detectors to be the most promising near-term candidates for the 1-minute, nonspecific detection portion of the system.

Specific Detection and Identification Technologies

Four potential methods of identification are discussed that can provide specific information on what type of biological organism or toxin may be present in a sampled bioaerosol cloud: nucleic acid sequence-based detection; molecular recognition of identifying structures on the surface of the organism or toxin; unique chemical attributes of the organism or toxin; and biological responses to the organism or toxin. The committee's conclusions regarding these identification approaches are summarized below.

Sequence-Based Detection

Sequence-based detection uses the genetic information contained in a pathogen's DNA or RNA to detect and identify the pathogen. In laboratory studies, such techniques have shown the best sensitivities (detecting as few as 5 cells) and very low false alarm rates (about 10-5 for a single signature and lower for multiple signatures). Typical analysis times range from 15 to 60 minutes. The key issue for detect-to-warn applications is the extent to which this sensitivity and specificity must be sacrificed in moving to the demanding detect-to-warn time lines of less than 5 minutes and preferably of 1 minute or so. The



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