as well as physical disruption by thermals or irregular air currents found in most large installations. At the right time of the day, more humans will be more vulnerable to a respirable aerosol threat in an enclosed space than outside, and indoor "weather" will almost always favor the attacker. Fortunately, however, enclosed spaces can be more easily provided with collective artificial protection than open spaces.

In view of the comparative simplicity and advantages to the attacker in the indoor scenario, as well as the richness of potential defensive responses, the committee focused most of its attention on detect-to-warn technologies for this scenario. However, it also considered the applicability of these technologies to the outdoor release scenario.

The committee found that Task 4 was the most challenging. This is because most of the technologies discussed in this report are not yet mature enough to support a discussion of specific materials, system design, or manufacturing issues. However, for the more mature technologies discussed, the committee attempted to highlight the key enabling technologies that will facilitate their deployment as effective detect-to-warn sensor systems.

Looking beyond the tasks assigned by DTRA, the committee also notes that the overall system to counter potential biological agent attacks must balance the advantages and limitations of detect-to-warn systems with other alternatives for protecting people. The best overall biodefense architecture will likely be a system-of-systems that includes not only detection systems but vaccines, therapeutics, collective protection, and other means of protecting personnel in facilities and installations. The analysis and balancing of these major defensive components is as essential as the pursuit of the promising detection-based architectures addressed in this report.

CONCLUDING THOUGHTS

Although the United States could some day be faced with such "designer" agents as bacteria containing genes coded for new virulence factors, viruses designed to target cell populations that they ignore in nature, or small, bioactive natural molecules not currently on any threat list, many of the principles discussed in this report will still apply 50 years from now. Infection and effective doses will remain the same. Most biological threats will be neither volatile nor dermally active; therefore, unless the agents are easily spread from person to person, the attacker will most often find it necessary to disseminate them in particles small enough to hang suspended in the air long enough to be inhaled by the intended victim in quantities large enough to cause disease.

For outdoor releases, weaponeers may achieve breakthroughs in ultraviolet protection that will allow stabilization of bacteria, viruses, proteins, and peptides, but the sun's heat will always spawn thermals and winds that move particles along with them, lending uncertainty to the intended cloud trajectory. Resting humans will still ventilate to obtain oxygen and expire CO2 in volumes ranging from 5 to 10 liters per minute. The cost and effectiveness of the sensors, the configuration of the facilities they protect, and the threat itself will continue to evolve. The most stable variable in this complex equation may be the human body and the life we seek to protect.

In the future, it will be necessary to make hard decisions regarding the protection of humans from biological threat agents. The purpose of this study is to assist homeland defense officials, hardware and policy developers, and, specifically, program managers within the Department of Defense to understand key state-of-the-art technologies, barriers, and enablers that might lead one day to effective biosensor systems for protection of buildings and installations.



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