• Portable sensors to allow first responders to assay levels and types of hazard at a distance—that is, to allow them to make correct initial assessments without themselves becoming casualties (see Chapter 8);

  • Mobile sensors to be used in mapping the extent of a cloud of a volatile agent and to guide civil authorities in controlling population movements;

  • Sensors to assist physicians in determining the extent of exposure of patients presenting at hospitals (see Chapter 3); and

  • Sensors to assess the level of contamination following an attack and, more importantly, to determine when a site is safe and can be returned to normal function.

As can be seen from the above list, the use of sensors is not limited to the detection of chemical agents; the detection of biological agents and of fissile and radioactive materials is discussed in Chapters 2 and 3. Current sensor capabilities are fairly limited; in many cases, the best “technology” for practical use continues to be trained dogs. Manufactured sensors are often designed for use in specific environments and to be selective for only one or two chemicals. Yet because there is a spectrum of possible threats, sensor systems are needed that can detect a large number of possible chemicals. And, given the ultrahigh toxicity of some of these chemicals, detection systems’ sensitivities must be significantly increased. In addition, sensor systems will need a number of different subsystems, including sample collection and processing, presentation of the chemicals to the sensor, sensor arrays with molecular recognition, sophisticated signal processing, and amplification of the transduction events.

Sensor programs funded by the government have not yet produced significant increases in counterterrorism capabilities, in part because the focus has been on the sensor itself and not on the overall system for detecting threats. There is a strong need to focus on systems approaches here—to explicitly consider how the sensor system will be used, by whom, for what purpose, and at what cost. While the common goals for virtually all sensors are that they be less expensive, more versatile, more reliable, and more compact, each of the potential applications listed above will have a different set of most-desired characteristics for a sensor system, and development efforts should recognize what trade-offs (between, say, size and versatility) each application demands (see Chapter 11).

One example of a factor that needs to be considered in sensor development is the relevant time scale. Chemical agents have a broad range of times required for their toxic effects to appear. One of the most plausible types—nerve agents—acts rapidly; evidence of toxicity can appear in seconds to minutes, depending on concentrations, exposures, and agent. Similarly, many industrial chemicals (e.g., chlorine, hydrochloric acid) that might be used as improvised chemical weapons are immediately apparent through smell or effects on eyes or mucus membranes at concentrations well below that required for serious toxicity (but if escape from them is not possible, the resulting damage to lungs becomes evident over time).

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