Historically, most terrorist attacks on civilian targets have involved the use of firearms or explosives, and current defensive strategies are aimed at preventing attacks perpetrated by such means. However, the use of the nerve agent sarin in 1995 to attack the Tokyo subway system, the use of the U.S. mail in 2001 to distribute letters containing anthrax spores, and the discovery in 2004 of the biological toxin ricin in U.S. Senate Office Buildings in Washington, D.C., demonstrate that chemical and biological agents have been added to terrorists’ arsenals. Attacks involving chemical/biological agents are of great concern, not only because of the potential for mass casualties but also because there is no strategy or technology fielded today that can respond adequately to this threat. As the United States and other countries reassess the security measures they have in place to prevent or defend against such attacks (particularly in areas where large numbers of people gather and then widely disperse), the risks to the air transportation system as a primary target become clear.
While potential attacks on all modes of transportation are of concern, the Committee on Assessment of Security Technologies for Transportation believes that the U.S. air transportation system continues to have a high priority for counterterrorism resources, both because of its economic importance and because of the intensified public perception of risk following the September 11, 2001, attacks. The air transportation system can also serve as a testbed for the development of defensive technologies and strategies that can subsequently be applied to other transportation modes.
Finding 1: The U.S. air transportation system is an attractive target for attacks with chemical or biological weapons, yet no federal agency has been assigned clear responsibility for developing a strategy for defense against such attacks.
The large numbers of people gathered in air terminals—perpetually coming and going—provide anonymity to the terrorist, and the fact that most passengers carry luggage makes the detection of threat agents concealed in luggage more difficult. The rapid dispersal of passengers from air terminals to destinations around the world means that those who become infected with communicable diseases could spread the diseases widely in a short time, a situation that was demonstrated in 2003 in the case of the severe acute respiratory syndrome (SARS) virus. Finally, a chemical/biological attack on the U.S. air transportation system would raise the already high level of public anxiety about travel risks and would likely result in significant economic disruption.
Considerable research on defensive concepts against chemical/biological attacks is being funded by the Department of Homeland Security (DHS), the Department of Defense (DOD), the Department of Energy’s (DOE’s) national laboratories, the Technical Support Working Group (TSWG),1 the Centers for Disease Control and Prevention, and other agencies. The DHS has supported preliminary studies aimed at improving the defenses of airports against chemical/biological attacks, but the future funding and scope of these efforts remain in doubt. Within DHS, the Transportation Security Administration (TSA), which has the lead in defending the system against concealed weapons and explosives, has not been assigned responsibility for leading the defense against chemical/biological attack.
Recommendation 1: The Transportation Security Administration, with its responsibility for the federal oversight of security operations at U.S. airports, should integrate strategies for defense against chemical/biological attacks into its broader security plan for protecting the U.S. air transportation system. The line of authority and
accountability for implementing these strategies should be clearly defined.
Air transportation spaces range from very large spaces (e.g., terminals) to very small spaces (e.g., aircraft), with unique characteristics that will help shape defensive strategies for the respective areas. For instance, given the likely dissemination of threat agents through the air in a chemical/ biological attack, the air-handling systems in these spaces are likely to be a particularly important factor in the mitigation of the impact of any attack. Because of its ongoing work to address threats from explosives in air transportation environments, TSA is the agency most knowledgeable about the unique characteristics of these spaces; it is well positioned to extend this work to encompass chemical/biological threats.2
Finding 2: No specific strategies, approaches, or procedures have been developed to defend the U.S. air transportation system against chemical/biological attacks.
Plausible scenarios for terrorist chemical/biological attacks include point releases of plumes of threat agents in various locations in a terminal or on air transportation vehicles, or releases directly into the air-handling systems of these spaces. Many different chemicals or biological organisms might be used—each with its own physical, chemical, and biological properties, which would influence the dispersal of the agent and the exposure of potential victims. Attacks might involve fast-acting agents (generally chemicals), causing victims’ symptoms to appear within seconds to minutes, or slower-acting agents (generally biological toxins or microorganisms), exposure to which occurs rapidly, although symptoms may not appear until after a delay of hours to days. The latter include infectious as well as non-communicable viral and bacterial agents.
The DHS has funded preliminary studies to elaborate specific chemical/biological threat vectors, to increase understanding of airflows, and to demonstrate chemical/biological detection systems in several terminals and boarding areas in two airports (San Francisco International Airport and Albuquerque Airport). The DHS has also conducted an exercise in which the many airport decision makers (e.g., in areas of operations, security, fire control and prevention, public health and safety, and environmental issues) come together to try to formulate a coordinated response to a simulated chemical/biological attack. Such studies and exercises are valuable and should result in useful guidance that can help many airports to begin to think about their own response plans. However, the work thus far is preliminary and of limited scope, and TSA itself appears to have had little involvement. Although TSA does participate in interagency groups that address homeland security issues (e.g., the Technical Support Working Group), the specific requirements of air transportation systems have not been given a high priority, according to briefings to the committee by TSA personnel who have attended these meetings.
Recommendation 2: The Transportation Security Administration, in collaboration with other appropriate entities within the Department of Homeland Security,3 should create a high-level task force to perform the following functions:
Create a validated threat assessment document for air transportation spaces and keep it updated;
Take advantage of ongoing research aimed at the development of models of the airflow within aircraft, terminals, and so forth, based on empirical studies of specific facilities, and explore the dispersal of chemical/biological simulants under various release scenarios;
Create guidance to help air transportation facilities develop a threat defense strategy; and
Determine unique air transportation requirements for dealing with chemical/biological threats and coordinate closely with other agencies that are active in the chemical/biological threats area to ensure that these requirements are given visibility in their programs.
This defensive strategy should include elements such as the following: contingency plans for responding to scenarios involving the release of various threat agents, clearly defined areas of responsibility for key decision makers, training for first-responders, plans for the evacuation of potential victims of attacks, plans for the isolation of contaminated areas, strategies for the early treatment of exposed individuals, timely remediation of affected areas, and rapid restoration of flight operations to ensure minimal adverse impact to the air traffic system.
The scale of the response to a chemical/biological attack must be commensurate with the level of confidence that an attack has indeed taken place. This is particularly important for attacks involving slow-acting agents, in which a detector alarm may be the only indication that an attack has occurred. For cases in which the detector has a relatively high rate of false-positive alarms, there may be a range of “low-regret” responses that could provide some measure of protection without producing the degree of disruption that might be justified if the certainty of attack were higher. An example of a low-regret response might be choosing to shut down the heating, ventilation, and air conditioning (HVAC) system to reduce the potential spread of agent while the validity of an
alarm is being assessed, rather than immediately evacuating an air terminal. This response could be augmented, again without causing disruption to airport operations, by dynamically fast-acting HVAC pressure control in which air spaces adjacent to the potentially contaminated air space would be positively pressured to protect them from any hazardous agents.
Finding 3: Many alternative chemical/biological detection technologies are being investigated in university, industry, and government laboratories, and various military prototype systems have been developed; however, it is very difficult to independently evaluate all of the performance claims for these technologies.
A staggering number of papers are published each year in the literature on various candidate chemical/biological detection systems. Researchers and manufacturers make diverse claims of detection limits, sensitivity, false-alarm rates, and robustness for these systems. The committee believes that in many cases, researchers emphasize the strengths of their particular detection systems while minimizing or ignoring their flaws. This practice makes it virtually impossible to evaluate the likely performance of a detection system in real-world air transportation environments.
The committee received briefings on numerous research programs around the country aimed at developing various chemical/biological detection systems. Several of these show promise, including some evaluated by an earlier NRC panel4 and one (mass spectrometry) evaluated previously by this committee. However, each technology appears to require substantial development and verification testing before it could be deployed in an airport or other transportation space.
Recommendation 3: The Transportation Security Administration should keep abreast of ongoing research on chemical/biological detector technologies without starting an in-house research and development activity. Rather, it should seek to leverage the research programs of other agencies, and it should consider supporting a vendor-independent testing capability in order to verify performance claims made for chemical/biological detection systems.
The primary technology mission and expertise of TSA’s laboratories and personnel involve the detection of weapons and concealed explosives. The TSA does not have the resources or expertise to develop detection and identification systems for chemical/biological agents. However, TSA’s technology-monitoring effort might involve funding a third party to survey the spectrum of detection technologies in order to identify those that might be most appropriate for the air transportation environment.
The TSA should maintain close liaison with other agencies (e.g., DOD, DOE, and the TSWG) to leverage their chemical/biological research efforts and to ensure that air transportation requirements are given a high priority. The TSA should also consider supporting an independent body to develop test criteria and to conduct standard tests to evaluate the performance of chemical/biological detection systems and to verify the claims of prospective manufacturers. Such an independent testing body would benefit the ongoing research efforts of many government agencies.
Finding 4: Although the rapid detection of a chemical/ biological attack and identification of the agent used are worthwhile objectives, a defensive strategy that depends exclusively on a detection-system alarm before action is taken (i.e., employment of a “detect and react” strategy) has several serious limitations.
In an attack with fast-acting agents, the chemicals would reach the victims and begin producing symptoms in approximately the same amount of time that these same chemicals would take to reach and produce a response from a technology-based detector. Thus, the best “detector” of a fast-acting agent may be visual evidence that people are collapsing or behaving in unusual ways. Visual recognition of symptoms cannot be relied on to detect delayed-acting chemical agents or biotoxins (which may not produce symptoms for several hours), and here chemical-detection systems may offer more promise, provided their cost and performance are acceptable.
For the detection of slow-acting biological agents (which may not produce symptoms for several days), the system response time would depend on the frequency of sampling and analysis. The frequency of sampling and analysis would be determined by factors such as the cost of the assay, the frequency with which critical reagents need to be replaced, the robustness of the detector, and so on. The minimum response time would be determined by the time required to collect a sample, prepare it for analysis, conduct the assay, and report the results. In the event of an alarm from a detector with a significant false-alarm rate, additional time would be required to determine its validity and to decide on an appropriate response.
The lengthy response time of such a “detect and react” approach might make it impractical for mitigating the immediate impacts of slow-acting agent attack in the air transportation environment (where passenger residence times are about 1 hour). It could, however, have benefits such as enabling subsequent notification of passengers regarding possible exposures, facilitating forensic investigations, and so on.
Recommendation 4: Given the limitations of sensor- and assay-based chemical/biological agent detection and identification technologies, the Transportation Security Administration should pursue a baseline defensive strategy against chemical/biological attacks that does not depend solely on the technological detection of threat agents to initiate action. Such a strategy would be based on elements such as the following:
Protective and preventative steps and enhanced security;
Improved visual surveillance of air transportation spaces;
The establishment of a separate air supply for spaces that have a critical function (e.g., cockpits, flight-control towers, emergency-response centers); and
Continuous air treatment to neutralize and/or remove agents or contaminants.
It is the judgment of this committee that the very large number of candidate sensor-based and assay-based detection systems have received a great deal of attention and research money but that none currently has the effectiveness, technical maturity, reliability, sensitivity, and selectivity to many different agents, nor the low cost, needed for deployment in the air transportation environment. In contrast, a variety of existing technologies that do not involve the detection of an agent could prevent or mitigate the consequences of chemical/biological attacks; such technologies are available today and would be arguably less costly to deploy. These “non-technological-detector-based” defensive measures (e.g., air cleaning, better security, better control of airflows) have not received the attention and analysis that the committee believes they deserve. Generally, the committee believes that technologies associated with non-detection-based strategies are nearer term, whereas technologies associated with detection-based strategies (with the exception of video-camera surveillance) are longer term or more speculative. Appropriate protective steps might include ensuring that the air-handling systems inside a terminal are balanced to reduce airflow between regions that could spread chemical and biological agents more rapidly. Enhanced security would include limiting physical access to the intake of air-handling systems—both for terminals and for aircraft on the ground.
To combat attacks with fast-acting agents in the terminals, continuous visual surveillance of densely populated areas and observation of behavior patterns may be as useful as any detector. The TSA should study the feasibility of the widespread deployment of surveillance cameras in populated areas, coupled with behavioral-pattern-recognition software, as an alternative to chemical agent detectors. Such cameras could also provide a dual-use value in improving the overall security environment. In addition, many critical nodes in the air transportation system (control rooms, emergency-response centers, and so on) are supplied with air that is recirculated from publicly accessible areas; this makes them vulnerable to being disabled by the release of chemical/biological agents in these public areas. Thus, it may be prudent to ensure that these critical nodes have an independent air supply and are kept at a positive pressure with respect to surrounding areas.
To combat attacks with slow-acting agents, TSA should study the feasibility of promoting the use of “clean air” systems that would continuously treat the air to remove respirable biological particles and chemicals, both in terminals and in transportation vehicles. This approach might involve a combination of technologies including improved air filtration, ultraviolet irradiation of filters, and/or passing the air through plasma cleaners or other treatment devices. A feasibility study would include the costs (both first cost and maintenance/replacement cost), number and optimum placement of air-cleaning units required, their effectiveness in removing threat agents from the air under various release scenarios, the number of likely exposures prevented, and so on. This defensive strategy would not prevent the exposure of people in the immediate vicinity of a point biological or chemical agent release, but it would limit the exposure of people in surrounding areas resulting from recirculation of the agent through the HVAC system. The provision of “clean air” to passengers would be analogous to municipal water treatment systems that provide clean drinking water to city residents, and it could have the ancillary benefit of reducing the spread of common ills such as cold and flu viruses. Coordination with related programs in other agencies, such as the Immune Building Program of the Defense Advanced Research Projects Agency, should be encouraged.
In conclusion, it appears that, given the need to maintain convenient public access to an efficient air transportation system, a terrorist attack on the system with chemical/biological agents would be difficult to prevent and would likely result in a significant number of casualties. Because there are a very large number of possible chemical and biological agents that might be used in a future terrorist attack and because the specific type of agent to be used would not likely be known in advance, the development and deployment of chemical sensors and bioassays for arbitrarily selected specific agents offer little real protection. By contrast, the deployment of video monitors and/or of biology-based “functional” detectors (analogous to the canary in the mine) that indicate the effects of any fast-acting toxic chemical agents would be beneficial in some attack scenarios. These systems could be deployed in a complementary way with non-detection-based defensive strategies. Thus, preventing or mitigating the overall impact of chemical/ biological attacks may depend less on the development of technologies for the detection of threat agents than on prudent protective measures that can be implemented before such an attack ever takes place. The TSA should explore the feasibility of these options and should help local authorities and facilities develop contingency plans for responding to chemical/biological attacks on the U.S. air transportation system.