the use of technology in preparing subway systems for

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ABSTRACT

Recent events have raised concern among public transitagencies regarding the potential for chemical or biologicalterrorism. In particular, the sarin attack in the Tokyo subwayin 1995 revealed that a significant number of casualties couldresult from even a small release of a chemical agent, due tothe high density of people in the subway and the spread ofagent by the movement of trains. In an attack, people arelikely to be impacted in stations, in trains, and above theground. The Department of Energy PROTECTS Program(Program for Response Options and TechnologyEnhancements for Chemical/Biological Terrorism inSubways) is aimed at developing and applying technologiesthat can save lives in a chemical or biological incident in asubway. The program is pursuing a systems approach to theproblem, including consideration of detection, modelsimulation for transport and fate, communications, decisionsupport systems, decontamination, training tools, andexercise and response planning.

This paper describes technologies that can be put intoplace in a subway system in an attempt to save thousands oflives in an incident. Detection technologies are relativelyimmature, and will therefore require testing and possiblyfurther development work before they can drive automatedsystem responses. System reliability may be enhancedthrough the use of support systems such as closed-circuitTV and redundancy of technologies. Such system conceptswill be evaluated and demonstrated in a subway environmentin the next 2-3 years by the Department of Energylaboratories.

Current incident response recommendations involve“containing” the chemical or biological agent rather than“venting” it. A release in a subway system is carried belowground by train movement, any operating fans, and themovement of people. The time to action after an agent isreleased is the key to saving lives and determines whichtechnologies are best suited for mitigating the impacts ofan agent release. The development of more specificrecommendations awaits improved modeling techniques, and

the exercise of such models for a wide range of attackscenarios.

INTRODUCTION

The network of a subway system, with its tunnels,moving trains and ventilation shafts, can distribute achemical or biological (C/B) agent throughout manystations and tunnels below ground, and up throughventilation shafts and station egresses above ground to anentire city. In fact, the release of a biological agent in asubway could lead to the exposure of more than 100,000people, counting those in the subway and those in the cityabove. If no rapid and inexpensive biological agent detectioncapability is available, as may remain the case for the nextfew years, it could be 48 hours or longer before anyonewould be aware that a biological incident had occurred.Hospitals would likely be the first to see the evidence of anattack. Figure 1 illustrates a similar problem with photographstaken during the Tokyo subway sarin incident, in which 12people died and 5,500 people sought medical attention.

The terrorist threat to U.S. subway systems comes bothfrom rogue nations that can pass weapons of massdestruction to terrorists for use inside the United States,and from homegrown terrorists who have their own politicalagendas. Although U.S. intelligence sources can track andlimit the activities of known terrorist groups; they areunlikely to be aware of all potential terrorist activities inthe United States. Intelligence, law enforcement, andemergency management communities face a unique challengein addressing the terrorist threat considering the ease inaccessing C/B agent “recipes” on the Internet and therelative ease in constructing some weaponized agents.

This paper briefly describes PROTECTS (Program forResponse Options and Technology Enhancements forChemical/ Biological Terrorism in Subways), the U.S.Department of Energy (DOE) initiative that represents anintegrated approach toward dealing with such incidents.PROTECTS covers preplanning as well as emergencyresponse during an event in a subway system. The current

The Use Of Technology In Preparing Subway Systems ForChemical/Biological Terrorism

Dr. Anthony J. PolicastroArgonne National Laboratory

Argonne, IL

Dr. Susanna P. GordonSandia National Laboratories

Livermore, CA

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PROTECTS team is led by Argonne National Laboratoryand Sandia National Laboratories for DOE.

PROTECTS is developing new technologies foremergency planning before incidents occur and advancedemergency management tools for emergency response.PROTECTS will illustrate the use of these technologies ina demonstration program at a U.S. subway system inapproximately 2-3 years.

UNDERSTANDING THE PROBLEM

A terrorist can attack a subway system in three generalways with C/B weapons:

1. release in a station,2. release in a train car, and3. release in a tunnel, perhaps from a grating at street

level down though a ventilation shaft.

Spreading of the C/B plume is driven by moving trainsthat disperse the agent throughout stations and into thetunnels to subsequent stations. In any of these releases,time is lost before system-wide action can take place, andduring that time the agent can spread and impact newvictims. Response time includes the time before theincident is reported, the time when decision makers at theOperations Control Center (OCC) determine what to do,and the time when OCC implements its emergencymanagement decision. The addition of detectors in thesubway system and use of closed-circuit TV (CCTV) withdecision analysis software in the OCC could significantlyreduce the length of this lost time in the future.

There are many possible terrorist attack scenarios. Aterrorist may release either chemical or biological agents.Chemical agents include nerve, blood, choking, blister, andincapacitating agents. Biological agents are even more

Track 5 - Safety and Security Response to Critical Incidents and other Emergencies

Figure 1. Problem of a Chemical Attack on a Subway System

The Problem:U.S. Subway Systems are Currently Unprepared forChemical or Biological Releases

Tokyo Sarin IncidentMarch 1995

Detection and Supporting Technologies Shorten System-wide Response Time and Saves Lives

PROTECTS(Program for Response Options and Technology Enhancements for

Chemical/Biological Terrorism in Subways)

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diverse. C/B agents may be released by evaporation, withan aerosolizer, or with an explosive, and they may bereleased from a single location or multiple locations.

Two chemical attacks that occurred help illustratelikely attack scenarios. In March 1995, the famous chemicalattack by Aum Shinrikyo on the Tokyo subway involved theevaporative release of sarin gas in multiple trainsconverging on a single target station. In May 1995, anattempted cyanide gas attack was narrowly averted bysubway guards. A small fire was used to create and dispersehydrogen cyanide gas from a restroom that ventilated to astation platform [1]. These two attacks demonstrate thediversity of possible attack scenarios, since they involvedifferent gases, release mechanisms, release locations, andsingle versus multiple releases.

To develop a holistic solution to the problem, a numberof issues need to be evaluated, as shown in Figure 2. Theyinclude assessment of vulnerabilities, detection, crisismanagement, modeling and simulation to determineresponse strategies, control options, and after-eventdecontamination. These issues are categorized into fourmain thrust areas for PROTECTS, also depicted inFigure 2.

To date, much of the work on PROTECTS has been inthe areas of effects modeling and engineered responses.

Model simulations made as of March 1999 with an ANL-developed C/B post-processor to the well-known SubwayEnvironment Simulation (SES) Model have been supportedby some GASFLOW 3-D Computational Fluid Dynamics(CFD) Model predictions. Those results have indicated thatthe following factors are important:

Piston Effect

After an agent release has started, the piston effectcaused by the movement of trains spreads an agent intotunnels and neighboring stations and up ventilation or fanshafts to street level.

Train Car Effect

The operating heating, ventilating, and air conditioning(HVAC) system in a train car can entrain C/B agent intothat train car from outside leading to exposure of thepatrons riding the train. Continued operation of the HVACsystem can then remove the agent over time, moving it intothe tunnels and stations as the train car operates. Trainsopening their doors at a station and people exiting andentering the train cars lead to additional exchanges of theagent with subway station air.


Figure 2. Major Thrust Areas for PROTECTS

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Effect of Stopping Trains

Shutting trains down does not immediately stop thespread of an agent since the inertia of the air generated bytrain movement does not halt immediately when trains arestopped but rather continues until it is dissipated by friction.This additional air movement can spread 300 feet or morebeyond the stopping point of the trains. The additional timelag and additional distance for dispersion must beconsidered in implementing emergency response actionsand recognizing which patrons are at risk. Also, it is notlikley that subway air motion stops completely when trainshalt due to wind blowing into stations, elevation differences,and temperature gradients. More research is needed here.

Venting Versus Containing

Generally, turning fans on to ventilate the agent to thestreet and dilute agent concentrations is not a good ideabased on current simulation modeling results. Someoperational staff at transit authorities have recommendedthat the emergency ventilation fans (used for fire and smoke)be turned on to rapidly vacate the subway of the agent,thereby allowing the system to be decontaminated andrecovered to revenue service as quickly as possible. In fact,venting may be useful for less lethal agents; lives may besaved when fresh air and oxygen are added to the cloud.However, venting is not wise for nerve agents; even verylow concentrations may injure or kill many more people atstreet level than in the subway. Additional model simulationswill determine if the above findings are true in allcircumstances.

Venting of a biological agent could cause even moredisastrous consequences. Biological agent plumes emittedfrom stations and tunnels through vent shafts to street levelmay travel 7 to 10 kilometers or more above ground withdeadly impact, depending on the agent, amount released,and train movement. Agents emitted from subway vent shaftsand station egresses can also enter nearby buildings andtheir HVAC systems, and impact people in those buildingsas well. The choice to vent or contain a C/B release shouldbe based on site-specific computer model runs. Thus, withoutknowing the identity of an agent, a default policy ofcontaining the release and preventing its spread appears tobe better in terms of lives saved than a policy of venting therelease to street level.

It may be one year or more before chemical sensorsare tested and validated for subway use and installed insubway systems. A 3 to 5 year period is likely for chemicalsensors that can detect many types of agents and forbiological agent detectors. Until such products are available

and proven, a general policy of containment appears to bebetter than a policy of ventilation. As noted above, runningadditional computer simulations to compare such optionsshould be done to determine how widely thisrecommendation is likely to apply.

Single Versus Multi-Track Tunnels

Trains can move 50 to 60 miles per hour in a tunnel;however, agent-filled air moves, at most, only about 30miles per hour. A series of trains can pass through an agentcloud as it travels in the direction of train movement in asingle-track tunnel. Net air movement is slower indouble-track tunnels than in single-track tunnels becausethe air shifts in direction as trains move in oppositedirections. Air movement in four-track tunnels reveals avery small piston effect; air is mainly stirred up each timea train passes. This small piston effect is due to the factthat a train’s cross-section may fill only about 20% of thetunnel’s cross-section. Each track section needs to beevaluated separately to be certain of the optimal responsestrategy for that section.

Response Time Is Critical

Reducing the amount of time it takes to implement asystem-wide response is critical to saving lives. The fasterthe emergency response, the more lives can be saved. Figure1 shows the results of computer modeling that estimatesthe number of lives that could be saved as a result of asystem-wide action, in this case stopping trains and shuttingall fans for both a chemical and biological agent. For thebiological release, no action refers to the current situationof no detection, and the future situation refers to detectioncapability leading to responses in 30, 15, and 6 minutes.For the chemical agent release, no action refers to thecontinued operation of trains (benchmark case) ascompared to stopping trains in 30, 15, and 6 minutes.

THE PROTECTS TECHNOLOGYSOLUTION

Detection may provide a key technology solution inpreparing subway systems for C/B attacks. The best use ofdetection technologies will vary among subway systemsand will likely change as new detection technologiesbecome available. A typical example of a detection systemis shown in Figure 3. Chemical or biological detectorsreport to the OCC and to the station manager in the kiosk.When an alarm sounds at the OCC, personnel can confirmfor example, that an incident is occurring, and that it is not


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a false alarm, by inspecting closed-circuit TV images ofthe affected area. Once the incident is confirmed, automatedresponse strategies, developed through pre-planning, basedupon modeling are implemented. On the basis of whichdetectors report agent concentrations and which agent isidentified, an estimate of the spread of the plume in the systemas well as emissions amd plume spread to the street will beperformed and made available to OCC personnel.

There are other supporting technologies to be evaluatedthat could be used to save lives in such an incident. Theircost-effective use in terms of lives saved as compared tocost is yet to be evaluated. Among the technologies are:

a. an inflatable barrier that can block the spread ofagent to the remainder of the subway system;

b. station mitigation methods such as water curtains,air curtains, and water or foam sprays;

c. first responder and incident commander supporttools; and

d. training and exercises.Training materials could be in the form of virtual reality

simulation tools, including the visualization of modelpredictions during a hypothetical incident. Most subwaysystems have the infrastructure to permit the implementationof this plan for stations in terms of their communicationsystems, supporting software, and OCC hardwarecapabilities.

First responders might uncover an incident in locationsnot covered by detectors. First responders such as transitpolice would not likely have personal protection equipmentor portable detectors but they would probably become thefirst line of reporting to the OCC. The plan is that they wouldbe given a flashcard to carry with them, reminding them ofactions to take. The flashcards would be backed up by amore detailed manual and supported through training andexercises. Firemen and hazmat (hazardous material) teamsresponding to the incident would be armed with a hand-helddevice that could receive the same information as thatavailable in the OCC. The information would includeprediction of the hazard zone below ground, prediction ofthe hazard zone above ground, and CCTV images of thestation under attack broadcast from that station’s own CCTVhookup.

The following sections further explore key componentsof PROTECTS in the areas of C/B warning, decision supportand response.

C/B Warning and Response

In the event of a C/B terrorist attack on a subway, rapiddetection and response can play a critical role in savinglives and reducing casualties. As discussed above, the longerit takes to detect and respond to an attack, the more areleased C/B agent will spread through the system, harming


Figure 3. PROTECTS Plan for Emergency Response During a Chemical Agent Attack

Closed Circuit TV Cameras

Chem / Bio Detectors

Terrorist Releases Agenton Passenger Platform

First Responder:•Flashcard•Supporting Manual

All Subway Systems:• First Responder Guidance• Transit Authority Response Guidance• Training / Simulation Tools (SES, GASFLOW)

CB-ARACPrediction of Above-ground

ContaminationAgent Released inSubway Station

IncidentCommanderand OCC:• automatedresponse strategy• below-groundcontamination• above-groundcontamination• CCTV link

SES Prediction of Contamination in Subway System

Operations Control Center (OCC)

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more people and exacerbating decontamination. As trainsmove through the system, in the absence of platform edgedoors, air pushed by the trains spreads a released agentthroughout a station and beyond to neighboring stations. Adetection and warning system, together with a well-orchestrated response strategy, can significantly reduce theconsequences of an attack. As discussed earlier, ourpreliminary modeling results indicate that overall casualtiesare reduced by stopping train movement within minutes ofan attack thereby vastly reducing the rate of agent spread.

An effective warning and response system to handlesuch attacks must incorporate two important components.First, a detection system must be developed that employsavailable technologies and methodologies to rapidly detecta C/B attack on the subway system. Second, a response planmust be developed that will allow the subway employees atthe stations, on the trains, and at the OCC to minimize thenumber of casualties in an attack without causing risk tothemselves in the process. The response plan should includethe use of available mitigating technologies to minimizethe impacts of an attack.

Detection

A detection system that monitors for signs of a C/Battack must include technologies and procedures designedto detect and confirm that an attack is taking place within ashort time of when the agent was released and with a low orno false alarm rate. We are currently evaluating the utility ofsensing, artificial intelligence, and video technologies forsuch a detection system. Thus, detection does not includeonly chemical and biological agent sensors but alsoincorporates other technologies and procedures that mayhelp to rapidly identify and confirm an attack. For example,artificial intelligence algorithms may be developed that arecapable of recognizing patterns of sound or motion in astation that are characteristic of a panic situation [2,3]. Suchalgorithms could cue the OCC that an attack is suspected,shaving precious minutes off the emergency response time.

More classical detection methodologies would employchemical sensors that alarm in the presence of chemicalagents. We are investigating the use of sensors, such asIMS, SAW, and FTIR sensors, for this purpose. The dreamdetector for detection and warning would be small, cheap,sensitive (sensing small concentrations), selective(distinguishing between agents), reliable, and autonomous.It would have a fast response time and a low false alarmrate. It would detect many types of agents, require littlemaintenance, and use no consumables. Although the sensorsavailable today are not yet capable of meeting all of these

requirements, detection of some chemicals may be possiblewith small, autonomous sensors. PROTECTS is currentlyinvestigating how best to apply the technology availabletoday to enhance subway security.

Whether an attack is initially detected by chemicalsensors, artificial intelligence algorithms, or throughsimple word of mouth from individuals at the suspect site,a methodology must be developed that will allow the OCCto confirm whether an attack is, in fact, occurring and tocharacterize the attack. In the event of a C/B attack, anytrains in the area should be stopped by the OCC or sloweddown to minimize air movement at the attack location. Sincethis is often considered a high consequence response, it iscritical to prevent frequent false alarms. One obviousmethodology to confirm attacks by fast-acting chemicalagents such as nerve gas is to enable the OCC to view stationplatforms by video. When an attack is suspected in an area,the OCC can immediately view that area to look for thesigns of an attack, and mitigating responses can beimplemented if the event is confirmed. This detectionmethodology would prevent initiating high consequenceresponses when no real attack has occurred.

Consider the following scenario which could indicate apossible nerve agent release on a subway platform, similarto the attack in Tokyo. Several people run out of a station,coughing and telling the station attendant that people arecollapsing down on the platform. How could the systemrespond to this? If an actual nerve gas attack were underway, people on the platform would indeed be affected, andit would be clear to a trained observer that a chemicalrelease had occurred. However, in the absence of a videoview of the station platform from a remote location suchas the OCC, there would be no safe method to quicklyinvestigate the situation. Since a gas cloud of this type isnot visible, it would not be constructive (nor would it besafe) to require an employee to enter the area for aninspection. Anyone entering the vicinity of such an attackwithout proper protective equipment would likely become acasualty. Likewise, if video views of the platform were onlyavailable from within the station itself, such as a stationkiosk, the employee monitoring those video screens wouldbe at risk in the event of a gas attack. Everyone within thestation should immediately be evacuated, since dangerousgases can spread throughout a station within minutes of arelease. Response procedures preferably should not requireemployees to enter or remain within a station where an attackis suspected. Thus, having CCTV images available at theOCC would be invaluable during a suspected attack, allowingfor evaluation of the situation from a safe location.


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It is important to note that the detection technologiesand methodologies mentioned here are still to be provenin the context of protecting subways from C/B attacks.Chemical sensors that are commercially available todayhave been developed for military applications, which havesomewhat different sensor requirements than subwayprotection applications. Important issues such as sensorreliability and maintainability in the subway environmentare under investigation by several agencies. Automaticpattern recognition of attack events from video or audiosignals is a promising concept but requires somedevelopment. For any detection method, the potential offalse alarms is an area of great concern, so combinationsof multiple detection methods are under consideration forimproved system reliability.

Response

Once a chemical attack has been detected in a subway,two decisions must be made immediately: how to controltrains in the area and whether to turn on the emergencyventilation fans. Based on preliminary model simulations,trains running near the area of the attack should be stoppedto reduce air movement and contain the C/B agent near therelease location. The trains should then be moved at a crawlto the nearest safe station to allow passengers to evacuate.If a train is in a tunnel approaching the affected station, itcould be reverse-railed back (if possible) to the previousstation for evacuation. It is far less clear how to avoidcontamination of trains that are either entering the affectedstation or in it. If ventilation of the trains can be controlled,then the HVAC system should certainly be shut down.Likewise, control strategies for releases that occur on-boardtrains are also more difficult. We have developed responseguidance for the various situations that may arise and arecontinuing work to refine the guidance.

The control of ventilation fans after detection of a C/B attack can also involve some complex decisions. Preliminarycomputer simulations indicate that it would be unwise touse ventilation after the release of a very toxic biologicalagent. In most cases, ventilation would not significantlyreduce the number of casualties within a station but wouldspread the contamination outside the station and therebycause more casualties. Likewise, in the event of a large lethalchemical release in a station, ventilation might causecasualties on the street level without significantly helpingthose within the station. In both of these cases, containingthe agent within the release location would probably be thebest option. On the other hand, for a small chemical agentrelease, such as in the Tokyo attack, ventilation might be an


attractive choice. If the release is sufficiently small, thenventilation fans might be able to dilute the gas enough toreduce casualties at the release location without riskingsignificant contamination outside. Likewise, if anincapacitating agent such as pepper spray were released ina station, the ventilation fans might be employed to drawfresh air toward the egress routes and thereby aid a safeevacuation. Thus, optimal decisions regarding ventilationcontrol require an evaluation of the subway system andtype and quantity of released gas. On the basis of thesimulations that were conducted to date, it is recommendthat the more conservative containment strategy initially beemployed in all cases, and that ventilation fans be turned ononly after the situation has been characterized well enoughto justify their use. These alternatives are now beinginvestigated using simulation models to determine the beststrategy under various scenarios.

Additional response options available on some systemstoday include the control of escalators and fare gates. Toreduce the number of people entering a station after anattack and allow for an efficient station evacuation, stationescalators should all be either switched to the outgoingtravel direction or shut down. In addition, fare gates shouldbe switched to the open position if possible to allow forrapid egress. These methods may be employed to aid stationsecurity staff in conducting a rapid and orderly evacuation.

Engineered Response [2,4]

In addition to applying the best available responseoptions, additional engineering options such as those inFigure 4, may be applied to further mitigate attackconsequences by containing and detoxifying contaminatedair. Air curtains, aqueous foam, fume burners, high-temperature catalytic converters, wet scrubbers, carbonbeds, and water sprays have been considered for thispurpose. Initial investigations indicate that it may bepossible to deploy a specialized sprinkler system withmodified sprinkler heads to remove agents from the air duringan attack. Standard water sprinkler systems would probablynot be very effective for agent removal but instead makeevacuation even more problematic. The effectiveness of evena modified sprinkler system has not yet been proven andrequires further investigation.

A more effective engineered response option forprotecting enclosed stations is presented by a combinationof platform edge doors (PEDs), canopy hoods, and activatedcarbon beds. PEDs prevent air movement in the stationscaused by trains and could thereby significantly reduce thespread of a released agent. In many subway systems today,

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ventilation is primarily achieved through train motion-induced air movement. Any C/B agent released in a stationwould be spread rapidly by this air movement. PEDs, whichisolate the air pushed by the trains from the stations, wouldsignificantly reduce the spread of an agent. PEDs alsoprovide safety and security benefits, preventingunauthorized access to the track area, avoiding unsafeconditions on crowded platforms, and maintaining a cleanerenvironment. Installing PEDs is a more appropriate solutionwhen building new subway stations than when retrofittingolder ones because of cost considerations.

To further contain an agent within a release location attrain or platform level, deployable canopy hoods could beinstalled. They could duct the contaminated air through anexhaust manifold to a detoxification system consisting ofactivated carbon beds. If an attack were detected before itspread significantly through the station (which is more likelyin a system equipped with PEDs), canopy hoods could bedeployed above the release to prevent further agent spreadand to decontaminate the area.

Maintaining fresh air in the station egress routes isalso an important mitigation objective but will probably

require a special engineering design for this purpose. Fullstation venting during a C/B attack may not be possible ordesirable, as discussed above. In any case, venting from thestation would not be very effective in keeping the egressroute clear, given the toxicity of many C/B agents. Instead,the escape route could be enclosed and equipped withauxiliary ventilation fans, which would isolate the air fromthe rest of the station and allow for separate ventilationand decontamination.

Decision Support Systems

No matter what detection and response options areavailable in the event of a C/B attack, rapid response isrequired to minimize casualties. Developing responseprocedures and training employees often can greatly helpin achieving this goal. In addition, computer-based decisionsupport tools can be developed to help decision-makers atthe OCC and the Incident Commander rapidly follow theresponse procedures based on the best availableunderstanding of the situation. For example, decisionsupport tools to aid in responding to smoke/fire events areavailable today. On the basis of the fire’s location and train

First Responder Emergency Response PC Tools:• Communication with OCC• Chem / Bio Symptomology Software• Virtual Visualization of Stations / Tunnels

Post-Event Decontamination

Emergency Planning and Exercises

Equipment to Stop Tunnel Air Flow(e.g. Inflatable Barrier)

Equipment to Reduce Station Impacts(e.g. Sprinklers / Water Curtains)

Figure 4. Sample Advanced Technology Components

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locations, and by using tabulated results generated fromsimulated fires, computer calculations can lead torecommended optimal ventilation control strategies. Thesestrategies can be implemented with a single confirmationby an OCC operator. This method clearly improves theresponse time over that of a manual system in which theoperator must choose and implement a ventilation strategy“on the fly.” Similar decision support tools will bedeveloped for response to C/B attacks using whateverresources and information are available more quickly andefficiently. The OCC and Incident Commander would sharethe same information on their separate computers. Figures3 and 4 illustrates technology tools being developed byPROTECTS for coordinated system-wide chemical agentemergency response.

CONCLUSIONS

The preparation of subway systems to deal withterrorist chemical or biological agent attacks is animportant and difficult problem. Early warning, rapidresponse, and engineered mitigation methods can save manylives in such an incident. A well-prepared system cansignificantly reduce the impacts of an attack and maydiscourage such attacks from taking place. The U.S.Department of Energy PROTECTS program is working withsubway systems to develop and implement technologicalsolutions for improved preplanning and for more effectiveresponses to such terrorist incidents.

ACKNOWLEDGEMENTS

The submitted manuscript has been authored bycontractors of the United States Government, SandiaNational Laboratory under Contract DE-AC04-94-AL85000and Argonne National Laboratory under Contract W-31-109-Eng-38. Accordingly, the United States Governmentretains a non-exclusive, royalty-free license to publish orreproduce the published form of this contribution, or allowothers to do so, for United States Government purposes.

REFERENCES

1. R. Purver, “Chemical and Biological Terrorism: TheThreat According to the Open Literature,”HYPERLINK http://www.csis-scrs.gc.ca/eng/miscdocs/tabintre.html, Canadian SecurityIntelligence Service, 1995.

2. W. A. Swansiger and J. E. Brockmann, “Mitigationof Chemical Attacks in Enclosed PublicTransportation Facilities,” Enforcement andSecurity Technologies Proceedings, SPIE Vol.3575, 1998.

3. W.A. Swansiger of Sandia National Laboratories,Livermore California, personal communication toS. Gordon, 1999.

4. W. A. Swansiger, “Defending Subways againstChem-Bio Terrorism,” SAND98-8210, SandiaNational Laboratories, 1997.

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