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Responding to Chemical Threats: A Case Study

Article, 13 November 2007
Domestic Security, Technology
The possible options for the use of chemical weapons by terrorists are both many and varied. This article looks at one specific scenario - the discovery of an explosively configured device located in an urban area.

Should an explosively configured device be found in an urban area, one of two situations will arise: either the emergency response teams will arrive before the device functions or they will arrive after the device has functioned. Each of these situations will require a different response and will face the emergency teams with a different set of priorities.

The Device has Not Functioned

In this situation the first priority must be, where possible, to prevent the device from functioning. The decision on whether such an action is feasible will be dependent on the initial assessment of the explosives experts (EOD). A second priority will be to try and contain or limit any release should the device function. A key factor here will be the location of the suspect device. If it is located within a building, for example, it may be possible to seal the room or building containing it. Simple procedures such as sealing windows, air vents and doors with adhesive tape can be of considerable help in limiting any release, should the device subsequently function. In situations where the device is located in the open air the use of foam blankets generated by the Fire Services, for example, can do much to limit the spread of contamination. Even a simple water curtain, strategically placed, may significantly help to reduce the level of downwind contamination. The success of any containment measures will of course depend to an extent on the amount of explosives present in the device.


It is assumed that, in accordance with standard practice, the area in the immediate vicinity of the suspect device will have been evacuated as soon as it was discovered. In situations where the device is suspected to contain a chemical agent, however, a subsequent decision will need to be made on the size and location of any additional areas that will require evacuation.

In the best-case scenario functioning of the device may result in contamination being limited to an area similar to that covered by the assessed explosive risk. Alternatively, in a worst-case scenario, the downwind plume of contamination may spread for many kilometres. In reality, the resulting area of contamination will lie somewhere between these two extremes. A further complication is that, at this stage in the operation, the identity of the agent, assuming one is even present in the device, will, in the majority of cases, be unknown. While it may sometimes be possible to gain information on the chemical fill from the detection of trace residues on the outside of the device, this will certainly not be possible in every case. In the latter situation it will be necessary to make assumptions based on the size and configuration of the container present in the device.

A number of factors will determine the actual location and size of the area of contamination arising from the functioning of the device. These will include, inter alia:

  • the quantity and physical properties of the agent present, e.g. gas, liquid or solid, volatility, etc.;

  • the rate of release, e.g. instantaneous or over a period of minutes;

  • wind direction and speed;

  • atmospheric conditions, e.g. dry, raining, stable, neutral, or unstable air conditions; and

  • location of the device, e.g. in the open, in a building, partially contained, etc.

Computer programmes have been developed which, when provided with the above type of data, can calculate the probable size and shape of the resulting area of contamination. Where the identity of the agent is known they may also differentiate between areas where fatalities are likely to occur and those where non-fatal casualties might be expected. When making evacuation decisions based on such computer plots it is important to understand their limitations. The data on which such programmes are based was originally generated from chemical agent trials carried out on military ranges. As you might expect these are generally large open areas, sometimes forested but with very few buildings. While the software may have been updated to take account of the effect of the presence of buildings, etc. our knowledge of the dispersion patterns of chemical agents in a heavily built up area remains severely limited. Having said this, providing the limitations of these programmes are fully understood they remain an invaluable tool in dealing with an emergency of this type.

It does not automatically follow of course that total evacuation of the predicted contamination area will either be necessary or even practicable. People outside the immediate high-risk area may be safer left inside buildings with the doors and windows closed. This is particularly the case where buildings are of a 'sealed', air-conditioned, design. In such situations, however, the buildings air intakes should, wherever possible, be shut down. Any final decision on evacuation will need to maintain a balance between the assessed risk from the chemical threat and the known risks associated with the evacuation of large numbers of people.

The Device has Functioned

In situations where the device has functioned before the emergency response teams arrival, the first priority will be to identify the chemical or chemical agent that has been released. Without this basic information the treatment of casualties and the control of the further spread of contamination will be exceedingly difficult. The range of toxic chemicals theoretically available to a terrorist is very large. However, for the purpose of response planning they can be divided, if somewhat crudely, into three basic categories:

  • toxic industrial chemicals such as, chlorine, ammonia, phosgene, hydrogen cyanide, etc.;
  • classical chemical agents such as, the mustard gases (sulphur and nitrogen mustards), lewisite, the nerve agents (G and V agents); and
  • the "exotic agents" such as, toxins (Botulinum, Ricin, etc.), incapacitants such as BZ, LSD etc.
  • Toxic industrial gases of the type mentioned above are generally highly volatile and one, or even two, orders of magnitude less toxic than the classical chemical agents. When released from a terrorist device they can be expected to disperse rapidly. Residual contamination is unlikely to be a problem and serious casualties are likely to be limited to those in the immediate vicinity of the release.

    The classical chemical agents can be further divided into two sub-groups, namely, persistent and nonpersistent agents. Non-persistent agents such as the nerve agent Sarin can be expected to disperse fairly rapidly. Given the volatility of this agent and its high toxicity, however, it should be noted that the downwind hazard area immediately following a release could be very large. Persistent agents such as, mustard gas and the nerve agent VX, on the other hand, may remain as a source of contamination for a considerable period of time after a release. The impact of the final category of more exotic agents will be much more dependent on the actual agent involved. Many of them are solids and, although they may be extremely toxic, their effective dispersion is much more difficult to achieve. Where the use of this category of agents is suspected the need to identify the particular agent used is even more crucial.

    Handling of Casualties

    The treatment regime for casualties suffering from exposure to chemical agents is to a large extent agent specific. Once again, this highlights the importance of the early identification of the specific agent used in an attack. Initial symptoms exhibited by casualties, while they may often be indicative of the agent used, cannot be relied upon to provide confirmation of the agent used. In the case of exposure to mustard gas, for example, erythema or blisters may not develop for several hours and evidence of exposure may, initially, be minimal. Similarly by the time a person suffering from percutaneous exposure to VX begins to demonstrate external symptoms they may be beyond help.

    The handling and initial treatment of casualties in contaminated areas poses an additional problem. Apart from the provision of life saving first aid, decontamination of casualties must be given the highest priority. This is particularly important where nerve agents or vesicants have been used, as the casualties' level of exposure will continue to increase due to the percutaneous effect of these agents. In addition a contaminated casualty is not only a danger to themselves but also to any unprotected people who subsequently come in contact with them.

    Contamination Control

    Contamination control procedures will be necessary whenever it is suspected that chemical agent contamination remains present. Its adoption will be of particular importance in situations where a persistent chemical agent has been used or is suspected of being used. In order to establish good contamination control it is essential that the boundaries of the contaminated area are identified and marked and that entry and exit to the area is fully controlled. Entry and exit points must be located on the upwind edge of the contaminated area. People entering the contaminated area will need to wear full protective clothing and everything leaving the contaminated area will need to be monitored and, where necessary, decontaminated. The use of handheld monitoring devices such as the British chemical agent monitor (CAM) or the French AP2C is ideally suited for this purpose. It should also be noted that, whilst special decontaminating solutions for chemical agents have been developed for use by UK Forces, the majority of agents likely to be used in any terrorist attack can also be degraded by solutions containing bleaching powder or sodium hypochlorite. For details contact Valerie Seefried at


    In a short article of this nature it is only possible to provide a broad overview of the problems associated with dealing with an incident involving or suspected of involving chemical weapons. Hopefully, however, it will have highlighted the importance of emergency response teams having the necessary skills and training to deal with this particular kind of threat. In the event of a terrorist attack involving chemical weapons, success in dealing with resulting emergency may well depend on this.

    Ron Manley is former Special Advisor for CW Destruction to the Ministry of Defence and former CW inspector for Unscom and OPCW

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