Chemical Agent Terrorism: A Refresher in Strategic Approach

It wasn’t many years ago that the acronym WMD, which of course we all today know stands for weapons of mass destruction, would have been unfamiliar to most of the population. Today, my eleven-year-old son, Tommy, has on numerous occasions asked me about the WMD threat over breakfast. This is certainly related to the events of September 11, 2001 and the subsequent articles in popular press relating to biological, chemical and nuclear devices that might be available or constructed by non-state sponsored terrorist groups. The Aum Shinrikyo event in the Tokyo subway in 1995 initially focused much detection and mitigation effort on the chemical agent threat. That incident confirmed what all of us in this industry had long feared: a non-state entity could manufacture a viable chemical agent and deliver it in a public location. Discussion abounds on the quality of the Aum Shinrikyo agent and the crudeness of its delivery method; however, what is indisputable was the agent was manufactured in relatively inexpensively facilities and after this first attempt, the effectiveness of the delivery system may be easily improved. Much historical concern about the chemical agent threat has been directed at non-state groups.1 This is certainly born out by the demonstrated fact that under forty minutes, the chemical formula for sarin (GB) can be obtained through a simple Internet search. However, recent events have brought state-sponsored groups into much clearer focus, especially now as the UN inspectors have entered Iraq under the current and more forceful UN resolution.

Fortunately, from a security designer’s prospective, dealing with the chemical agent threat is in some ways similar to facility threats that we deal with on a day-to-day basis. Our response can be cast in terms of detection and response.

Much effort has been expended by numerous government agencies in the development of reliable chemical agent detectors suitable for long term, stationary use in facilities. These devices are normally located either in the mechanical equipment rooms close to the air inlet duct-work or in proximity to large highly populated assembly areas. In the former case, these devices draw air samples from the inlet and, in some cases, return ducts, analyze the sample for potential chemical or toxic industrial gases (TIGs), initiate an alarm signal if necessary, purge the analysis chamber and repeat the process. For those devices located near large population assembly areas, the process is the same; for aesthetic purposes, the detection device is likely to be located in an adjacent closet or equipment room.

Two technologies are commonly utilized in these types of devices. First, surface acoustic wave (SAW) detects changes in the characteristics of a piezoelectric crystal in the presence of potential chemical contaminants. In a clean, uncontaminated airflow, the crystal behaves in a specific, repeatable manner. However, in the presence of chemical agents, and most recently, TIGs, the crystal behavior is modified to the extent that it can be reliably detected and used as an indicator of chemical agent or TIG presence. This process can also differentiate between the types of nerve agents and TIGs.

The ion mobility spectrometry works slightly different in that it measures the drift tube transit time of a ionized sample of unknown composition. Chemical agents and TIGs have characteristic and repeatable transit times under controlled conditions. The process time for the IMS type detectors is on the order of three to five seconds.

One of the most cost effective ways to protect a facility against a potential exterior chemical agent attack is to focus on the fresh air intakes. A quick survey of any urban area will reveal that we have placed these critical HVAC features in a variety of locations ranging from grates in the sidewalk, waist-high grates in the sides of buildings and elevated ducts at various locations on the face of the building and even on the roof. It is also fairly evident that the age of the building does not necessarily determine where the ducts might be located. A recent survey of a new Federal building found that the air inlet ducts were at ground level around the perimeter of the facility in a public access area. Some of the remedies can be relatively straightforward. A recent publication by the Centers for Disease Control (CDC) entitled, Guidance for Protecting Building Environments From Airborne Chemical, Biological, or Radiological Attacks, presents a number of very common-sense approaches to intake protection. For those intakes that are located close to the ground, enclosures can be built around them to raise the effective inlet height to an elevation above that which would be considered optimum for a dispersal device. This type of upgrade is obviously dependent upon the specifics of the facility such as location of windows, esthetics, and characteristics of surrounding buildings.

Another mitigating factor used in addressing the chemical agent threat is filtration. Absorbent filters (i.e., activated carbon or absorbent type media) are effective in stopping and retaining chemical agents. Unfortunately, there are a number of common engineering issues associated with this approach. First, the pressure drop associated with an activated carbon type filter is greater than that associated with normal building system filters. This will require an increase in the size of the air handling motors requiring not only up-front capital expenditures but increased operating costs as well. Using the activated carbon filters in a bypass filtration duct, which is used only in the event that a chemical agent is detected in the airstream, can alleviate this difficulty; unfortunately, most mechanical rooms do not have the space available for an additional duct run. Some manufacturers are trying to address the pressure drop problem by packaging their charcoal filters with supplemental air-handling units, resulting in a pressure neutral configuration. Again, the concerns with regard to operating costs and maintenance issues still remain. Second, continual use of the activated carbon filters will result in increased maintenance costs due to replacement and disposal of the used filters.

A final issue with regard to this topic that must be resolved is the one of response: what are the appropriate building and protective personnel responses in the event of the detection of chemical agent in the HVAC inlet stream? Some of the responses are obvious:

One of the more difficult issues to wrestle with in this regard is whether or not the contaminated air should be exhausted to the exterior of the building. This practice would serve to quickly reduce contaminate concentration within occupied areas; it also raises the grave potential of endangering the safety of any individuals that might be near the exhaust duct-work from the building. Again, one of our recent site surveys discovered a major HVAC inlet duct for a building located immediately adjacent to the exhaust duct from another. In this example, exhausting the contaminated air would undoubtedly result in its introduction into the second building.

There are clearly devices available on the market that can reliably and quickly detect the introduction of chemical agents and TIGs into a building HVAC system. Due to the nature of current projects, there is very little operational data available to the private sector regarding nuisance alarm rates and maintenance costs. Filtration is also a mitigating measure that is available either as an independent approach or as a complimentary feature with detection. Again, there are costs and space issues associated with this approach that must be resolved on a facility-specific basis. Finally, the appropriate response to a chemical agent attack is one that will have to be considered carefully depending upon the particular nature of the building and its surrounding environment.

Randy Nason, PE, VP