Department of

Architectural Engineering

 


Bioterrorism and Immune Building Technology

Recent events have exposed the vulnerability of our cities and infrastructure to attacks by suicidal fanatics who are determined to cause mass destruction and inflict record numbers of casualties. Airplane security may have been tightened, but various opportunities for such attacks remain open to any single-minded individuals bent on taking revenge for US foreign policy or the actions of our allies. High profile terrorist attacks and mass casualties would seem to be the order of America's future, and until the root causes are addressed and resolved, it will be up to designers, engineers, and building managers to prepare as best they can for the worst possible consequences.

Of all the activities that could be undertaken by terrorists in the attempt to gain world attention, perhaps none has the potential for causing as many mass casualties as the use of biological and chemical weapons. This report summarizes the basics of biological weapons (BW) and what measures might be taken to protect buildings.

Most microorganisms that cause disease or produce toxins may be used as biological weapons, and these include viruses, bacteria, fungal spores, and toxins, but not all will cause casualties or even infections. Toxins are biological poisons and may include those produced by bacteria, called endotoxins, or those produce by fungi, called mycotoxins.

Not all of these agents would necessarily be effective, as some have low infection rates, or ar unlikely to cause fatalities or incapacitation. Table 1 lists all of the major biological weapon (BW) agents that have ever been used, developed, or mentioned as biological weapons in the literature (see References).

Chemical Weapons

Table 2 lists all those chemical weapons which are considered by the various indicated references to be prime choices as chemical weapon (CW) agents. Some of these agents have been used previously in both war and terrorism.

Release Mechanisms

Two basic types of disseminators are possible for both chemical and biological weapons. The aerosolizer is a spraying device that comes in many forms and can be used to spray a liquid or a powder into the air. Microorganisms in liquid form would be aerosolized by such a mechanism. Toxins in powdered or liquid form could also be aerosolized, provided they are ground to sufficiently small sizes.

Aerosolizers can be powered by pressurized gas, such as CO2 or air, to produce vapors. They can also use techniques such as ultrasonic atomizers or “spinning top” aerosolizers to produce clouds of vapor. Although atomization might reduce water to molecules, microbes would be reduced to elementary bacteria, viruses, or clumps of the same. The aerosolization process can destroy some or all of the microbes, depending the species fragility, pressure, and nozzle type.

Most chemicals tend to evaporate under normal room temperature and pressure, so pressurized mechanisms may not be necessary. anyway. Such was the approach used by the Aum Shinrikyo cult in Japan. The potential for mass destruction inherent in the use of chemical weapons is eclipsed by the potential of biological weapons. Although more difficult to create, biological weapons have an order of magnitude more destructive potential on a per mass basis.

Bombs for disseminating chemicals or biological agents can be driven by two mechanisms – explosives or pressurized gas. The main problem with using explosives is that explosives tend to destroy the BW agents. The approach used by Iraq, the Soviets, and the Japanese when they constructed anthrax bombs was to use a slow detonating explosive.

Dissemination in the outdoor air is considered the least likely scenario due to the quantities that would be required, and the fact that wind would tend to disperse airborne agents to harmless concentrations in short order. Such was the case with the Aum Shinrikyo cult; their attempts to disperse anthrax outdoors all failed.

Dissemination in buildings

Figure 2: Explosive or slow release of a BW agent in a building would recirculate throughout the ventilation system.

The most likely approach that would be used by terrorists would be to employ an aerosolizer to release the agents, either into the ventilation duct or into general areas of the building. Release of a BW agent on a single floor would heavily contaminate that floor, as illustrated in Figure 2, but the remaining floors would see much lower concentrations since the agent would arrive from the supply duct in lower concentrations. Both the mixture of normal outside air and the passage through the air handling unit would tend to reduce contminant levels in the supply air. Figure 2: Explosive release of a BW Agent in one zone of a building would result in heavy local contamination but have less effect on other floors.

Passive dissemination of toxins and pathogens is another possibility. An agent like anthrax could be dusted on interior surfaces where aerial dispersion would result. Terrorists would be likely to contaminate themselves by such an act, but may not care.

Anthrax is one of the few BW agents for which infectious properties are known. The ID50, or dose that would infect 50% of exposed people, is 10,000 spores (Cieslak 1999). The LD50, or lethal dose for 50% of people, is 28000 spores (Inglesby 1999). This figure illustrates how varying doses of inhaled anthrax spores might impact a population. It is based on a normal bell curve distribution with a standard deviation of 1.0.

The filtration of anthrax spores can be predicted using the logmean diameter of 1.12 microns, or more precisely, by using the full size distribution curve (Kowalski et al 1999). The UVGI rate constant for anthrax spores on surfaces is approximately 0.000031 cm2/microW-s (Knudson 1986, Dietz 1980). The airborne rate constant is not known but can be extrapolated. The decay curve for anthrax spores is typically a two stage curve with a shoulder.

Explosive dispersion inside a building is one possible approach that terrorists may use, but this would cause immediate alarm and mitigate the effects. The most likely approach would be to use an aerosolizer to release the agents into the return air ventilation duct or air handling unit.

Immune Building Technology

The principle behind immune building technology is to integrate the ventilation system, the building envelope, detection and control systems, zone isolation systems, and air treatment systems so as to provide maximum protection to building occupants. Much of the technology may be advanced and expensive to implement, but when scaled down such approaches can lead to commercial building systems that protect occupants not only against BW agents but also against naturally occurring airborne diseases. Immune building technology may consist of proven technologies like dilution ventilation, filtration, and UVGI, and developmental technologies like PCO, pulsed light, and biosensors.

The effect of any building ventilation system is to recirculate airborne contaminants and purge them over time. Most buildings bring in some 20-25% outside air and mix it with recirculated air. Most buildings do not use any kind of filters, other than dust filters, or any air treatment systems. Although the use of high efficiency filtration or UVGI systems would not have much effect at the point of release of an agent, it would have a major effect on any contaminants that were recirculated. The result would be that casualties in other areas of the building would be reduced. The results of the study on a simulated anthrax attack on a 50 story building in Figure 3 indicate that even a modest system composed of (non-HEPA) filters and UVGI lamps is capable of seriously diminishing the impact of any biological weapons attack.

The cost of implementing air treatment systems must be weighed against the actual risk. Of course, the risk can’t be readily estimated for any given building, and so a more practical approach is simply to allocate a budget for such systems. A system based on filtration, UVGI, increased outside air flowrate (with energy-saving heat exchangers), or other technologies (i.e. PCO) could then be sized to provide some minimal level of building immunity.

Addendum: Anthrax in the Mail

Passive distribution of anthrax in the mail has raised the question of how this might be defended against. UV irradiation can be used to sterilize the outside surface of envelopes with an exposure of 2-30 minutes, depending on the intensity of the UV lamp and the distance the envelope sits from the lamp. Typical medical or dental equipment UV sterilization units can be used for this purpose.

Alternatively, a typical 1420W 2450 MHz microwave oven can be used to sterilize mail (Cavalcante & Muchovej 1993). Since the microwaves penetrate paper they sterilize the inside and the outside of the envelope. It takes longer, perhaps 30-50 minutes, and can start a fire if the envelope contains any metal (i.e. charge cards, reflective tape, or CDs), but may be a convenient option for concerned citizens. Suspect mail should be bagged immediately, them microwaved.

Decontamination of buildings in which anthrax tainted mail has been found can be accomplished using chemical disinfectants or even ozone (Hibben et al 1969, Ishizaki et al 1986). Buildings can be pumped full of the vapors or gases and left to sit for hours or days, then exhausted to atmosphere. Ozone will, however, destroy some organic materials like rubber.

References

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