With the threat of terrorism greater now than ever, building and facility owners must consider possible terrorism threats to their properties. Possible threats may be chemical, biological, radiological, or explosive in nature. Although a terrorism attack on a building is a rare event, all buildings represent potential targets to a terrorist group. As a prudent safety measure, building owners and engineers can take steps to make their buildings less attractive as potential terrorist targets. Building owners and engineers must be selective in choosing those building features that are most feasible and appropriate for their building. Typical threat mitigation and response actions may include increasing security, upgrading mechanical systems, developing emergency response plans, and educating building occupants.

Designing to accommodate potential terrorism threats represents a new frontier for engineers. As part of this new frontier, design engineers may be asked to incorporate terrorism mitigation features into their designs. Facility managers now must consider terrorism implications in how they operate their buildings. The realm of mechanical engineering and facility operations has now expanded to include the new frontier of terrorism mitigation and response.

As part of an overall terrorism mitigation and response program, engineers and facility managers have the greatest input in the area of design and operation of mechanical systems. Although an attack with an explosive device against a building is possible, there are few mechanical system features that can mitigate the effects of such an attack. However, the design of a building's mechanical system can lessen the effects of an attack with a chemical, biological, or radiological (CBR) agent. Consequently, engineers should focus mitigation efforts on CBR terrorism events.

Assessing the Potential Threats

If a building owner wishes to implement a terrorism mitigation and response program for an existing or new (i.e., under design) building, an important first step is completion of a threat assessment of his building. A threat assessment considers potential terrorist attacks against a piece of property.

Many factors can influence the impact of a CBR agent release, including the agent's physical and chemical properties, toxicity, concentration, mode of transmission, and location of release. When conducting a threat assessment for CBR agents, one must also consider environmental and building factors, such as ambient conditions (temperature, relative humidity, wind, and weather) and layout of the mechanical system.

Biological Agents

Biological agents can be subdivided into several classifications, including, but not limited to: airborne vs. non-airborne; infectious vs. non-infectious; contagious vs. non-contagious; and virus vs. bacteria vs. fungi.

One must understand the differences between these classifications in order to effectively design or alter a mechanical system to prevent, control, or lessen the effects of a biological agent release.

Airborne biological agents pose a greater risk to building environments as opposed to non-airborne agents. Because of their aerosolization capabilities, airborne biological agents tend to be easier to disseminate and can contact a larger population of building occupants. Non-airborne agents are usually transmitted via surface-to-hand or hand-to-hand contact routes; hence, they are more likely to affect a smaller pool of building occupants.

Infectious biological agents are those that can cause disease in a host (e.g., a human). A non-infectious disease is one not capable of being spread from one host to another. From a terrorism mitigation and response standpoint, building owners and engineers should be concerned with infectious biological agents. A contagious disease is one that may be transmitted from one host (e.g., a human) to another host (e.g., a human). A non-contagious disease is transmitted from its natural reservoir (e.g., rodent) or a vector (e.g., mosquito) to a host. Host to host transmission is not possible with non-contagious diseases. All contagious diseases are infectious; however, not all infectious diseases are contagious.


Viruses are smallest of the three types of pathogens: virus, bacteria, and fungi. Most viruses have diameters less than 0.23 microns1.Although many viruses cause disease, only a few represent potential bioterrorism agents, including Variola Major (smallpox) and hemorrhagic fever agents (e.g., Ebola, Marburg, Lassa, Hantavirus, etc.).

Variola Major is a contagious pathogen spread primarily via direct contact with an infected person. Airborne transmission is unlikely unless individuals are within close proximity to one another. There is no treatment for smallpox and the only prevention is vaccination. Smallpox symptoms include fever, malaise, headaches, and body aches, followed by its characteristic spotted red rash. Approximately 30% of smallpox cases are fatal.2

Most hemorrhagic fever viruses are zoonotic, which implies their natural hosts are not human. The natural hosts for most hemorrhagic fever viruses are known, with a few exceptions including Ebola and Marburg. Hemorrhagic fever viruses are typically transmitted to humans, when humans come in contact with urine, feces, saliva, or other bodily excretions from infected animals. Person-to-person transmission is possible with some hemorrhagic fever viruses, including Ebola, Marburg, and Lassa, through close contact with infected individuals or their body fluids. Symptoms include fever, fatigue, and bleeding from body orifices. Some outbreaks have resulted in relatively high (>75%) mortality rates.3


Bacteria vary in shape and size. Most spherical bacteria, cocci, have diameters ranging from 0.2 to 2 microns4 . Rod-shaped bacteria, bacilli, typically have diameters ranging from 0.5 to 1 micron and lengths ranging from 1 to 4 microns. There are several potential bacterial bioterrorism agents, including Bacillus anthracis (Anthrax), Vibrio cholerae (cholera), Yersinia pestis (plague), Francisella tularensis (tularemia), and Coxiella burnetii (Q fever).

Of all the bacterial agents, Bacillus anthracis is the most likely agent to be used in a terrorist event. While in the spore form, they have diameters ranging from 2 to 6 microns and can survive in the environment for long periods of time. Anthrax is not contagious, and Anthrax spores can be transmitted via airborne routes. Its size and aerosolization characteristics make Bacillus anthracis a viable bioterrorism agent. Symptoms may include fever, malaise, respiratory distress, shock, and possibly death.5


Fungal spores typically range in size from 1 to 20 microns6. Although some fungal organisms are infectious, cause disease and/or have aerosolization potential; they are not typically considered bioterrorism agents.

Chemical Agents

Chemical agents are typically distributed as aerosolized liquids, vapors, or gases for warfare or terrorism purposes. Chemical agents can be subdivided into several categories, including the following:

  • Blister agents (sulfur and nitrogen mustards)
  • Blood agents (hydrogen cyanide, cyanogens chloride, and arsine)
  • Choking agents (phosgene and diphosgene)
  • Nerve agents (sarin, VX, tabun, and soman).7

In addition to these traditional chemical warfare agents, one may also want to consider chemicals from nearby industrial sites, rail lines, and highways. Conceivably, a terrorism group could ignite an explosive device at such sites, which may cause a release of one or more industrial chemicals into the nearby community. A well-executed threat assessment should identify potential hazards from such scenarios.

Radiological Hazards

There are three primary terrorism scenarios regarding radiological agents that may affect nearby buildings: conventional explosive used to spread radioactive materials (e.g., "dirty bomb"); attack on a nuclear facility; or a nuclear weapon.

Following detonation of a conventional explosive contaminated with radioactive materials, small, contaminated particles will be released into the environment. Some of the particles will remain airborne; others will settle on the ground immediately. The released particles may emit radiation if they are contaminated with the radioactive material.

Health Effects

Health effects from exposure to biological agents are not likely to appear until several days following the initial exposure. Health effects from exposure to chemical agents are likely to occur immediately (i.e., less than one hour) following the initial exposure. Health effects from exposure to radiological agents may not be noticed for years following the initial exposure (e.g., cancer or genetic effects); however, in cases where individuals receive relatively high doses of radiation, symptoms may appear within a few hours following the initial exposure.

Building Design: Engineering Controls

Each building has certain features and characteristics that determine its attractiveness as a potential terrorist target. Buildings should undergo a site-specific vulnerability and threat assessment to identify these characteristics and determine the corresponding possible terrorism scenarios. For existing buildings, such an assessment will also identify existing conditions of mechanical system components or features, including filtration, dampers, supply/ exhaust/return fans, ductwork, controls, equipment location, pressure relationships, smoke control, and fire alarm or detection system interaction.

Based on such an assessment, a terrorism mitigation and response program may be needed. Engineering controls may be an important component of such a program. When designing engineering controls, building owners and engineers should consider a variety of factors, including CBR agent(s) likely to be used, modes of distribution (e.g., aerosolization), building location, and layout of the mechanical system.

When implementing engineering controls and other security features into a building, owners and engineers will have to consider financial implications and may have to prioritize the implementation of specific engineering controls, security measures, and other actions.


Filtration is one key means of mitigating the effects of a CBR release within a building. Standard commercial buildings may have particulate filters with efficiencies that are not ideal for capturing airborne biological agents. For example, a MERV 9 filter removes less than 50% of 1 to 3 micron size particles from an airstream. If filtration of potential biological agents is needed for a building's HVAC system, a higher-grade filter will be necessary. For example, a MERV 16 filter removes more than 95% of particles in the following three size ranges: 0.3 to 1 micron, 1 to 3 micron, and 3 to 10 microns.8 If filtration upgrades are performed on an existing building's AHUs, one must first determine if the newer, higher efficiency filters are compatible with the existing AHU. Filters must not only fit into the housing, but the fan must also be able to sustain proper airflow with the increased pressure drop created by the new filters. Filters must be installed correctly within the housing; otherwise, air bypass may occur between the filters and housing. Air bypass allows some contaminants in the airstream to pass uninhibited around the filters.

Particulate filters will not remove gas or vapor chemical agents from an airstream. Typically, sorbent filters are needed for chemical agents. The use of sorbent filters in commercial building air-handling systems is a rare practice; however, it may be warranted for some high profile buildings.

Filtration can be effective in removing particulates contaminated with radioactive materials from an airstream. For example, following detonation of a dirty bomb, contaminated dust will be airborne. Proper filtration can remove some of the contaminated dust particles from the airstream. The effectiveness of the filtration will depend on the efficiency of the filter and particulate size.

Outdoor Air Intakes

Outdoor air intakes should ideally be located on or near roofs, where they are not easily accessible to the public. When outdoor air intakes are located away from public access, they are less likely to be used as an entry point for a CBR agent release into the building. For existing buildings, it may not be feasible to relocate outdoor air intakes away from public areas. In such cases, building owners and engineers may consider extending the intake to a higher elevation. Another option includes erecting a barrier, such as a fence, around the intake to minimize public access.

Building/Room Pressurization

Buildings should be positively pressurized with respect to the outdoors. This pressurization prevents outdoor contaminants from entering the building via doors, windows, and breeches in the building envelope. Ideally, outdoor air should only enter a building through the outdoor air intakes.

Building owners and engineers may also consider the pressure relationships of public areas. For example, public areas such as lobbies, waiting areas, and entrance foyers should utilize separate air-handling systems, be negatively pressurized with respect to adjacent areas, or placed under direct exhaust to the outdoors. Because these areas are easily accessed by the public, a CBR agent release may be more likely to occur within these areas. By exhausting these areas directly to the outdoors, a CBR agent release in these areas is less likely to spread to other areas of the building. If these public areas are negatively pressurized with respect to surrounding areas and air within these spaces is returned to an AHU, one's ability to contain a CBR agent in this area will be dependent upon the quick discovery of such a release. If the CBR agent can be identified in a timely manner, building engineers may be able to shut down AHUs or place the affected unit in 100% exhaust mode.

Other possible entry points for CBR agents include mailrooms and loading docks. Building owners and engineers may also want to consider utilizing separate air-handling systems for these areas, placing these areas under negative pressure with respect to adjacent areas, or placing them under direct exhaust to the outdoors. A fume hood may be useful in mailrooms and dock areas for inspecting and opening suspicious packages.

Building owners and engineers may consider a pressurization system for one or more exit stairs in order to provide a safe exit path for building occupants. By utilizing a stair pressurization system, the pressurized stair may not be affected by a CBR agent release within the rest of the building. The pressurization system would have to draw air from non-contaminated areas, such as the outdoors. The pressurized stair would represent a relatively safer route for escaping from contaminated areas within the building.

Building Envelope

If the building envelope can be properly sealed, it becomes easier to maintain the positively pressurized building with respect to the outdoors. Common locations for building envelope breeches include doors, windows, pipes, and joints.

Air Returns

Return diffusers should be located away from public areas. Return diffusers can serve as an entry point for a CBR agent. If return diffusers are placed in public areas, they should be conspicuously located so that they may be observed by building employees or security personnel.

Non-ducted returns offer more opportunities for introduction of a CBR agent into the HVAC system. By utilizing ducted returns, it becomes more difficult for a terrorist to release a CBR agent directly into a return airstream.

HVAC Controls

Modern HVAC systems have controls, which permit relatively quick shutdowns of fans and closing of dampers. Such controls may be associated with smoke control systems. Depending on the situation, it may be appropriate to shut down the HVAC system and close dampers following a CBR agent release in or near a building. By shutting down systems, further spread of the CBR agent will be minimized. For outdoor CBR agent releases, shutting down fans, and/or closing outdoor air dampers minimizes entry of the CBR agent into the building.

Another useful option may include being able to set an HVAC system to supply 100% outdoor air. If a CBR agent is released in a building and is allowed to spread throughout the building, the building engineer may need to ventilate the building quickly. A 100% outdoor air setting for an HVAC system permits such a scenario.

HVAC controls should be secured to prevent public access. Ideally such controls should be placed behind locked doors or cabinets. Building engineers should also consider securing access to other key utility (e.g., water, electrical, and gas) controls and valves.

HVAC System Information

Information regarding MEP systems should be secured, including engineering drawings, diagrams, and specifications. Building owners should only release such documents to trusted contractors and designers. Individuals plotting a CBR agent release within a building may gain valuable information from such documents.

MEP system controls should be properly labeled for quick identification. Proper labeling will expedite possible shutdowns or other appropriate response actions in the event of a CBR agent release.

Equipment Access

Areas containing MEP equipment should be secured to prevent unwanted access. Accesses to roofs, mechanical rooms, electrical rooms, pump rooms, and other critical areas should be locked.

Maintenance Staff

An important component of a terrorism mitigation and response plan should include training for members of the maintenance staff and other key building personnel. Staff should be trained on how to respond to a CBR incident. Response procedures may include shutting down HVAC systems, closing dampers, and/or evacuating affected areas. Some buildings or facilities may choose to develop and train an emergency response team, which may also be responsible for responding to hazardous materials incidents, fire alarms, and building evacuations. Regardless of the level of responsibility for maintenance staffs or emergency response teams, drills should be performed to simulate actual CBR incidents. Some facilities may choose to coordinate drills with local government emergency response officials.


Several years ago, building designs often did not address terrorism response and mitigation issues. However, based on recent U.S. and world events, it becomes clear that CBR agent releases represent real threats to buildings. Owners must now consider these possibilities when constructing a new building or operating an existing one. Building engineers' responsibilities may now include maintaining terrorism mitigation and response equipment and keeping maintenance staffs informed and trained on response procedures. Design engineers may need to expand their designs for some types of buildings to address the mitigation and response issues. For all of us (owners, building engineers, and design engineers), terrorism mitigation and response represents a new frontier in building operation and design. ES