Chemical/Biological/Radiation (CBR) Safety of the Building Envelope  

by Rob Bolin, PE
Syska Hennessy Group



The design and construction of safe and secure buildings continues to be the primary goal for owners, architects, engineers, and project managers. Security and safety measures, such as those for anti-terrorism and force protection (ATFP), must be considered within a total project context, including impacts on occupants and the environment, regardless of the level of protection deemed appropriate. This includes the consideration of impacts to the facility HVAC systems in general, and those systems interacting with the building envelope specifically. Of particular concern are airflow patterns and dynamics both inside and outside of buildings, especially pertaining to the internal release or external release of chemical, biological or radiological contaminant, and the measures necessary to limit airborne contamination.


The main objectives of Chemical/Biological/Radiological (CBR) safety and protection are to provide building systems and controls that provide a safe and secure indoor environment in the event of a CBR release inside or outside of the building.

Because of the fairly recent attention to this topic the definition of what constitutes safe and secure building systems and controls is evolving, there are several fundamentals that should be implemented.

  • Provide a Tight Building Envelope — A tight building envelope can significantly reduce the entrainment of CBR agents in to the building through careful control of infiltration.

  • Incorporate Dedicated Ventilation and/or Exhaust Systems — Dedicated ventilation and exhaust systems located on roofs or high above grade level are easier to keep secure from CBR release and entrainment.

  • Use Dedicated HVAC Systems for Perimeter Zones — Dedicated HVAC systems serving perimeter zones adjacent to building envelopes enhance the control of positive building pressurization and CBR mitigation.

  • Evaluate the HVAC System Controls — HVAC system controls should have the ability to monitor outdoor air intakes for the presence of chemical agents but should also provide emergency shutoff and control.

  • Consider the Physical Security of HVAC Components — HVAC equipment and outdoor air intakes should be physically secure from tampering and the potential release of CBR agents in close proximity.

  • Incorporate Higher Levels of Filtration — While not specifically impacted by the design of the building envelope, the filtration provided as part of the HVAC system can help protect a building in the event of a CBR agent release.


Provide a Tight Building Envelope

Illustration of air leakage through a building enclosure

Figure 1. Air Leakage through a Building Enclosure

In traditional construction, infiltration occurs through gaps and cracks in the building envelope. Excess infiltration of cold air in the winter and hot humid air during the summer can create uncomfortable indoor environments and raise heating and cooling costs by 20% to 40%. Such unintentional infiltration is also a concern for an exterior chemical, biological, and radiological (CBR) release at some distance from a building, such as a large-scale attack. Decreasing infiltration improves comfort, saves energy, controls moisture, reduces indoor pollution, and makes forced ventilation necessary. Also, tight building construction in combination with building pressurization can be an effective CBR-protection strategy.

Commission Envelope Elements—the building commissioning process should include commissioning of the building envelope to insure that all performance requirements are being met. Commissioning of the building envelope can identify areas of concern related to air infiltration and leakage, moisture diffusion, surface condensation, and rain water entry—all issues that can negatively impact the building's energy performance and indoor environmental quality. Of particular importance is to begin commissioning of the building envelope during design when design modifications can be easily incorporated, rather than waiting until construction when remediation can cost significantly more.

While the LEED® Green Building Rating System requires buildings to undergo Fundamental Building Commissioning of systems to achieve certification, it merely recommends that some form of building envelope commissioning be incorporated. Lemieux and Totten have proposed a Building Envelope Commissioning process that could supplement the Fundamental Building Commissioning required for LEED® certification.

Incorporate Dedicated Ventilation and/or Exhaust Systems

Exposure of building occupants to potentially hazardous chemical, biological, and radiological (CBR) agents negatively impacts the indoor environment and can pose serious health threats. To help maintain superior indoor air quality and protect people's health, dedicated ventilation systems (aka. dedicated outdoor air systems [DOAS]) and dedicated exhaust systems can be installed. DOAS use separate air handlers to condition and deliver the minimum required constant volume of outdoor air. Be sure to protect all outdoor air intakes and locate away from all exhaust openings. For more information, see the Department of Health and Human Services Guidance for Protecting Building Environments from Airborne Chemical, Biological, or Radiological Attacks, May 2002 and Guidance for Filtration and Air-Cleaning Systems to Protect Building Environments from Airborne Chemical, Biological, or Radiological Attacks, April, 2003.

Use Dedicated HVAC Systems for Perimeter Zones

Provide dedicated HVAC systems to serve perimeter zones and maintain positive pressurization with respect to the building envelope. This strategy will help mitigate external and internal release of chemical, biological and radiological contamination.

Evaluate the HVAC System Controls

Photo of Ion mobility spectrometry (IMS) chemical detector designed for installation in HVAC Systems

Figure 2. Ion Mobility Spectrometry (IMS) chemical detector designed for installation in HVAC Systems
Courtesy Smiths Detection

Many HVAC systems incorporate energy management and control systems that can regulate air flows within a building on an emergency response basis. Consider utilizing the control system for this purpose. In some cases, simply shutting off the building's HVAC and exhaust systems might be the most effective means of avoiding introducing a CBR agent from outside the building. In other cases, pressurization and airflow control between zones within a building may be effective in minimizing the spread of a CBR agent released within the building. This strategy should include consideration of using pressure sensors inside and outside the building (and between zones) to measure pressure levels and control HVAC equipment to maintain pressure relationships.

If an HVAC control plan is pursued, building personnel should be trained to recognize a terrorist attack quickly and to know when to initiate the control measures. All procedures and training associated with the control of the HVAC system should be addressed in the building's emergency response plan.

Consider the Physical Security of HVAC Components

Three examples of protecting outdoor air intakes

Figure 3. Protecting Outdoor Air Intakes
Courtesy of Guidance for Protecting Building Environments From Airborne Chemical, Biological and Radiological Attacks, NIOSH

Outdoor Air Intakes—One of the most important steps that can be taken to protect a building's indoor environment from CBR attack is the security of outdoor air intakes. Introducing CBR agents into outdoor air intakes allows for the dispersion throughout the building via the HVAC system. Intakes at or below grade are most at risk due to their accessibility and because most CBR releases will be near the ground and remain there. Locate outdoor air intakes above the third floor of the building, and preferably at roof level whenever possible.

Examples of vulnerable outdoor air intakes

Figure 4. Vulnerable Outdoor Air Intakes
Courtesy of Guidance for Protecting Building Environments From Airborne Chemical, Biological and Radiological Attacks, NIOSH

Rooftop HVAC Equipment—Roofs are similar to other entrances of a building and should be physically secured. Since HVAC equipment, including air handling units and their outside air intakes, are often located on rooftops and are susceptible to CBR attack, it is especially important to restrict access to the roof.

Photo of HVAC equipment mounted on a building's rooftop

Figure 5. Example of Rooftop HVAC Equipment

Incorporate Higher Levels of Filtration

Filtration—This is an overall HVAC system issue, not only specific to the Building Envelope Design Guide. Please refer to BIPS 06 / FEMA 426 Reference Manual to Mitigate Potential Terrorist Attacks Against Buildings for a thorough discussion of filtration for CBR mitigation. Some consideration should also be given to the associated energy cost impact of installing the additional filtration equipment. Energy simulation and life cycle analysis tools can and should be used to assess the energy cost impacts.



The CBR Safety fundamentals previously described should be considered for and applied to the building envelope design and construction, and to HVAC systems interacting with the building envelope, for all buildings.

Related Issues

Balancing Security/Safety and Sustainability Objectives—Providing for sustainable designs that meet all facility requirements is often a challenge to the building design, construction and operation community. With limited resources it is not always feasible to provide for the most secure facility, the most architecturally expressive design, or energy efficient building envelope. From the concept stage through the development of construction documents, it is important that all project or design stakeholders work cooperatively to ensure a balanced design. Successful designs must consider all competing design objectives.

Integrated Design—Designers are moving away from the conventional building design approach that has historically resulted in little interaction between all parties involved in the project. There is a movement to embrace integrated building design, fostering communication amongst all parties that could be involved in the project, and facilitating working together from the start to coordinate and optimize the design of the site and the building.

Relevant Codes and Standards

Federal Mandates

Standards and Guidelines

Additional Resources

Federal Agencies


  • Building Ventilation and Pressurization as a Security Tool, Andy Persily, ASHRAE Journal, September 2004.
  • Building Security Through Design: A Primer for Architects, Design Professionals, and their Clients, The American Institute of Architects, Washington DC, 2001
  • Protection of Federal Office Buildings Against Terrorism, Committee on the Protection of Federal Facilities Against Terrorism, Building Research Board, National Research Council, Washington DC, National Academy Press, 1988.
  • Uses of Risk Analysis to Achieve a Balanced Safety in Building Design and Operations, by Bruce D. McDowell and Andrew C. Lemer, Editors; Committee on Risk Appraisal in the Development of Facilities Design Criteria, National Research Council, Washington DC, National Academy Press, 1991.