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Reducing the Risk of Occupants' Exposure to
Biological Threats: Practical Steps for Building Owners

References cited on this page:

1.Hitchcock PJ, Mair M, Inglesby TV, et al. Improving performance of HVAC systems to reduce exposure to aerosolized infectious agents in buildings; recommendations to reduce risks posed by biological attacks. Biosecur Bioterror. 2006; 4(1):41-54.
2.Office of the Director of National Intelligence. National Intelligence Estimate: The Terrorist Threat to the US Homeland. Washington, DC: Office of the Director of National Intelligence; July 2007.
3.Kowalski, W. and W. Bahnfleth. Immune building technology and bioterrorism defense. HPAC Engineering 2003;75(1):57-62.
4.ASHRAE. Report of Presidential Ad Hoc Committee for Building Health and Safety under Extraordinary Incidents on: Risk Management Guidance for Health, Safety, and Environmental Security under Extraordinary Incidents. Atlanta: ASHRAE; January 26, 2003.
5.Lichtblau E, Meyer J. Response to terror; the investigation; jihad training details heighten attack fears; tactics: in disclosures to FBI, imprisoned Bin Laden associate describes lessons in chemical weapons, explosives and other acts of terror. Los Angeles Times. September 28, 2001; Part A; Part 1; National Desk: pg. 22.
6.U.S. EPA. An introduction to indoor air quality - Biological Pollutants. Washington, DC: U.S. EPA; November 14, 2007.
7.Myatt, TA, Milton, DK. Indoor pollutants: endotoxins. In: Spengler JD, Samet JM, McCarthy JF, eds. Indoor Air Quality Handbook. New York: McGraw-Hill; 2001: 42.1–42.14.
8.Platts-Mills, TA. Indoor pollutants: allergens derived from arthropods and domestic animals. In: Spengler JD, Samet JM, McCarthy JF, eds. Indoor Air Quality Handbook. New York: McGraw-Hill; 2001: 43.1–43.15.
9.Muilenberg, ML. Indoor pollutants: pollen in indoor air: sources, exposures, and health effects. In: Spengler JD, Samet JM, McCarthy JF, eds. Indoor Air Quality Handbook. New York: McGraw-Hill; 2001: 44.1–44.18.
10.Burge, HA. Indoor pollutants: the fungi. In: Spengler JD, Samet JM, McCarthy JF, eds. Indoor Air Quality Handbook. New York: McGraw-Hill; 2001: 45.1–45.33.
11.Rao, CY. Indoor pollutants: toxigenic fungi in the indoor environment. In: Spengler JD, Samet JM, McCarthy JF, eds. Indoor Air Quality Handbook. New York: McGraw-Hill; 2001: 46.1–46.19.
12.Nardell, EA. Indoor pollutants: Tuberculosis. In: Spengler JD, Samet JM, McCarthy JF, eds. Indoor Air Quality Handbook. New York: McGraw-Hill; 2001: 47.1–47.13.
13.Barry, BE. Indoor pollutants: Legionella. In: Spengler JD, Samet JM, McCarthy JF, eds. Indoor Air Quality Handbook. New York: McGraw-Hill; 2001: 48.1–48.15.
14.U.S. EPA. Indoor air facts no. 4 (revised) sick building syndrome. Washington, DC: U.S. EPA; August 9, 2007.
15.15. Persily, A. et. al. Building Retrofits to Protect Against Airborne Chemical and Biological Releases (NISTIR 7379). Washington, DC: NIST. March 2007.
16.Burroughs, HE. Improving Filtration Effectiveness. HPAC Engineering.
47.Office of the Director of National Intelligence. Unclassified report to Congress on the acquisition of technology relating to weapons of mass destruction and advanced conventional munitions, 1 January-31 December 2004. Washington, DC: Office of the Director of National Intelligence.
Full list of references


Most commercial buildings are not configured and maintained in ways that effectively reduce occupants’ risk of exposure to biological threats. As a result, the majority of people in high occupancy buildings are unnecessarily vulnerable to both aerosolized biological attacks and naturally occurring biological threats.

Types of Biological Threats

Intentional threats: bioterrorism
Bioterrorism is an urgent and growing threat to U.S. national security, and the lethality of biological weapons mirrors that of nuclear weapons. A covert biological attack on U.S. civilians could potentially cause tens of thousands of casualties and immense social and economic disruption. Furthermore, there are no significant technical barriers to prevent the development and use of biological weapons, and tactical warning of a biological attack is unlikely.

Several states potentially hostile to the United States are suspected of having biological weapons programs, and there are groups, such as al-Qa’ida, that are both committed to attacking the U.S. and are pursuing biological weapons capabilities.2,47 According to the July 2007 U.S. National Intelligence Estimate regarding the terrorist threat to the U.S. homeland, al-Qa’ida “would not hesitate” to use biological weapons “if it develops what it deems is sufficient capability.”2

Commercial buildings are attractive targets for biological attacks because they present a potential for wide-scale agent dispersal through air handling systems, as well as the potential to cause mass casualties and great economic loss, as a result of productivity losses, decontamination costs, etc.3,4 It is known that al-Qa’ida is aware of the potential to use air handling systems to attack building occupants: as early as 1998, training in Afghanistan-based al-Qa’ida camps included the use of ventilation systems as a "delivery system" to attack buildings with poisons.5

Naturally occurring threats: bacteria, molds, allergens, etc.
Building occupants face threats from numerous, naturally occurring biological contaminants that can be spread through the air, including viruses, bacteria, molds, toxins produced by bacteria/molds, and allergens such as pollen, pet dander, and pest droppings.6-13 The health effects associated with naturally occurring indoor biological air pollutants include disease, toxicoses, and hypersensitivity (i.e., allergic) diseases.6-13 In addition, exposure to indoor biological air pollutants has been associated with "sick building syndrome," a set of non-specific symptoms that may include upper-respiratory irritative symptoms, headaches, fatigue, and rash, and "appear to be linked to time spent in a building, but no specific illness or cause can be identified.”14

Indoor biological pollutants can find their way into and spread around buildings in varying ways depending on the contaminant. Many bacteria, molds, and allergens are ubiquitous in the outdoor environment and can enter a building and spread throughout via the air handling system.6 Bacteria, molds, and allergens also can be brought into a building on myriad sources, including building materials, carpets, clothing, food, or by pets and pests (e.g., rodents, cockroaches).6 Once in a building, certain bacterial and mold species can get into the HVAC system and grow in damp or wet places such as cooling coils, humidifiers, condensate pans, and filters, or may grow in places where water has collected, such as ceiling tiles, carpeting, or insulation, any of which can then serve as a continued source of contamination throughout a building.6

Inadequate Protection for Most Buildings

Problems in configuration and maintenance
Most commercial buildings are not configured and maintained in ways that effectively reduce occupants' risk of exposure to biological threats:

  • The HVAC systems of most commercial buildings do not have air filtration systems sufficient to effectively remove biological contaminants from the air; this is particularly true for bacteria and viruses.
  • Filters are frequently installed improperly and/or maintained improperly, resulting in reduced filtration efficiency as a result of filter bypass
  • Many commercial buildings have infiltration problems that can reduce the effectiveness of air filtration systems because air that enters a building via infiltration bypasses filtration systems
  • Many commercial buildings have outdoor air intakes located at or near ground level, making them more accessible; this increases occupants' vulnerability to biological attacks and also can increase vulnerability to naturally occurring threats when the air intake is near a potential contamination source.3,4,15,16

Expert Recommendations for Building Owners

On June 13–14, 2005, the Center for Biosecurity convened the Working Group on Reduction of Exposure to Infectious Agents during a Covert Bioterrorism Attack. The Working Group was charged with recommending practical steps that building owners can take to reduce the risk of occupant exposure to harmful pathogens after an aerosol release of a biological weapon.1 The Working Group was composed of subject matter experts in air filtration, building ventilation and pressurization, air conditioning and air distribution, biosecurity, building design and operation, building decontamination and restoration, economics, medicine, public health, and public policy.

Practical measures and available technologies
The Working Group focused on developing practical recommendations  for improving or adjusting the functions of the heating, ventilation, and air conditioning (HVAC) systems in commercial and public buildings that could reduce the risk of occupant exposure to aerosolized pathogens following a biological attack. The Working Group placed an emphasis on currently available technologies that would not be prohibitively expensive or require major retrofits and that could provide additional benefits, such as improved indoor air quality.
A caveat: do no harm
It is imperative that building owners and operators fully assess the potential consequences of any risk reduction  measures prior to implementation. Changes should not be made to buildings, building systems, and operations if they will degrade indoor air quality and comfort under normal operating conditions or if they will in any way interfere with the proper operation of fire protection and life safety systems.

Seven recommendations for immediate implementation
The following recommendations can be immediately implemented by building owners and operators, as appropriate; they are based on those of the Working Group on Reduction of Exposure to Infectious Agents during a Covert Bioterrorism Attack. 

  • Minimize filter bypass: Seal, caulk, and gasket everything (filter cartridge, retainer bank, tracking, etc.) to minimize filter bypass.
  • Commission: Commission buildings during design and construction, and re-commission routinely to ensure that ventilation systems are operating in line with design intent. 
  • Enhance filtration efficiency: Increase air filtration to the maximum economically justifiable MERV level to improve the removal of particulate matter from the air.
  • Maintain filter systems: Conduct regular inspections and maintain filter systems correctly to ensure that the HVAC system functions properly.
  • Train staff: Ensure that maintenance staff has the appropriate training to operate and maintain the HVAC system.
  • Tighten the envelope: When economically feasible, tighten the building envelope to reduce the infiltration rate.
  • Pressurize: When economically feasible, pressurize the building to reduce infiltration rate.

Potential collateral benefits 
Available scientific data suggest that implementing measures to reduce building vulnerability and the related risk of occupant exposure to bio threats may have additional (“dual-use”) benefits, including:

  • improved HVAC system cleanliness, which can potentially improve building energy efficiency and reduce operating/maintenance costs over time; and
  • improved indoor air quality, which may improve the health and increase the productivity of building occupants.15
Last updated:  July 22, 2011