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Protecting Building Occupants from Exposure to Biological Threats

Building Vulnerabilities

Most commercial buildings are not configured and maintained in ways that effectively reduce occupants’ risk of exposure to biological threats. Biological threats include both dangerous biological agents (e.g., anthrax) that may be intentionally introduced into a building (bioterrorism), and naturally occurring allergens, molds, and bacteria that are introduced into a building unintentionally or by natural processes. Because many commercial and public buildings are not configured and maintained in ways that effectively reduce their vulnerability to biological threats, the people who live, work, and shop in them are at risk of unnecessary exposure to intentional and naturally occurring biological threats, the effects of which may reduce productivity and cause illness, or even death.


Buildings are attractive targets for terrorists because the intentional release of an aerosolized biological agent in an inadequately protected commercial or public building offers the potential for wide-scale agent dispersal through the air handling system, which can cause mass casualties and high economic costs (i.e., productivity losses and decontamination costs).[3,4]

Four building attack scenarios: There are 4 general scenarios whereby a biological agent could be intentionally introduced into a mechanically ventilated building.[3,15] Open interactive illustrations of attack scenarios.

  • Large-scale outdoor release of a biological agent: In this scenario, some portion of an aerosolized biological agent released up-wind of a building would enter the building through the outside air intake and be distributed throughout by the HVAC system. In addition, the aerosolized biological agent could enter the building via infiltration, which is the passage of air into a building through leaks in the building envelope.   

  • Outdoor release of a biological agent directly into an air intake: In this scenario, a biological agent would be introduced directly into the HVAC system from outside of a building via the air intake, and then distributed throughout the building.  

  • Indoor release of a biological agent directly into an HVAC system: In this scenario, a biological agent would be introduced directly into the HVAC system from within the building and distributed throughout. Distribution and spread of the biological agent would depend on the HVAC system design and the agent's point of entry (e.g., air handling unit, supply air distribution system, return air distribution system).

  • Indoor release of a biological agent in common area or special use space: In this scenario, an aerosolized biological agent would be released in a common area or special use space, such as a lobby, auditorium, or mail room. While such an attack would most directly affect the people present in the common area during the attack, some portion of the aerosolized biological agent could be distributed beyond the release area via airflows created by pressure relationships that exist within different parts of the building relative to each other. The biological agent also could be distributed beyond the release area as a result of its entry into HVAC system through the return air system (if present).

The likelihood of any of these 4 scenarios is impossible to determine and depends on the intentions and capabilities of a particular attacker relative to the existing level of vulnerability to the specific type of attack.[20]

Naturally Occurring Biological Threats

Sources: Sources of indoor biological pollutants vary depending on the contaminant.[6] Many bacteria, molds, and allergens are ubiquitous in the outdoor environment and can enter a building through the air intake and spread throughout via the air handling system and/or infiltration. They also can be brought into a building on myriad sources including building materials, carpets, clothing, food, etc. or by pets and pests (e.g., rodents, cockroaches).

Certain bacteria and mold species that get into a building—through the air intake system, or infiltration, or carried by a vector—find their way into the HVAC system, where they can grow in damp or wet places, such as cooling coils, humidifiers, condensate pans, and filters, and then serve as a continued source of contamination throughout the building. Some bacteria and mold species can grow in places where water has collected, such as ceiling tiles, carpeting, and insulation, and serve as a continued source of contamination.

Bacteria and viruses that are spread by person-to-person transmission (e.g., mycobacterium tuberculosis, influenza virus) can be brought into a building by infected individuals, and potentially can enter the return air system and be spread throughout a building by the HVAC system. However, the extent to which HVAC systems contribute to the spread of such diseases is unclear. Additionally, indoor air conditions such as temperature and humidity, which are a direct result of an HVAC system’s conditioning of the air in the occupied space of a building, can affect the efficiency of person-to-person transmission of certain organisms, such as influenza virus.[25] However, the extent to which indoor air conditions contribute to the spread of such diseases is unclear.

Potential adverse health effects of exposure; Building occupants can be affected and even sickened by 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] There are several health effects associated with indoor biological air pollutants:

  • Disease: Exposure to airborne bacteria, molds, and viruses can cause diseases such as legionellosis, pontiac fever, tuberculosis, histoplasmosis, aspergillosis, and influenza.

  • Toxicoses: Exposure to airborne microbiological toxins (e.g., endotoxins, mycotoxins) can cause direct toxic effects. For example, humidifier fever—a flu-like illness—has been associated with exposure to microbiological toxins, although its etiology remains uncertain.

  • Hypersensitivity (i.e., allergic) diseases: Exposure to airborne allergens such as pollens, pet dander, dust mites, fungi, and pest droppings can cause allergic diseases such as asthma, hay fever, and hypersensitivity pneumonitis.[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] The costs to the U.S. economy in performance losses as a result of sick building syndrome has been estimated at $76 billion.[21]

Poor indoor air quality was the 6th most common complaint among corporate tenants in 2003.22 It increases absentee rates and results in 2% to 4% performance losses on average.[23] Furthermore, there have been several major lawsuits related to indoor air quality that have named builders, equipment manufacturers, and building owners as codefendants, and have resulted in multimillion-dollar settlements.[24]

Inadequate Building Protection and Vulnerabilities to Biological Threats 

The HVAC systems of most commercial buildings are not configured and maintained in ways that effectively reduce occupants' risk of exposure to biological threats.

Insufficient air filtration: The HVAC systems of most commercial buildings are not configured with air filtration systems that effectively remove biological contaminants (particularly bacteria and viruses) from the air.[3,4,15,16] The widely used Standard 62.1 (Ventilation for Acceptable Indoor Air Quality) produced by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) requires use of an air filter with a minimum efficiency reporting value (MERV) not less than 6; this is intended to protect HVAC mechanical equipment from contamination with relatively large particles such as lint and dust.[4,26]

Most commercial buildings use filters ranging from MERV 5 to MERV 8, which provides building occupants with little protection from biological threats because this level of filtration does not effectively remove micron-sized particles from the air.[4,27] It is not until MERV 13 or higher that particle removal in the size range of concern for biological threats begins to become significant (i.e., >90%).[4] For example, Table 1 (below) shows the estimated filtration rates for 4 different biological agents for MERV 6, 8, 10, 13, 15, and 16 filters. As can be seen, at MERV 13 >95% of Bacillus anthracis organisms, the causative agent of anthrax, are removed from the air.[3]

Table 1: Estimated Filtration Efficiency of Pathogen Removal for MERV-Rated Filters (%) a
OrganismMean diameter (μm)MERV 6MERV 8MERV 10MERV 13MERV 15MERV 16
Bacillus anthracis1.11815.536.739.296.399.9+99.9+
TB bacilli 0.6377.418.119.578.698.098.1
Smallpox virus0.2243.77.47.939.668.070.6
Influenza A Virus 0.0986.

Source: Adapted from Kowalski WJ, Bahnfleth WJ. Immune Building Technology and bioterrorism defense. HPAC Engineering 2003(Jan):57-62. Available at
a Based on models of MERV test results from two filter manufacturers

Improper installation and/or maintenance of filtration systems: Filters are frequently installed improperly and/or improperly maintained, resulting in filter bypass, which reduces filtration efficiency.[15,16, 28] One study that assessed filtration effectiveness in 57 commercial buildings located in 5 cities (Atlanta, GA; Birmingham, AL; Gainesville, FL; San Francisco, CA; and Washington, DC) found that “[n]one of the buildings . . . exhibited what could be considered “good” or “thorough” filter seal in all systems.”[16] The Environmental Protection Agency’s (EPA) Building Assessment Survey and Evaluation (BASE) Study, which was conducted over a five-year period from 1994-1998 to characterize determinants of indoor air quality and occupant perceptions in 100 randomly selected public and commercial office buildings in 37 cities in 25 states, found that among the buildings assessed, 44% of the study space air handling units had fair or poor general conditions (i.e., state of filter and filter rack frames), 23% had fair or poor filter fit into frames, and 47% had filters in fair or poor condition.[29]

Infiltration: Infiltration reduces the effectiveness of air filtration systems because air that enters a building via infiltration bypasses filtration systems.[15] Available evidence indicates that most commercial buildings are leaky relative to homes and that newer buildings are not more airtight than older buildings.15, 30,31 A “reasonable first order assumption for a typical leaky commercial building” is that the infiltration rate is equal to the air intake rate at the air handler.[15] The filtration efficiency in such a situation would be greatly reduced. Measurements in commercial buildings indicate that windows and doors account for a relatively small amount of air leakage (~10-20%).[15] Most leakage appears to occur between wall sections, wall-floor connections, corners, and the roof-wall interface.

Accessible air intake: Many commercial buildings have outdoor air intakes located at or near ground level, which increases the risk of occupant exposure to both intentional threats (bioterrorism), because of the air intake’s accessibility, and naturally occurring threats, because of the air intake’s proximity to potential contamination sources such as streets, alleys, parking lots, loading docks, and standing water.[3,15,32] EPA’s BASE study found that among the buildings assessed, 14% of the study space air handling units had air intakes located at ground level.[33]

*Note: The information that appears on the pages collectively known as "Protecting Building Occupants" was up-to-date and accurate when published in 2008; the materials have not been updated since original publication. Please be sure to check current resources for the most up-to-date information on this topic.



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