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Protecting Building Occupants from Exposure to Biological Threats

Reducing Risks

Reducing the Risk of Occupant Exposure to Biological Threats

Security Upgrades: Physical security upgrades can prevent access to the HVAC system and other vulnerable building components, thus minimizing the opportunity for a deliberate internal release of a biological agent. Risk also may be reduced through adoption of new technologies, changes to buildings and building systems, and improved operational strategies, which include the following:

  • Ventilation system shutdown: Quickly shutting down a ventilation system to reduce occupant exposure after a release.
     
  • Outdoor air purge (dilution ventilation): Bringing 100% outdoor air into a building, circulating it, and purging it to the environment to dilute and remove contaminants from the air after a release.
     
  • Commissioning (and re-commissioning): Assessing a building’s ventilation system operation to ensure that it is operating in line with design intent.
     
  • Envelope tightening: Increasing a building envelope's air tightness to reduce the movement of air contaminants into a building via infiltration.
     
  • Pressurization: Over-pressurizing a building interior relative to outdoors to reduce the movement of air contaminants into a building via infiltration.
     
  • Enhancing filtration efficiency: Improving air filtration efficiency to increase removal of particulate matter from the air.
     
  • Isolating the air intake: Locating the air intake to reduce accessibility (e.g., high elevation).
     
  • Isolating special-use spaces: Maintaining common use spaces considered vulnerable to internal releases (e.g., lobbies, mail rooms, auditoriums) at a lower pressure relative to adjacent spaces to limit potential spread of a biological agent beyond the immediate release area.[1,3,4,15, 28, 31,34,35]

Applicability of protective mechanisms: Not all protective mechanisms are applicable to all building attack scenarios. Of note, proper use of ventilation system shutdown and outdoor air purge depends on the specific characteristics of a building and its HVAC system and the agent that has been released. If applied improperly, these techniques may increase exposure instead of reducing it (Table 1).[15]

Table 1: General Applicability of Protective Mechanisms for Risk Reduction in Four Attack Scenarios*
 Building Attack Scenario
Protective Mechanism

Large-Scale
Outdoor Release 

Air Intake
Release

Internal Release:
HVAC System

Internal Release:
Common Area/ Special Use Space

Ventilation System Shutdown

+/-

+/-

+/-

+/-

Outdoor Air Purge

+/-

+/-

+/-

+/-

Commissioninga

+

+

+

+

Envelope Tighteningb

+

N/A

N/A

N/A

Pressurizationb

+

N/A

N/A

N/A

Enhancing Filtration Efficiencyc

+

+

+d

+e

Isolating Air Intake

N/A

+

N/A

N/A

Isolating Special-Use Spaces

N/A

N/A

N/A

+f

*The extent to which any risk reduction measure will be protective is building specific and situationally dependent.
+/- Potential to increase exposure following a biological attack
N/A: Not applicable
a Is protective to the extent that it improves the reliability of HVAC system in support of other protective mechanisms; protective capacity is limited to design intent.
b Requires effective air filtration.
c Must address infiltration and filter bypass to realize full effectiveness
d Extent to which the distribution and spread of a biological agent can be reduced by filtration under this scenario depends, in part, on the HVAC system design and the point of entry of the agent.
e Will not protect the people present in the release area under this scenario, but it can reduce further exposures by reducing the distribution and spread of a biological agent beyond the release area as a result of its entry into HVAC system through the return air system (if present).
f Will not protect the people present in the release area under this scenario, but it can reduce further exposures by reducing the distribution and spread of a biological agent beyond the release area as a result of airflows created by pressure relationships that exist within different parts of the building relative to each other.

Source: Adapted from Persily, A. et. al. Building Retrofits to Protect Against Airborne Chemical and Biological Releases (NISTIR 7379). Washington DC: National Institute of Standards and Technology. March 2007. Available at https://www.nist.gov/publications/building-retrofits-increased-protection-against-airborne-chemical-and-biological.

Comparison of protective mechanisms: Each protective mechanism has its strengths and weaknesses, which should be taken into account when considering their implementation (Table 2).

Table 2: Comparison of Protective Mechanisms for Risk Reduction
Protective Mechanism AdvantagesDisadvantages
Ventilation System Shutdown
  • Potentially protective against all attack scenarios under certain circumstances
  • Requires timely situational awareness
  • Has potential to increase exposure if timing is off
  • May be difficult to implement
  • May require ventilation system modifications
Outdoor Air Purge
  • Potentially protective against all attack scenarios under certain circumstances
  • Requires timely situational awareness
  • Has potential to increase exposure if timing is off
  • May be difficult to implement
  • May require ventilation system modifications
Commissioning
  • Improves reliability of HVAC system; supports implementation of other protective mechanisms
  • Potential indoor air quality (IAQ) benefits
  • Potential for improved energy efficiency
  • Not able to offer additional protection beyond design intent
Envelope Tightening
  • Protective against outdoor releases of biological agents when combined with effective filtration
  • Potential IAQ benefits
  • Potential for improved energy efficiency
  • Requires effective filtration
Pressurization
  • Protective against outdoor release of biological agent when combined with effective filtration
  • Potential IAQ benefits
  • Improved envelope durability
  • Can be difficult to implement and maintain
  • May require ventilation system and building modifications
  • Increased energy consumption associated with conditioning additional outdoor air
  • Can lead to envelope moisture problems in cold climates
  • Requires effective filtration
Enhancing Filtration Efficiency
  • Protective against all attack scenarios
  • No potential to increase exposure
  • Always on
  • Relatively simple/available
  • Part of current practice
  • Potential IAQ benefits
  • Potential for improved ventilation system efficiency over time
  • Potential to improve building cleanliness
  • Potential to reduce extent and cost of decontamination
  • May require ventilation system and building modifications
  • Must address infiltration and filter bypass to realize full effectiveness
  • Increased pressure drop may result in increased energy consumption
Isolating Air Intake
  • Protective against air intake attack scenario–essentially removes threat
  • Potential IAQ benefits
  • Can be costly and impractical in retrofits
Isolating Special-Use Spaces
  • Can limit contamination spread throughout building after internal release
  • Does not protect those in isolated spaces
  • May require building modifications
  • Dedicated ventilation system may be required

Source: Adapted from Persily, A. et. al. Building Retrofits to Protect Against Airborne Chemical and Biological Releases (NISTIR 7379). Washington DC: National Institute of Standards and Technology. March 2007. Available at https://www.nist.gov/publications/building-retrofits-increased-protection-against-airborne-chemical-and-biological.

Expert Recommendations for Building Owners

Practical measures and available technologies to reduce risk: On June 13–14, 2005, the Center for Biosecurity of UPMC convened the Working Group on Reduction of Exposure to Infectious Agents during a Covert Bioterrorism Attack. The group was charged with recommending practical steps that building owners can take to reduce the risk of occupant exposure to biological agents following 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. With a focus on developing practical recommendations for improving or adjusting the functions of the heating, ventilation, and air conditioning (HVAC) systems in commercial and public buildings, the Working Group emphasized 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: Before implementing any measures intended to reduce the risk of exposure to occupants following a biological attack, it is imperative that building owners and operators fully assess the potential consequences of any risk reduction measures.[15,34] This includes understanding the specific building layout, activities (inside and out), and building systems and operations. Changes should not be made to buildings, building systems, and/or 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 are based on the Working Group on Reduction of Exposure to Infectious Agents during a Covert Bioterrorism Attack and can be immediately implemented by building designers, owners, and operators as appropriate:

  1. Minimize filter bypass: Seal, caulk, and gasket everything (filter cartridge, retainer bank, tracking, etc.) to prevent filter bypass.[1,15,28,35] The extent to which air filtration can reduce occupant exposure following a biological attack depends upon where the contaminant is released relative to the location of the filters and the occupants and the extent to which air flows through the filters.[15] The efficiency of any filter can only be realized if the air actually enters the ventilation system and passes through the filter media. If infiltration is a problem and filter systems are not properly sealed, air will bypass the filters and efficiency will be reduced.
     
  2. Commission: Commission buildings during design and construction, and re-commission routinely to ensure that ventilation systems are operating in line with design intent.[1,15] The extent to which a building’s ventilation system can be relied upon to reduce occupant exposure following a biological attack depends, in part, on the extent to which the ventilation system is operating within its design intent. Commissioning and re-commissioning is necessary to ensure that the building can provide the intended level of protection.
     
  3. Enhance filtration efficiency: Increase air filtration to the maximum MERV level that is economically justifiable to improve the removal of particulate matter from the air.[1,15,28,35] MERV 13 should be considered the minimal target MERV level because that is where particle removal in the size range of concern for biological threats becomes significant. For example, MERV 13 filters capture >90% of 1–3-micron particles entering the filter.[26]
     
    If retrofitting, existing filters should be replaced with the highest MERV filters possible within design/economic constraints. If the ventilation system cannot accommodate at least MERV13 filters, upgrading the HVAC system should be considered. For new construction, buildings should be designed for the maximum MERV level that is economically justifiable.
      
    Of note, the effectiveness of enhancing filtration efficiency can be greatly decreased by filter bypass, which can result from infiltration and/or poor air seal of filter installation.15 Accordingly, filter bypass issues must be addressed when enhancing filtration efficiency.[1,15,28,35]
      
    When enhancing filtration efficiency, pre-filters should be utilized when reasonable, relative to contaminant load conditions, operating life cycle costs, and design/economic constraints.[1,15,28,35] Pre-filtering is the use of lower MERV filters upstream of higher MERV filters to remove large particulate matter from the air before it reaches the higher MERV filters. Effective use of pre-filters can protect the performance and increase the life cycle of higher MERV filters, which can have positive economic benefits.[28,35] In retrofit situations it may not be possible to incorporate pre-filters into the building’s ventilation system due to design constraints.[15] However, if the ventilation system needs to be upgraded to accommodate enhanced filtration efficiency, then pre-filtration should be considered as a component of enhancing filtration efficiency.[15] For new construction, pre-filtration should be considered as part of an overall strategy of maximizing filtration efficiency to the extent economically justifiable.
      
  4. Maintain HVAC systems: Ensure that the HVAC system functions properly by conducting regular inspections and performing proper maintenance.[1,15,35]
      
  5. Train staff: Ensure that maintenance staff has the appropriate training to operate and maintain the HVAC system.[1,35]
      
  6. Tighten envelope: Tighten the building envelope to reduce the infiltration rate when economically feasible.[15,34,35] Of note, to be optimally effective in reducing potential exposures to biological threats, envelope tightening requires sufficient air filtration.[15]
      
    If retrofitting, the impact of infiltration should be assessed and the building envelope tightened when economically feasible.15 For new construction, buildings should be designed and constructed to have a tight envelope.
       
    Currently, there are no national standards for envelope tightness to guide the implementation of this recommendation. However, the ANSI/ASHRAE Standard 62.1-2007, Ventilation for Acceptable Indoor Air Quality and the ANSI/ASHRAE/IESNA Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) offer qualitative guidance on envelope tightening.
      
  7. Pressurize: Pressurize the building to reduce infiltration rate when economically feasible.[1,15,31,/] Of note, to be optimally effective in reducing potential exposure to biological threats, pressurization requires sufficient air filtration.[15]
      
    Whether a building can be pressurized depends upon the building’s geometry, HVAC system design, and envelope tightness, as well as weather conditions.15 Building pressurization requires that the HVAC system be able to deliver more air to the occupied space than is being exhausted and lost due to exfiltration.[15,31] It may not be possible to pressurize a leaky building without first addressing envelope leakage.[15]
      
    If retrofitting, the potential to achieve and maintain pressurization should be assessed, and the building should be pressurized when economically feasible.15 For new construction, buildings should be designed and constructed to achieve pressurization within design and economic constraints. There are currently no standard requirements established to guide the implementation of this recommendation and building pressurization strategies should be individually designed based on climate, building height, and envelope leakage.[31]

*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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