6.6.3 Return-Air Systems

Similar to the outdoor-air intake, HVAC return-air systems inside the building can be vulnerable to CBR attack. Buildings requiring public access have an increased vulnerability to such an attack. Design approaches that reduce this vulnerability include the use of ducted HVAC returns within public access areas and the careful placement of return-air louvers in secure locations not easily accessed by public occupants.

The second objective is to design to minimize the impact of an attack. For many buildings, especially those requiring public access, the ability to prevent a determined terrorist from initiating a CBR release will be a significant challenge. Compared to buildings in which campus security and internal access can be strictly controlled, public-access buildings may require a greater emphasis on mitigation. However, even privateaccess facilities can fall victim to an internal CBR release, whether through a security lapse or perhaps a delivered product (mail, package, equipment, or food). Examples of design methods to minimize the impact of a CBR attack are listed below.

• Public access routes to the building should be designed to channel pedestrians through points of noticeable security presence.
• The structural and HVAC design should isolate the most vulnerable public areas (entrance lobbies, mail rooms, load/delivery docks) both physically and in terms of potential contaminant migration.
• The HVAC and auxiliary air systems should carefully use positive and negative pressure relationships to influence contaminant migration routes.

Further discussion of some of these prevention methods is provided below.