Secure Buildings: Pre-Event – Immediate Advice: Air Intake
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Last Updated
April 27, 2005

Pre-Event – Immediate Advice

Prevent unauthorized access to air intakes; air exhausts might also be a concern.

A terrorist can quickly contaminate a building with a chemical or biological agent by introducing it into the building's ventilation system. This can be done even without access to the interior of the building through the building's air intakes. Some buildings have air intakes that are inaccessible (e.g. on the roof), but others have intakes that are vulnerable.

Even keeping the public a short distance away from the air intake may not provide complete security. For instance, a plastic bag containing anthrax spores could be tossed into an air intake from some distance away. Baffles over the air intake can make this type of attack less likely to succeed, but they might also affect the amount of outside air the building can pull in, and the energy efficiency with which this can be done, so they should not be installed without careful evaluation.

Contrary to the expectations of building designers and operators, many buildings take in large amounts of air through the building exhausts; this phenomenon occurs during warm weather, in buildings that don't have a return fan or that use volume-matching to control the return fan. Since building designers assume there will be no inward flow through the exhausts, exhaust registers are often placed on loading docks or adjacent to sidewalks, so they may be particularly vulnerable. Determine whether your building is subject to this phenomenon; if it is, consider restricting access to the exhausts, or modifying the return fan controls. Ensuring that there is never significant inward flow through the exhaust registers will prevent introduction of chemical or biological agents through this route.

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