6.6.1 Air intakes
Air intakes may be made less accessible by placing them as high as possible on the building exterior, with louvers flush with the exterior (see figure 6-9). All opportunities to reach air-intakes through climbing should be eliminated. Ideally, there is a vertical smooth surface from the ground level to the intake louvers, without such features as high shrubbery, low roofs, canopies, or sunshades, as these features can enable climbing and concealment. To prevent opportunities for a weapon to be lobbed into the intake, the intake louver should be ideally flush with the wall. Otherwise, a surface sloped at least 45 degrees away from the building and further protected through the use of metal mesh (a.k.a. bird screen) should be used. Finally, CCTV surveillance and enhanced security is recommended at intakes.
In addition to providing protection against an air-borne hazard delivered directly into the building, placing air-intakes high above ground provides protection against ground-based standoff threats because the concentration of the air-borne hazard diminishes somewhat with height. Because air-blast pressure decays with height, elevated air intakes also provide modest protection against explosion threats. Furthermore, many recognized sources of indoor air contaminants (e.g., vehicle exhaust, standing water, lawn chemicals, trash, and rodents) tend to be located near ground level. Thus, elevated air intakes are a recommended practice in general for providing healthy indoor air quality.
In the event that a particular air intake does not service an occupied area, it may not be necessary to elevate it above ground level. However, if the unoccupied area is within an otherwise occupied building, the intake should either be elevated or significant precautions (tightly sealed construction between unoccupied/occupied areas, unoccupied area maintained at negative pressure relative to occupied area) should be put in place to ensure that contaminants are unable to penetrate into the occupied area of the building.