Tiers of cages (90 are spaced above the floor (10,11). The ceiling (6) of the room (7 or 8) has several separate vent apertures (26) spaced apart, with sliding doors (27) for adusting their effective open areas. Exhaust fans (15) are recessed in the ceiling (6). They expel air from the room to the exterior of the building (1). Expelling air from the room reduces the pressure and so outside fresh air is drawn into the room through the vent apertures (26). The apertures coordinate with the exhaust fans (15).
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