Vertical circulation in facilities without direct access to ground-level transportation and multilevel animal facilities should include a minimum of two dedicated freight elevators, one for transporting "clean" items and one for "soiled" items, and more importantly, one for backup, while the other is being serviced. The focus of traffic flow in an animal facility revolves around the cage sanitation facility and the flow of cages between it and the animal rooms. The horizontal circulation pattern to be used is one of the early decisions to be made in the facility planning process. There are two basic horizontal circulation patterns, single corridor and dual corridor. Dual corridors are also known as "clean-dirty" corridors. The objective of the dual corridor circulation pattern is to decrease the potential for cross contamination between animal rooms. Theoretically, dual corridors are superior to single corridors in terms of reducing cross contamination; however, as compared to single corridors, they come at a high cost in terms of the ratio of animal housing space to circulation space. Figure 8.2 illustrates this point. Whether or not dual corridors are cost-effective is a complex issue, and the answer will vary according to the relative weight assigned to each of the many pros and cons.9 Clearly each has advantages, disadvantages, and limitations (Table 8.1). Few would disagree that a dual-corridor plan is the best choice if cost and space are not an issue. However, many single-corridor barrier and containment facilities appear to function effectively in terms of providing adequate contamination control.

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