The caging or housing system is one of the most important single elements in the physical environment of laboratory animals. Criteria have already been established to evaluate such systems. Except for systems that use laminar flow principles, most cages are passive because air exchange or ventilation occurs by natural rather than mechanical means. Thus, the air exchange rate in a cage is related to the room air distribution pattern, the air exchange rate, and the amount of heat and contaminants generated in the cage.

Temperature, humidity and ventilation rates (Table 1) for animal facilities have been specified on the assumption that the same environmental conditions of a room are maintained in a cage. Mainland and Herrera reported in 1954 that it may be statistically invalid to assume no difference between cage and room environments and that confounding of experimental treatments with cage effects may invalidate an experiment. Since then, other reports have identified thermal, gaseous, and particulate concentration differences between animal cages and the surrounding space.

Because no data exist to determine air exchange rates and other parameters of cage microenvironments relative to room conditions, we undertook the work reported here.