Objective design criteria for control of cage microenvironments can only be obtained after the required cage conditions are specified and the functional relationships between the cage microenvironment and the surrounding macroenvironment are known. Cage air exchange is the primary means of maintaining gaseous and particulate concentrations within acceptable limits while heat generated in the cage can be dissipated by convection due to air exchange or by heat· transfer through the cage surfaces. Since the heat transfer coefficients of the cage surface areas are dependent on the air velocities at the inner and outer surfaces, few cages available today can be assumed to provide microenvironments independent of the room macroenvironment. If conditioned air is supplied directly to the cage, room dependency of the microenvironment will be relatively small. However, if cage air exchange occurs due to natural convection currents or by room air distribution patterns, the cage microenvironment will be highly dependent on the room conditions. The former system has been identified as a supply-coupled system (SCS) and is used in special laboratory situations such as housing systems for germfree and gnotobiotic animals. The latter system has been identified as a room-coupled system (RCS) and is commonly used in most laboratory situations.

The air exchange rate in an SCS can be explicitly determined, but in the RCS, the air exchange rate is dependent of the cage heat load, room air distribution system, and cage location within the room. Because of the complexity of the variables, cage air exchange rates in prototype dog cages were determined experimentally. Resultant performance comparisons are described below.