The microenvironmental conditions of a commercial rodent filter top caging system was evaluated when housing mice for a 2 week testing period, with an open cage with no filter used as a control. The results were compared to the effect of utilizing a novel developed forced-air ventilated system (Vent A Cage, Barriärteknik, Brösarp, Sweden) for filter cages, delivering filtered air directly to the cage and providing approximately 150 air changes per hour.In an animal room maintained at 21—22°C, 50—55 0/0 RH and 12—14 airchanges per hour the following experimental settings were evaluated: Micro-Isolator® cages of Macrolon ® type III without forced air (Standard), housing 4, 8 or 16 female mice with body weights ranging between 23—28 g were compared to two cages housing 16 mice each: a Micro-Isolator provided with an individual Vent A Cage system (Forced-air) and a cage without a filter lid (Control).Intracage RH, C02 and N113 levels were evaluated once a day in each caging system during a 2 week period. Bedding and feed were not removed during the testing period, but water bottles were Changed on day 6 and 12. Control measurements of intraeage temperature of filter cages showed only small deviations from actual room temperature throughout the study (data not shown).RH and carbon dioxide levels ranged between 70—100 % and 4,000—17,000 ppm respectively in standard cages with 8 and 16 mice, and between 60—75 % and 2,000—5,000 ppm respectively in the standard cage with 4 mice. The open control andthe forced-air cages housing 16 mice showed levels of RH and C02 similar to the maeroenvironment (animal room); the forced-air system averaged 40—60 % and 1,000—2,700 ppm, respectively.No appreciable ammonia levels were detected in either system until day 14. In the standard filter cage housing 16 mice the ammonia levels increased from about 7 ppm (traces) on day 13 to 650 and 700 ppm on day 14 and 15, respectively.The results indicate that housing of Z 8 adult mice in a Macrolon 111 Micro-Isolator cage within a day leads to a buildup of humidity and C02 levels in excess of recommended levels for mice. The comparatively late occurring ammonia levels detected in the present study as compared to other studies of filter top cages may be explained by the type and quality of bedding used. The forced-air system evaluated provided acceptable intracage air conditions with 16 mice for two weeks without cage changing and resulted in a remarkable improvement of the microenvironmental conditions as compared to a standard filter top cage
