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Ventilation Design Handbook on Animal Research Facilities Using Static Microisolators 
earlier reports, such as M.R. Gamble and G. Clough (1976), indicated ammonia 
values > 200 ppm, recent studies testing cage densities of four mice/microisolator 
cage, such as G.C. Choi, J.S. McQuinn, B.L. Jennings (1994), and J.J. Hasenau, R.B. 
Baggs, and A.L. Kraus (1993), are consistent with the observed data that ammonia 
concentration can be low. Two possibilities may explain why the ammonia 
concentrations were low. The first is that improved cage washing procedures and 
animal room cleanliness may reduce concentrations of ammonia-producing bacteria, 
resulting in lower ammonia concentration. In this facility, the cages are washed with 
water at 82 ±6 °C followed by a dry heat cycle at 118 °C. The second possibility is 
that formation of ammonia may be dependent on the strain of mice. 
• Temperature inside the cages did not change with increasing ventilation. The 
temperature inside the cages was 1 to 3 °C higher than the room temperature, an 
observation consistent with an earlier study that proved the use of a filter top results 
in a temperature increase of about 2 °C in the cage. 
• Humidity inside cages significantly decreased with increasing ventilation, from 55 
percent relative humidity at 5 ACH to 36 percent relative humidity at 20 ACH. 
Humidity inside the cage increased compared with humidity in the room, and the 
average difference between room and cage RH was about 10 percent, similar to the 
increased cage humidity previously observed by M.L. Simmons, D.M. Robie, J.B. 
Jones, and L.J. Serrano (1968). 
• Carbon dioxide concentration decreased from 2,500 ppm to 1,900 ppm when 
ventilation rate increased from 5 ACH to 10 ACH, but no further significant decrease 
was observed at 20 ACH. In conclusion, increasing the room ventilation rate higher 
than 5 ACH did not result in significant improvements in the cage microenvironment. 
Guide For The Care And Use Of Laboratory Animals , Institute of Laboratory Animal Resources, 
1996, National Research Council, National Academy Press, 23-55. 
Microenvironment and macroenvironment: The microenvironment of an animal is the 
physical environment immediately surrounding it, the primary enclosure with its own 
temperature, humidity, and gaseous and particulate air composition. The physical 
environment of the secondary enclosure, such as a room, a bam, or an outdoor habitat 
constitutes the macroenvironment. Although the microenvironment and the 
macroenvironment are linked by ventilation between the primary and secondary 
enclosures, the environment in the primary enclosure can be quite different from the 
environment in the secondary enclosure. It is affected by the design of both enclosures. 
Measurement of the characteristics of the microenvironment can be difficult in small 
primary enclosures. Available data indicate that temperature, humidity, and 
concentrations of gases and particulate matter are often higher in an animal's 
