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Ventilation Design Handbook on Animal Research Facilities Using Static Microisolators 
• Increasing the room ventilation rate for a given design appears to have a beneficial effect on 
the room’s breathing zone ventilation as measured by CO 2 and ammonia concentrations (see 
figure 0.2). For the single density racks parallel to the walls increasing the supply flow rate 
from 5 ACH to 20 ACH reduced the breathing zone CO 2 concentration from 140 ppm to 63 
ppm. This is a decrease of 55 percent. For the double density racks perpendicular to the walls 
the reduction is even more dramatic. A reduction of over 70 percent, from over 300 ppm to 
93 ppm, occurred when the flow rate was increased from 5 ACH to 20 ACH. 
However the high values of CO 2 concentration of about 300 ppm above the background 
levels are still significantly too low. American Conference of Governmental Industrial 
Hygienists (ACGIH) recommends a threshold limit value (TLV), time weighted average 
(TWA) of 5,000 ppm (9,000 mg/m 3 ) for carbon dioxide. ACGIH also recommended a Short 
Term Exposure Limit (STEL) of 30,000 ppm (5,4000 mg/m 3 ). 
Figure 0.2 Room Breathing Zone CO 2 concentrations (ppm) for single density racks with the 
Thoren change station and double density racks with both Thoren and Laboratory Products 
change stations 
Although 20 ACH appears to produce the best ventilation for the low induction diffuser with 
low level exhaust cases compared with over 50 percent of the 15 ACH runs, other diffusers, 
exhaust locations or rack layouts actually have even lower room CO 2 concentrations. 
Higher supply discharge airflow rates are usually accompanied by higher supply discharge 
temperatures since the air does not have to be so cold to extract the heat generated in the 
room. This allows animal facility designers/operators to choose between using low supply 
volumes at lower supply temperatures, with a lower cost for fans but higher cost for air 
conditioning, or high flow rates at higher temperatures, which result in higher fan but lower 
