Page HI ■ 22 
Ventilation Design Handbook on Animal Research Facilities Using Static Microisolators 
walls reduces the CO 2 concentration from 1,764 ppm to 1,667 ppm, a reduction of only six 
percent. For the double density racks perpendicular to the walls the reduction is larger, but still 
only from about 2,300 ppm to 1,800 ppm (about 20 percent). 
Figure 3.26 Comparison of Room Breathing Zone CO 2 Concentrations (ppm) 
Increasing the room ventilation rate has a continuous beneficial effect on the room breathing 
zone ventilation (as measured by CO 2 and ammonia concentrations). For the single density racks 
parallel to the walls increasing the supply flow rate from 5 ACH to 20 ACH reduces the 
breathing zone CO 2 concentration from 140 ppm to 63 ppm, a decrease of 55 percent. For the 
double density racks perpendicular to the walls the reduction is even more dramatic, from over 
300 ppm to 93 ppm (over 70 percent reduction) when the flow rate is 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. The American Conference of Governmental 
Industrial Hygienists (ACGIH) recommends a threshold limit valve (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 (54,000 mg/ m 3 ). 
