80 
standpoint. However, any consideration of use of such materials should be 
evaluated carefully from the standpoint of build-up to dangerous levels 
and problems of decontamination of the cabinet. 
The Class III cabinet is a closed-front ventilated cabinet of gastig’nt 
construction providing total protection for personnel and product from 
contaminants exterior to the cabinet. The cabinet is operated under a neg- 
ative pressure of at least 0.5 inches vater-guage. All supply air is HEPA- 
filtered. Exhaust air is HEPA-f iltered or incinerated to protect the 
environment. This cabinet, fitted with arm-length rubber gloves, provides 
the highest containment of these three classes of cabinets and is utilized 
for all activities involving high-risk agents (i.e., CDC etiologic agents, 
class 4). 
Now I will discuss the four levels of physical containment described 
in the guidelines. 
PI Level (Minimal) . A laboratory suitable for experiments involving 
recombinant DNA molecules requiring physical containment at the PI level is 
a laboratory that possesses no special engineering design features (Figure 
1 ). 
Figure 1 
PI Laboratory 
[221] 
