84 
Properly installed biological safety cabinets, meeting appropriate NIH per- 
formance standards are used for all transfer operations and for all proce- 
dures likely to produce aerosols. Operations that produce large amounts of 
aerosols, such as blending, sonication, vortex mixing, and vigorous agita- 
tion, are avoided where possible. Where necessary, they are carried out in 
enclosed systems that are not opened until adequate time has been allowed for 
aerosols to settle. Gloves are worn during the handling of biohazardous 
materials. Vacuum lines are protected by filters. 
P3-level research can be conducted in laboratories where the room air 
flow and general exhaust air conditions described above cannot be achieved, 
provided that this work is contained in closed systems which are maintained 
under a negative pressure of at least 0.25 inches water-gauge. Operations 
including centrifugation, fractionation, sonication, etc. , are contained in 
these systems. All materials, before removal from these systems, are steri- 
lized or transferred to a sealed unbreakable container which is then removed 
from the system through a chemical decontamination tank, or after the entire 
system has been decontaminated. 
P4 Level (High) . Experiments involving recombinant DNA molecules re- 
quiring physical containment at the P4 level shall be confined to work areas 
in a facility of the type designed to contain microorganisms that are extreme- 
ly hazardous to man or may cause serious epidemic disease (Figure 4). The 
Figure 4 
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