Safeguards provided by P3 physical containment have been used 
most extensively in the last two decades to ensure the safe conduct of 
research with organisms that cause serious disease in man. Bacteria 
that cause brucellosis, tuberculosis, and plague have been successfully 
contained, as have viral and rickettsial agents that cause rabies, 
lymphocytic choreomeningitis, Q fever, and typhus fever. These pro- 
cedures are also appropriate for the containment of research activities 
using smallpox virus and yellow fever virus when experimental animals 
are not involved. 
P4 Level ( High ). Physical containment providing the greatest safe- 
guards ToFTecfucmg the potential for accidental release of microorganisms 
are used at the P4 level. All research operations involving recombinant 
DNA materials are confined to Class III biological safety cabinets, which 
are gas-tight physical enclosures. Operations are performed through 
gloves attached to the cabinet. The cabinets are maintained under 
negative air pressure, and the exhaust air from the cabinets is either 
filtered through two sets of high -efficiency particulate air filters or is 
filtered once and incinerated. Under ordinary circumstances of operation, 
they provide an impenetrable barrier between the inside of the cabinet 
and the laboratory environment. 
The Class III biological safety cabinets are located in a maximum 
security facility providing secondary safeguards designed to further 
reduce the potential for escape of microorganisms to the environment. 
These features are shown in figures V-4 and V-5. The facility is 
either a separate building or a controlled area, within a building, that 
is completely isolated from all other areas of the building. Access to 
the facility is under strict control. 
Figure V-4 
P4 Laboratory 
