including centrifugation , fractionation , sonication, etc., shall b6 con- 
tained in these systems. All materials, before removal from these systems, 
shall be sterilized 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) - Work areas are in a special facility of the type de- 
signed to contain highly infectious and hazardous microbiological agents (15). 
These areas are isolated by airlocks, a negative pressure environment, 
personnel clothing change and shower rooms, and treatment systems to 
inactivate or remove biohazardous agents contaminating exhaust air, liquid 
and solid wastes. All persons occupying these areas should wear protective 
laboratory clothing and shower at each exit from the facility. The handling 
of the biohazardous agents should be confined to biological safety cabinets 
in which the exhaust air is incinerated or passed through Hepa filters. 
C. Biological containment levels - Biological barriers are specific 
to each host-vector system. Hence the criteria for this mechanism of 
containment cannot be generalized to the same extent as ^or physical con- 
tainment. This is particularly true at the present time when our experience 
with existing host-vector systems and our predictive knowledge about projected 
systems is sparse. The classification of experiments with recombinant DNAs 
that is necessary for the construction of the experimental guidelines 
(Section III) can be accomplished with least confusion if we use the host- 
vector system as the primary element and the source of the inserted DNA as 
the secondary element in the classification. It is therefore convenient 
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