8 
handling of recombinant DNA molecules shall be contained 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." 
P3 "properly installed biological safety cabinets, meeting appropriate 
NIH performance standards ," for the purposes of the comparative evaluations 
in this report, are approximately equal to cabinets referred to in the 
literature, safety manuals, and manufacturers' advertising, as (1) Class I 
cabinet (inward airflow of 60 to 100 linear feet per minute through a variable 
or fixed (8 to 10 inch) cabinet-wide opening for the hands; (2) Class II 
cabinet (partially recirculated sterile air in a downflow cabinet with an 
inward airflow and fixed or variable opening as in the Class I); (3) Bio- 
safety cabinet; (4) Biological safety cabinet; (5) Microbiological safety 
cabinet; (6) Biohazard safety cabinet; (7) Open face safety cabinet; or 
(8) Biogard hood. In all these, the exhaust air passes through a microbio- 
logical filter. 
Negative pressure environment . This is a design in which the air flows 
from outside the building, into the "clean" area, if any, into the laboratory 
corridors, and into the laboratory rooms, from which the air is exhausted. 
In some special situations, there also is an exhaust in airlocks, but usually 
not. 
At this point, because of the definition of P3 and P4, it seems pertinent 
to comment on the value of secondary barriers. Secondary barriers become 
especially important to personnel within the building when such agents as 
Coxiella burneti , experimental aerosols, or micronized dry powders are handled, 
or when there are centrifuging, griding, or similar aerosol -producing 
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