6 
four levels are associated with the "Classification of Etiologic Agents 
on the Basis of Hazard" (5), and with the "Summary Statement of the 
Asilomar Conference on Recombinant DNA Molecules" (3); and the National 
Cancer Institute uses three for research on oncogenic viruses (6). We 
emphasize that these are an aid to, and not a substitute for, good technique 
Personnel must be competent in the effective use of all equipment needed 
for the required containment level as described below. We define only 
four levels of physical containment here, both because the accuracy with 
which one can presently assess the biohazards that may result from recombi- 
nant DNAs does not warrant a more detailed classification, and because 
additional flexibility can be obtained by combination of the physcial 
with the biological barriers. Though different in detail, these four 
levels (PI < P2 < P3 < P4) approximate those given for the classification 
of etiologic agents (i.e., classes 1 through 4; ref. 5), in the Asilomar 
summary statement (i.e., minimal, low, moderate, and high; ref. 3), and 
by the NCI (low, moderate, and high; ref. 6), as is indicated by the P- 
number or adjective in the following headings. It should be emphasized 
that the descriptions and assignments of physical containment detailed 
below are based on existing approaches to containment of hazardous organisms 
We anticipate, and indeed already know of, procedures (14) which 
enhance physical containment capability in novel ways. For example, 
miniaturization of screening, handling and analytical procedures provides 
substantial containment of a given host-vector system. Thus, such 
procedures should reduce the need for the standard types of physical 
containment. 
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