.7 
PI level (minimal) - Requires standard microbiological practices 
(see A above) . 
P2 level (low) - In addition to the PI procedures, this level re- 
quires the following: (i) Access to the laboratory is controlled when 
handling potentially hazardous organisms requiring P2 containment, but 
not otherwise. During the controlled period appropriate biohazard signs 
should be posted at access points to the laboratory, and only authorized 
persons who have been advised of the potential biohazard should enter when 
such signs are posted. The signs may be removed upon completion of de- 
contamination after the hazardous procedures. Experiments of lass 
biohazard can be carried out concurrently in carefully demarcated areas of 
the same laboratory if necessary, given that adequate precautions are 
taken to avoid cross-contamination and no animals or plants are in the 
area.' (ii) Mouth pipetting is prohibited; mechanical pipetting devices 
are required, (iii) Specific precautions are required for those procedures 
that have a high potential for release of aerosols containing potentially 
hazardous material, e.g., centrifugation should be carried out in airtight 
cups or rotors (8); sonication and blending should be similarly contained 
or carried out in appropriate enclosed biological safety cabinets, (iv) All 
potentially hazardous material must be inactivated before it enters the 
drainage system. 
P3 level (moderate) - In addition to the P2 procedures, this level 
requires the following: (i) Operations with potentially hazardous organisms 
1 
For example, interaction between vectors containing cloned DNA fragments 
and permissive host strains used in the same laboratory during cell growth, 
storage, or disposal might lead to escape from the biological containment. 
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