8 
that require P3 containment should be carried out in a laboratory that 
is separated from areas where less than P3 containment is practiced. The 
ventilation system shall be balanced to provide for an inflow of supply 
air from the adjacent corridor into the laboratory through the access 
doorway. This supply air flow shall be in excess of 50 cubic feet per 
minute (15). The general exhaust air from these laboratories should be 
discharged outdoors and dispersed to the atmosphere to prevent re-entry 
into the building. If recirculated , the air must be decontaminated. 
Appropriate biohazard signs should be posted at access points to the 
laboratory, and only persons specifically authorized by the principal 
investigator should enter when these signs are posted. Normally, author- 
ized persons should be limited to those who work in the laboratory. 
(ii) Properly installed biological safety cabinets, meeting appropriate 
NIH performance standards (16), should be used for all transfer operations 
and for all procedures likely to produce aerosols. Operations that produce 
large amounts of aerosols, such as blending, sonication, vortex mixing, 
and vigorous agitation, should be avoided where possible. Where necessary, 
they must be carried out in enclosed systems that are not opened until 
adequate time has been allowed for aerosols to settle, (iii) Gloves 
should be worn during the handling of biohazardous materials, (iv) Vacuum 
lines should be protected by filters. 
P3 level research can be conducted in laboratories where the room air 
flow and general exhaust air conditions described above cannot be achieved, 
provided that this work is contained in closed systems which are maintained 
under a negative pressure of at least 0.25 inches water gauge. Operations 
[791 
