Basic Documentation

Room Pressurization Applications
Biosafety Level 4 (BL-4)
Biosafety Level 4 (BL-4) is the classification that applies to laboratories
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that present the
highest risk to individuals in the laboratory, the facility, and to nearby communities. As such,
they must be designed in accord with very strict safety requirements. BL-4 laboratories
require all of the BL-3 provisions plus use of the highest classification of biological safety
cabinet (Class III glove box) for all work performed. These labs must be geographically
isolated and functionally independent from the rest of the buildings associated with a facility
having this type of laboratory.
BL-4 laboratories require a 100% outside air, non-re-circulating dedicated ventilation system
with HEPA filtered intake and exhaust air. The laboratory itself must be at a relatively high
negative static pressure and the adjacent support areas, such as gowning, showering, etc.,
are somewhat less negatively pressurized. However, all areas associated with the laboratory
must also be at a negative static pressure (although at a somewhat lower level) with respect
to the other non-laboratory areas of the building.
Hospitals
Operating rooms, intensive care units, nurseries, and certain other areas in hospitals are
normally positively pressurized to prevent harmful pathogens (germs) from entering these
rooms. In these instances, transfer air can only flow out from the positively pressurized room
to prevent or at least retard airborne contaminants from entering the room. Certain patient
rooms, particularly those for treating AIDS patients and any patient that is at a high risk of
infection, are also maintained at a positive pressure (referred to as protective isolation rooms)
to ensure patient protection via proper directional airflow.
Hospital rooms may also be negatively pressurized (referred to a infectious isolation) to
prevent airborne pathogens from patients with a contagious disease, such as Tuberculosis,
from migrating out and infecting hospital workers or other patients. In these instances, the
room’s negative pressurization ensures that the direction of transfer airflow is always into the
infectious isolation room.
Animal Holding Rooms
In general, rooms used to house animals and perform research functions must be kept at a
positive static pressure level. A positive static pressure level prevents the animals from being
contaminated by airborne pathogens that can enter the room. In contrast, research facility
support areas, such as cage washing, or where contaminated or animal waste material is
present, should be kept at a negative static pressure. A negative static pressure level
prevents odors from emanating and prevents the airborne transmission of harmful
substances to other areas of the facility.
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Relatively few BL-4 laboratories exist since they deal with exotic, highly dangerous (with no known cure) infectious substances.
Such laboratory facilities are usually isolated from other buildings and protected with a sophisticated security system to ensure
against any unauthorized access.
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