To be effective, a fume hood must confine contaminants within the hood, remove them through the ductwork, and filter or disperse them so that they do not return to the building through air intake systems. Materials exhausted from fume hoods are often disposed of by dilution into the atmostphere. Filters must be located upstream of the exhaust fan and must permit replacement without contaminating the surrounding environment.
Filters present an initial high cost and are associated with high maintenance associated.
Carbon filters also present present a pollution source during disposal. The installation of a charcoal filter prior to the HEPA filter may be considered as a measure to protect the HEPA filter from deleterious effects from chemical vapours and to protect the individual performing maintenance and certification testing of the HEPA filter. The CSA Z316.5-94 standard for fume hoods requires the performance of tests conducted on all fume hoods upon initial installation and periodically thereafter to maintain them in good operating condition. All requirements for test conditions, equipment accuracy and calibration, and outlined in the CSA standard must be followed. In AP containment level 2 facilities, BSCs are used for procedures with potential for producing infectious aerosols and with high concentrations or large volumes of infectious material.