rather than providing replaceable filter elements in fixed permanent housings which require replacement by plastic bagging techniques , easily disposable and incineratable filter modules are removed and replaced as integral units while providing complete protection against contaminants from filtered exhaust air from enclosures containing airborne or potentially airborne hazardous materials , such as biological safety cabinets , fume hoods , cages for contaminated animals , nuclear facilities , etc . the disposable filter modules are removable and replaceable by a technique in which the modules are maintained under negative pressure via a filtered vacuum source at all times while the module inlets and outlets are open . sealing of the openings is by small sheets of polyethylene film , and no bagging such as in the known &# 34 ; bag in , bag out &# 34 ; technique is required .