Glossary
Negative Pressure Enclosure
Also: NPE
A temporary containment around an asbestos work area, maintained at lower air pressure than the surrounding space so that air flows in but not out, preventing fiber escape.
A negative pressure enclosure (NPE) is a temporary containment around an asbestos work area, built from polyethylene sheeting on a temporary framing or supported by the existing room structure, and maintained at a lower air pressure than the surrounding space by HEPA-filtered negative air machines.
The pressure differential (commonly at least minus 0.02 inches of water column) means that any air movement through tears or gaps in the containment is inward, not outward. Asbestos fibers released during the work stay inside the enclosure.
Negative air machines exchange the enclosure’s air at a rate that achieves at least four air changes per hour (project specifications often require more). The exhaust passes through HEPA filtration before being discharged outside.
A Decontamination Unit is attached to the NPE so workers can shower and change clothes when leaving the work area. Pressure differential is monitored continuously, often with manometers that alarm on loss of negative pressure.
NPE setup is required for Class I Work of significant scope. Smaller Class I jobs may use alternative work practices like glove bags or mini-enclosures.