Containment, Work Area Preparation, and Engineering Controls
What to know
Containment separates the work from occupied space. Critical barriers seal openings (HVAC, doors, windows), and a negative pressure enclosure uses HEPA-filtered negative air machines to keep the work area under negative pressure relative to adjacent spaces, typically with a pressure differential around -0.02 inches of water and enough air changes to clear the space. Polyethylene sheeting covers floors, and where required, walls and ceilings; seams are taped and fire-retardant plastic is often specified.
Three-stage decontamination units (clean, shower, dirty/equipment rooms) are standard for large or high-risk jobs. The regulated area is posted with warning signs and limited to authorized, trained workers. Engineering controls include wet methods, HEPA vacuums, glove bags for pipe work, and local exhaust. Negative air machines must be sized to the enclosure volume and monitored; exhaust must discharge to a controlled location, not into occupied space.
Traps
Questions test whether you know negative pressure is verified with a manometer or smoke test, not assumed. Another trap: putting the shower between the dirty room and the clean room in the wrong order, or forgetting that HEPA exhaust must be filtered before discharge. Also watch for claims that poly alone suffices without negative pressure.
Rule of thumb
Air moves from clean to dirty: place the HEPA exhaust so air is drawn from the clean side of the enclosure through the work zone and out through the filter.