Negative Pressure Reading Drifting During a Long Removal Shift
A crew is removing asbestos-containing floor tile and mastic from a 40 ft × 60 ft room using wet methods and manual scrapers. The containment has a HEPA negative air machine with a manometer at the entry. At the start of the shift the manometer read −0.02 inches of water; by mid-afternoon it reads −0.005, and the worker notices the machine sounds louder and the exhaust feels weaker. The room door has been opened several times for supply runs, and the plastic sheeting on one wall has a small tear near the floor. The crew is two-thirds finished and wants to push through to completion before the weekend.
1. Interpret the manometer trend and the machine behavior. What is happening, and what should be done before work continues?
Read the worked answer
The rising (less negative) pressure reading and the louder, weaker exhaust indicate the negative air machine is losing effectiveness, most likely from a loaded HEPA filter, a blocked intake, or a duct problem. Combined with the opened door and the wall tear, the containment is no longer maintaining the intended negative pressure relative to adjacent spaces.
Before work continues:
- Stop removal work.
- Inspect the machine: filter loading, intake obstruction, duct connections, and power.
- Repair the wall tear and re-seal; establish a controlled entry/exit so the door is not propped open.
- Restore and verify negative pressure (re-check manometer) before resuming.
- If the machine cannot be restored promptly, replace the filter or unit.
The judgment is that a drifting manometer is a leading indicator of containment failure; it must be corrected before more dust is generated, not after the job is finished.
2. The crew wants to finish before the weekend. How should the supervisor weigh schedule against the containment problem?
Read the worked answer
Schedule does not override containment integrity. The supervisor should:
- Treat the pressure loss and the tear as a containment breach that must be corrected before removal resumes.
- Consider whether the breach may have allowed migration; if so, inspect adjacent areas and sample if warranted.
- Re-brief the crew on entry/exit discipline so the door is not left open.
- If the machine cannot be fixed, stop work and reschedule rather than generate dust in a compromised enclosure.
The reasoning: the purpose of negative pressure is to protect adjacent occupied space; continuing removal with a failed system converts a controlled job into an uncontrolled release, which is a far larger problem than a delayed completion.