Day Three in the Office Containment: Air Changes, a Log Entry, and Clearance
A crew is removing 2,000 square feet of sprayed-on ceiling texture (RACM, friable, 4% chrysotile) from a 60 ft by 24 ft by 10 ft room in a Missouri office building. The contractor's plan requires at least 4 air changes per hour and a sustained pressure differential of -0.02 inches of water column, verified twice per shift. The setup uses two HEPA negative-air machines rated 1,200 cfm each, one magnehelic gauge, and a two-stage decontamination unit. On day 3 the day-shift supervisor turns off one machine because the containment is "too cold and drafty." The remaining machine's prefilter is loaded and measured flow falls to about 400 cfm. The gauge reads -0.005 in. w.c.; the supervisor writes "OK" on the log. A worker in a torn Tyvek suit is seen walking from the work area directly into the occupied hallway carrying a spray bottle.
1. Using the numbers, was the containment performing as the plan requires on day 3? Show the calculation.
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Run the numbers.
- Room volume: 60 x 24 x 10 = 14,400 ft3.
- Flow needed for 4 ACH: (4 x 14,400) / 60 = 960 cfm.
- Day-3 actual: one machine at 400 cfm gives (400 x 60) / 14,400 = 1.7 ACH - under half the plan minimum.
- Even at rated capacity, one 1,200 cfm unit gives 5 ACH, nominally above 4. But rated is not delivered: loaded prefilters cut flow well below the nameplate. The plan needs measured flow from a vane anemometer, not the label.
- Pressure: -0.005 in. w.c. is a quarter of the required -0.02 in. w.c. That differential is what keeps fibers inside; at that reading the barrier is barely negative, and the hallway may draw air from the work area whenever a door moves.
Conclusion: the containment was not compliant. Writing "OK" on the log was a falsification, not a judgment call. Work should have stopped when the second machine went off.
2. What is the correct response to the torn-suit exit, and what must be re-established before work resumes?
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Sequence matters.
- Stop work. No one enters or leaves by the dirty path; post someone at the decon unit.
- The worker in the torn suit goes through full decontamination: HEPA vacuum the suit, remove it, shower, clean clothes; bag the suit as asbestos waste. Notify him of the potential exposure and start exposure monitoring - under 29 CFR 1926.1101 he needs a respirator medical evaluation on file and results communicated to him.
- The hallway: assume contamination. Wet-wipe and HEPA vacuum the path he walked; never dry-sweep. Take PCM air samples before reopening and inform the building owner.
- Rebuild the containment: two working machines with fresh prefilters, measured flow at or above 960 cfm with one unit able to carry the load alone, a second gauge as a cross-check, and a leak check (smoke or tissue test) at the seams and decon joints.
- Retrain the crew on the decon protocol and file an incident report with photos and the log page.
- Resume only with both machines running and pressure recorded continuously, not once per shift.
3. Three aggressive PCM clearance samples come back at 0.006, 0.008, and 0.009 f/cc; the outdoor background is 0.007 f/cc; the supervisor reports the visual inspection passed. May the owner reoccupy?
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Not on this evidence.
- All three results are below the 0.01 f/cc PCM clearance criterion, but 0.008 and 0.009 leave almost no margin. PCM counts all fibers, and an outdoor background of 0.007 f/cc tells you the count is dominated by non-asbestos fibers. A result that close to the criterion against that background is not a defensible clearance.
- Visual inspection is a separate, non-waivable criterion: no visible dust or debris, documented by an accredited supervisor or inspector rather than by the crew that did the work.
- Clearance sampling must be done while the containment is still under negative pressure, with aggressive methods, and not in the middle of cleaning.
Next step: keep the containment up, re-clean, run the HEPA units, and either resample or go to TEM, which identifies asbestos fibers instead of counting everything. Because of the earlier breach, also sample the hallway and the decon exit. Reoccupancy happens on written clearance and the owner's acceptance - not because the crew is out of days.