Four Out of Five Isn't Clearance: Reading a Failed School Clearance
An accredited contractor finishes removing 800 square feet of vinyl asbestos floor tile and mastic from two classrooms in a Montana elementary school. The work is Class II, performed inside a containment with critical barriers over the HVAC diffusers. The project designer specified AHERA-style clearance: a thorough visual inspection, then aggressive air sampling analyzed by phase contrast microscopy, with clearance at 0.01 fibers per cubic centimeter. The technician collects five samples inside the containment 30 minutes after final cleaning, using pumps set at 2.0 L/min for 200 minutes. Results are 0.008, 0.006, 0.004, 0.009, and 0.012 f/cc. The outdoor blank reads 0.003 f/cc. Post-calibration shows four pumps between 1.98 and 2.02 L/min and the pump that produced the 0.012 result at 1.70 L/min. The cassette for the 0.004 sample arrived at the laboratory with its end cap detached. The contractor argues the school should accept clearance because 'four out of five passed.'
1. May the school accept clearance on these results? Analyze the numbers.
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The criterion. Clearance under the AHERA framework requires that air samples show fiber concentrations below 0.01 f/cc after the visual inspection has passed. The 0.012 f/cc sample is above the limit on its face, so the data set does not support clearance.
The average does not rescue it. The five results sum to 0.039, an average of 0.0078 f/cc. A contractor will argue that the average is below 0.01. That argument fails for two reasons: the 0.012 is not a marginal overage caused by counting noise, and the sample is one of the two compromised samples in the set. An average computed from invalid volumes is not evidence of anything.
The blank. The outdoor blank at 0.003 f/cc is high enough to be worth a second look. A blank in that range suggests either background fiber in the area, contamination during handling, or a lab counting issue, and it undercuts any claim that the 0.012 is a fluke. The lab should be asked for the blank handling record and the analyst's count sheets.
Conclusion. Clearance cannot be granted. 'Four out of five' is not a standard that appears anywhere in the regulations or the project design.
2. Which samples are technically invalid, and how does the pump calibration data affect sample volume?
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Flow tolerance. A pump calibration that drifts more than 5% between pre- and post-sampling invalidates the assumption of a constant flow rate. Four pumps moved from 2.0 L/min to between 1.98 and 2.02 L/min, a drift of 1% or less, so a 400 L volume (2.0 L/min x 200 min) is a fair approximation and those samples stand on volume, leaving the 0.012 result as a real exceedance rather than a calculation artifact.
The 0.012 sample. Its pump fell from 2.0 to 1.70 L/min, a 15% drop, well outside tolerance. The true volume is lower than 400 L and cannot be reconstructed reliably from end-point calibration alone. The result is analytically suspect and cannot be used either to clear or to condemn the space; it must be replaced.
The 0.004 sample. A detached end cap on arrival at the lab means the cassette was open to ambient air somewhere between the containment and the microscope. That sample is invalid regardless of its low value, because the low value itself may be dilution or loss.
Also check the method. Clearance in a school requires aggressive sampling to suspend settled dust. If the technician sampled quietly with the containment at rest, all five results understate what will be airborne when children return, and the entire set should be repeated.
3. What must happen before a valid clearance can be granted, and what should be documented?
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Sequence.
- Re-enter and re-clean: HEPA-vacuum all surfaces, wet-wipe the floors and ledges, and re-inspect visually. Clearance starts with a visual pass, and any dust or debris found means the air sampling is premature.
- Repair the containment and verify it: critical barriers intact, decon unit functioning, HEPA units running and measured.
- Run aggressive sampling properly, using forced air to stir settled dust before and during sampling, with a full set of pumps calibrated immediately before and after, each drawing a known volume, plus a field blank handled identically.
- Use calibrated pumps with flow that stays within 5%, and confirm cassette integrity at the lab bench, not after a truck ride.
Documentation. Record the visual inspection with date, time, and inspector; instrument identification and calibration records; sample locations sketched on the containment plan; pump start and stop times and pre/post flows; chain of custody and lab accreditation; the blank result; and the written clearance determination signed by the responsible party.
Communication. Tell the school in writing why the first set failed and what changed. The district will be asked about this at the next board meeting, and a one-paragraph explanation with the data attached is far better than silence.