Two HEPA Units, 43,200 Cubic Feet, and a Pressure Gauge That Won't Cooperate
An abatement contractor is removing 1,500 square feet of spray-applied acoustic ceiling material (2% chrysotile) from a closed school gymnasium measuring 60 feet by 40 feet with an 18-foot ceiling. The project design calls for a full enclosure, a minimum of 4 air changes per hour, and a pressure differential no less positive than -0.02 inches of water relative to adjacent spaces. The supervisor installs two HEPA filtration units rated at 2,000 cfm each, connected with 25 feet of 12-inch flexible duct; both discharge through the exterior wall. At 10:00 a.m. the air-monitoring technician reads -0.008 inches of water inside the enclosure and measures 1,100 cfm at each unit's exhaust. The gym has no dedicated make-up air, and the three-stage decontamination unit is the only intended air inlet. Two workers in full-face PAPRs are inside wetting and scraping. The supervisor wants to continue because 'negative pressure is negative pressure.'
1. Calculate the required and actual air changes per hour and determine whether the enclosure complies with the design.
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Containment volume. 60 ft x 40 ft x 18 ft = 43,200 cubic feet.
Required airflow. 4 air changes per hour means 4 x 43,200 = 172,800 cubic feet per hour, or 172,800 / 60 = 2,880 cfm delivered through the HEPA units.
Actual airflow. The technician measured 1,100 cfm at each exhaust, so 2,200 cfm total. That is 2,200 x 60 = 132,000 cubic feet per hour, or 132,000 / 43,200 = 3.06 air changes per hour.
Verdict. The enclosure fails on two independent criteria. It is at roughly 76% of required airflow (2,200 of 2,880 cfm), and the measured differential of -0.008 inches of water is less than half the -0.02 inches of water the design requires. Note also the gap between nameplate and field performance: the units were sold as 2,000 cfm each but delivered 1,100 cfm in place. Rated capacity is never the number to design or to defend with; only field measurement counts.
2. What is the most likely cause of the shortfall, and what corrective actions should be completed before removal resumes?
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Diagnosis. Air cannot leave a room faster than it enters. The only inlet is a decontamination unit with flap doors and small openings, so the fans are working against a starved enclosure; the pressure gauge confirms it, because a starved enclosure cannot hold a deep differential. Secondary contributors are the 25 feet of 12-inch flex duct, which is corrugated and adds substantial static loss, plus partially loaded HEPA filters and any crushed or kinked duct runs.
Corrections, in order.
- Provide engineered make-up air: frame a filtered opening sized for the full 2,880 cfm, with a HEPA filter on the supply side so unfiltered air is never drawn in.
- Add capacity: a third HEPA unit or larger units, so measured total exceeds 2,880 cfm with margin for filter loading over the project.
- Straighten and shorten duct runs, eliminate unnecessary elbows, and verify no flex duct is crushed against the wall penetration.
- Re-measure delivered cfm at each exhaust and re-read the differential at multiple points, including corners and the decon doorway, and confirm the gauge is calibrated and zeroed.
Do not restart until both the airflow and the differential meet the design and the numbers are recorded on the daily log with time, gauge readings, and instrument identification.
3. What are the exposure and program consequences if the supervisor continues at the current readings?
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Worker exposure. Reduced ventilation means fiber generated by scraping stays suspended longer and at higher concentration in the breathing zone. PAPRs protect the wearers, but respirator assigned protection factors are set assuming the enclosure performs; a containment running at 3 air changes per hour with a weak differential allows accumulation and secondary migration of settled dust back into the air as workers move.
Containment integrity. A differential of -0.008 inches of water is fragile. Door openings, worker traffic, and gusts against the exterior wall penetration can momentarily neutralize or reverse it, and any reversal pushes fiber into the corridor, the HVAC returns, or occupied parts of the building. The gym being closed protects occupants, but the escape path still reaches building spaces that will be reoccupied.
Program consequences. Continuing after the technician documented a design deviation converts an engineering problem into a knowing violation, and it destroys the project's defensibility at clearance. The supervisor should stop removal, keep the decon unit and enclosure intact, and report the deviation to the project designer and owner. Restart only after corrections are measured and logged, and consider inside air monitoring during the first hours of resumed work to confirm the fix worked rather than assuming it.