6.2 Negative Pressure Enclosures: Pressure Differentials, Air Changes, HEPA Air Filtration & Manometer Monitoring

6.2 Negative Pressure Enclosures & HEPA Air Filtration

Quick Answer: A negative pressure enclosure exhausts filtered air out of the work area faster than air leaks in, so every gap pulls clean air inward. OSHA (29 CFR 1926.1101(g)(5)(i)) requires at least 4 air changes per hour and at least -0.02 inch of water column (w.c.). New Jersey's subcode requires one complete air change every 15 minutes and at least 0.03 inch w.c. in unoccupied buildings or 0.05 inch w.c. in occupied buildings. The differential is monitored with digital manometers with continuous printout. HEPA filters capture 99.97% of 0.3-micrometer particles.

Why Negative Pressure Works

Air moves from higher pressure to lower pressure. When HEPA-equipped air-filtration units exhaust air from a sealed work area to the outside, the pressure inside drops slightly below the pressure in the surrounding building. Any leak then draws building air into the enclosure instead of letting contaminated air escape. You can see this when poly walls bow inward. A pressure differential alone does not remove fibers, though. The air changes carry suspended fibers to the HEPA filters.

Federal and New Jersey Numbers Compared

The subcode states that nothing limits maximum exhaust capacity, and project specifications may require more air changes. On a New Jersey school or public-building project, the subcode numbers control. On a private commercial job, OSHA's -0.02 inch and 4 air changes are the federal minimums, and project specifications often require more.

Units: 0.02 inch w.c. is about 5 pascals, 0.03 inch is about 7.5 pascals, and 0.05 inch is about 12.5 pascals.

Sizing HEPA Air-Filtration Units

The Formula

1. Volume: V = length x width x height (cubic feet).
2. Required airflow for 4 air changes per hour: CFM = (V x 4) / 60 = V / 15 One complete air change every 15 minutes gives exactly this figure.
3. Number of units: Required CFM / actual available output per unit, rounded up.

Actual Versus Rated Output

New Jersey requires the AST to calculate the required number of units based on the actual available output, not the rated output. The AST repeats the calculation whenever the work-area volume changes, and reports any shortfall to the owner, the contractor, and the project designer (N.J.A.C. 5:23-8.10(d)3). Units must be rated by the manufacturer for actual working capacity and field tested (N.J.A.C. 5:23-8.15(f)8). Loaded filters, duct runs, and bends all reduce airflow below the nameplate rating. In occupied buildings, the AST must verify exhaust capacity by field measurement. A filter pressure-drop gauge is not a substitute (N.J.A.C. 5:23-8.19(c)2).

Worked Example

A room is 80 ft x 50 ft x 10 ft = 40,000 cubic feet.

• Required airflow = 40,000 / 15 = 2,667 CFM.
• If field measurement shows each "2,000 CFM" unit actually delivers 1,400 CFM through its duct, then 2,667 / 1,400 = 1.9, so 2 units are needed.
• If the room expands to 60,000 cubic feet, the requirement is 4,000 CFM, and 4,000 / 1,400 = 2.86, so 3 units are needed.

Always round up. Having one more unit than the minimum helps hold the differential when filters load.

During Encapsulation

When encapsulants are sprayed on ACM (Section 7.3), the subcode requires enough HEPA units for one complete air exchange every 30 minutes (N.J.A.C. 5:23-8.16).

HEPA Filtration

• The subcode defines HEPA as a filter capable of 99.97% efficiency down to 0.3 micrometers (N.J.A.C. 5:23-8.2). Units must filter asbestos fibers to 0.3 micrometers at 99.97% efficiency (N.J.A.C. 5:23-8.15(b)9).
• Typical units use a coarse pre-filter and an intermediate filter to protect the more expensive HEPA filter. Replace pre-filters often. Change filters as needed to maintain airflow. In occupied buildings, the ASCM plan must provide for filter changes that keep the pressure differential around the clock (N.J.A.C. 5:23-8.19(c)6).
• Air-filtration units run at all times during the project (N.J.A.C. 5:23-8.15(b)10).

Air Flow Path and Exhaust

The subcode (N.J.A.C. 5:23-8.15(b)10) requires:

• Air flows into the work area through all openings: the decontamination chamber, waste-exit ports, any leak points, and controlled make-up air inlets.
• Air is exhausted through the HEPA units by flexible or solid duct leading outside the building.
• Air-filtration units are placed as far as possible from the decontamination chamber, so clean air sweeps through the entire work area.
• A HEPA-equipped vacuum truck or additional HEPA units may be used when needed to maintain the differential.

In occupied buildings, exhaust must go outside. If that is not feasible, it passes through a second set of in-line air filtration devices and discharges only into spaces the ASCM approves. In buildings over four stories, make-up air may not be drawn through openings in separation barriers unless devices allow flow only toward the work area (N.J.A.C. 5:23-8.19(c)2).

Monitoring the Differential

• Instruments: Digital manometers with continuous printout, or other approved low-pressure monitoring devices. The AST zeroes and levels the gauges each time a reading is taken.
• Placement: The ASCM firm installs one or more separate monitoring systems near the work-area entrances and between the work area and any interior spaces supplying make-up air. At least one system is required for every 10,000 square feet of separation surface next to the work area. Sensor tubes must be placed so air-filtration devices do not cause false readings.
• Frequency: The AST checks the differential at the beginning of each work shift, every four hours thereafter, and at the end of the contractor's workday (N.J.A.C. 5:23-8.10(d)4).
• Shift end: In occupied buildings, the AST and the contractor's supervisor confirm containment integrity before workers leave.

When Pressure Drops

Exam Traps

• Which number applies? OSHA uses -0.02 inch w.c. The New Jersey subcode requires 0.03 inch in unoccupied buildings and 0.05 inch in occupied buildings.
• Air changes: "Once every 15 minutes" and "4 per hour" are the same rate. During encapsulation spraying, the subcode rate is once every 30 minutes.
• Rated CFM: Units are sized on actual output.
• HEPA: 99.97% at 0.3 micrometers. Answers citing 95% or 1 micrometer are distractors.
• Units never shut off during the project. A power loss is an emergency condition.

Scenario

A gymnasium measures 80 ft x 60 ft x 20 ft = 96,000 cubic feet in an unoccupied school. The contractor brings two units rated at 2,000 CFM.

• Required airflow = 96,000 / 15 = 6,400 CFM.
• Field tests show each unit delivers about 1,500 CFM, so two units provide about 3,000 CFM, less than half of what is needed.
• The digital manometer reads 0.012 inch w.c., well below the 0.03 inch subcode minimum.

The AST does not issue the notice to proceed. Using actual output, 6,400 / 1,500 = 4.27, so the contractor needs at least 5 units. After the units are installed, ducted outdoors, and positioned away from the decon unit, the differential must reach 0.03 inch w.c. or more before abatement can begin.