Negative Air Machine Setup for Containment

A negative air machine is a HEPA air scrubber operated to create a pressure differential between a contained work area and the surrounding building. Done right, it pulls air INTO the work area through any leak path, and the only way air can leave is through the HEPA filter. This containment strategy is the standard for asbestos abatement, mould remediation and lead paint removal.

How Negative Pressure Works

The scrubber's exhaust is ducted out of the contained area (typically through a window or external wall opening). The intake stays inside the contained space. As air gets pumped out, the pressure inside drops below the surrounding building pressure. Outside air rushes in through any unsealed seam, but contaminated air cannot escape.

Pressure Target

OSHA and IICRC standards target 5 Pa (0.02 inches of water column) below ambient. Verify with a digital manometer. The reading is taken at the entry barrier where the worker passes through.

Equipment Needed

• HEPA air scrubber sized for 4 to 12 ACH
• 10 to 25 ft of flexible ducting (10 inch diameter typical)
• Window or exterior vent transition kit
• 6 mil polyethylene sheeting for containment walls
• Spray adhesive and duct tape for seam sealing
• Digital manometer for verification

Browse HEPA air scrubbers sized for negative air.

Sizing the Machine

Calculate room cubic feet, then determine airflow needed for target ACH. For asbestos, OSHA requires 4 ACH minimum. For mould, IICRC S520 recommends 4 to 12 ACH depending on Condition.

Example: 1500 cu ft bedroom at 6 ACH = 1500 / 60 x 6 = 150 CFM. Most professional 500 CFM scrubbers handle this with significant headroom for filter loading.

Step by Step Setup

1. Plan the Containment

Identify the work area perimeter. Determine where the scrubber will sit and where the duct will exit. Plan the worker entry/exit path through a decontamination unit if required.

2. Build Containment Walls

Install 6 mil poly walls floor to ceiling. Tape all seams. Use rigid frame supports if walls are over 8 ft. For ceiling work, hang poly from the ceiling joists.

3. Set Up the Air Scrubber

Position the scrubber inside the contained area. Connect the flexible duct to the exhaust port. Run the duct out through a window opening or pre cut hole in the containment.

4. Seal the Duct Penetration

Where the duct passes through the containment wall or window opening, seal tightly with tape. Any gap here defeats the negative pressure.

5. Power Up

Plug in the scrubber. Verify the fan starts. Check the duct is firmly attached and not collapsed.

6. Verify Negative Pressure

Use a digital manometer to read the pressure differential at the entry barrier. Adjust scrubber speed (if variable) until the reading sits at minus 5 Pa or lower (more negative).

7. Add Critical Barriers

If the work crosses multiple rooms, install secondary critical barriers in doorways. Each barrier creates a sub zone that helps maintain pressure across the whole containment.

Maintenance During the Job

• Verify pressure differential at start of each shift
• Replace pre filter when airflow drops noticeably
• Listen for unusual fan noise (bearing failure, debris in fan)
• Inspect duct for kinks or compression that reduce airflow

Common Mistakes

• Undersized scrubber (cannot maintain ACH)
• Leaky containment walls (negative pressure cannot hold)
• Kinked duct (reduces airflow 30 to 50 percent)
• Exhaust into another contained area (defeats the purpose)
• No pressure verification (no documentation of compliance)

Pair With Decontamination

For asbestos and Category 3 work, pair the negative air machine with a three stage decontamination unit. Workers enter and exit through the decon, never through unsealed openings.

The negative air machine is the first piece of equipment to set up and the last to tear down. Documentation of pressure differential at the start and end of each shift is required for OSHA compliance and supports the contractor in any post job inspection or air monitoring dispute.