Managing airflow direction is as important as filtration. Negative and positive air set the pressure difference between a work zone and adjacent spaces, controlling where dust travels. Use each deliberately to either contain contamination or protect clean areas.

Negative air: contain the work

With negative air, the work zone is slightly lower pressure than outside, so air flows in and dust does not escape. Use when stripping, chasing, sanding, or any task with high dust release near occupied areas. Create the pressure by ducting a filtered air scrubber to exhaust outside or to a safe area; provide make-up air via a controlled opening.

Positive air: protect the clean

Positive air keeps a finished or sensitive area at slightly higher pressure so dust cannot enter. Use to protect finished rooms, server spaces, or client areas adjacent to works. Supply clean, filtered air into the protected zone and let it leak outwards through door gaps or controlled vents.

Setting up and checking

Seal openings, establish a clear airflow path, and avoid short circuits. Provide make-up air so doors do not slam and fans are not starved. Aim for a small but stable pressure difference; use a manometer if available, or a simple smoke test to verify flow direction. Always attach HEPA (H14 for fine/carcinogenic dusts) when exhausting into occupied buildings.

When to choose which

Use negative air for dusty works inside a containment (typical in refurbishment and removal tasks). Use positive air to protect clean rooms during adjacent works. On complex projects, you may combine both—negative in the work enclosure and positive in the protected area—to control movement in both directions.

Practical takeaways

• Choose negative air to keep dust in, positive air to keep dust out.
• Seal, provide make-up air, and verify direction with smoke or a manometer.
• Duct to safe locations and maintain high-efficiency filtration on exhausts.
• Avoid short-circuiting; create a defined path from intake to discharge.

Used with good on-tool extraction and housekeeping, pressure control makes dust migration predictable and keeps neighbours and clients on side.