Right-sized airflow is the difference between meaningful dust control and a noisy box moving air. On UK sites the aim is simple: keep airborne particulate down so work stays under WELs and people can see and breathe safely. The method below gives you a fast, defensible calculation.

1) Choose a target air change rate

Air changes per hour (ACH) is a practical way to size air scrubbers or negative air machines. There is no single HSE-mandated ACH; choose a figure that will reasonably control dust for the task, then verify with a particulate monitor.

• Light maintenance or finishing: 4–6 ACH
• General construction and cutting with on-tool extraction: 6–10 ACH
• High dust tasks (grinding, chasing, demolition): 10–20 ACH
• Adjacent occupied areas when using negative pressure: 10–12 ACH

2) Measure the room volume

Volume (m³) = length × width × height. Where mixing is poor (nooks, mezzanines, heavy obstructions), assume only 80–90% of the volume mixes efficiently.

3) Convert ACH to required airflow under load

Required airflow (m³/h) = room volume × target ACH. Always work with under-load airflow (with filters and ducting fitted), not free-air figures.

Example: 8 m × 4 m × 3 m room = 96 m³. Target 10 ACH → 960 m³/h under load. Add a 20–30% allowance for filter loading and duct losses, so select equipment delivering around 1,200–1,300 m³/h under load.

4) Account for ducting, placement, and pressure

• Keep ducts short, straight, and sealed to reduce pressure drop.
• For negative air, exhaust outside the work zone and make-up air should enter from a cleaner space to maintain direction of flow.
• For recirculation, place the scrubber to create a loop across the dust source and the return path.

5) Verify and adjust

• Use a PM meter to check trend reductions in PM10/PM2.5 during and after work.
• Watch the unit’s pressure gauge; rising differential pressure signals filter loading and reduced airflow.
• Combine with capture at source (LEV, water suppression) and proper housekeeping using H-Class vacuums.

Practical takeaways

• Start with ACH, but prove control with real measurements.
• Specify airflow under load; free-air numbers are not control.
• Allow 20–30% headroom for filter loading and duct losses.
• Use negative pressure to protect adjacent clean areas.
• Layer controls: on-tool extraction, air cleaning, and no-sweep housekeeping.

HSE’s COSHH duty is to control exposure as low as reasonably practicable. Size your airflow, then confirm it actually reduces dust to safe levels and keeps working as filters load.