PM numbers appear on monitors and reports across sites, but what do they actually tell you about risk and control? Understanding particle size is central to choosing the right extraction, air filtration and housekeeping so you protect people and keep jobs moving.

What the PM numbers mean

PM refers to particulate matter sized by aerodynamic diameter. The number is the upper cut-off in micrometres for the fraction being measured.

• PM10: inhalable dust entering the nose and mouth.
• PM4: respirable fraction that can reach the gas-exchange region.
• PM2.5: fine particles that penetrate deep into the lungs.
• PM1: ultrafine particles with potential to translocate beyond the lungs.

PM is usually shown as mass concentration (e.g., µg/m³). It is size-based, not material-based; a PM2.5 reading cannot tell you if the particles are silica, wood, or diesel soot.

Why size matters for health and compliance

Different sizes deposit in different parts of the airway. HSE sets Workplace Exposure Limits for substances, not PM categories. For example, respirable crystalline silica (RCS) has a WEL of 0.1 mg/m³ (8-hr TWA). A PM number alone does not confirm compliance, but it helps you judge how far particles can travel and what controls to apply.

Using PM readings on site

Portable monitors reporting PM1/2.5/10 are useful for hotspot finding, task comparison and checking if controls are working. Place monitors at breathing height, near tasks but out of the direct jet, and log before, during and after work. Treat readings as trends and triggers, not legal proof. For compliance or exposure assessments under COSHH, use appropriate occupational hygiene sampling.

Match controls to particle behaviour

1) Capture at source

• Use tool-mounted local exhaust ventilation with the right shroud and maintain airflow under load; clean or change filters before they choke.
• Apply water suppression where suitable; slow cutting speeds to reduce generation.
• Select filtration to the hazard: M-Class for many wood and general dusts; H-Class with an H14 final stage for fine or carcinogenic dusts such as RCS and welding fume.

2) Capture in the air

• Deploy air scrubbers or negative air units to manage background levels. Position to pull air across the work area, not push dust through it.
• Use the full filtration chain (pre-filter, main, then HEPA). H14 is recommended for fine/respirable risks. Track performance as filters load.

3) Capture on surfaces

• Avoid sweeping and compressed air. Use industrial vacuums with high-efficiency filtration and antistatic tooling where needed.
• Empty and seal waste safely; bag changes are a common exposure point.

Limits of PM metrics

PM data does not identify composition. A low PM10 number does not guarantee low silica, and a high PM1 number might be dominated by diesel. Combine PM monitoring with knowledge of the task and materials, and escalate to personal sampling where required.

Practical takeaways

• Place a PM monitor at breathing height to check controls before and during dusty tasks.
• Prioritise source capture; confirm extractor airflow under load and keep filters clean.
• Use H14 final filtration when dealing with fine or respirable hazards.
• Do not sweep; vacuum with suitable M- or H-Class equipment and sealed disposal.
• Treat PM as an early warning; use COSHH-compliant sampling for exposure decisions.

Used well, PM categories help you predict how dust behaves and choose controls that prevent exposure, keep air clear and avoid rework.