Dust monitors display concentration as mg/m³. The figure is only useful if you know what it represents and how it compares with risk thresholds. In the UK, HSE EH40 sets an 8-hour Workplace Exposure Limit (WEL) for respirable crystalline silica (RCS) at 0.1 mg/m³. General workplace dust has guidance WELs of 10 mg/m³ (inhalable) and 4 mg/m³ (respirable). Your reading needs to be interpreted against the correct fraction and time-base.

Know what your monitor is measuring

Check whether the instrument reports PM10, PM4 (respirable), PM2.5 or total/inhalable dust. Most optical particle counters estimate mass using assumptions about particle density and shape, so treat spot values as indicative. High humidity, welding fume, or very dark particles can bias readings. Where accuracy matters, confirm trends with a gravimetric sample.

Time-weighting matters

WELs are usually 8-hour TWAs. A two-minute spike of 1.2 mg/m³ respirable dust does not equal an over-exposure if the shift average stays below the limit. Use logging intervals (e.g., 1 minute) and review both peaks and averages. Some tasks also have short-term limits; if none apply, set your own site action thresholds.

Placement and setup

Place area monitors at breathing height, near the task but outside direct jets. Zero the instrument before use and perform a flow or zero-filter check daily. Record weather, ventilation state and processes running. Keep a site log so you can explain anomalies.

Turn numbers into controls

Use readings to verify controls, not replace them. If levels rise, act in this order: capture at source (tool extraction, water suppression, slower feeds), capture in the air (air scrubbers or negative pressure with H13/H14 HEPA, sized on under-load airflow), then capture on surfaces (H-class vacuums, never dry sweeping).

Practical takeaways

• Match the mg/m³ reading to the correct dust fraction and the relevant WEL.
• Track both peaks and 8-hour averages; set site action levels at 50% and 75% of the limit.
• Verify monitor setup daily; note factors like humidity and nearby ventilation.
• Escalate controls in the capture-at-source, in-air, then housekeeping order.
• For high-risk RCS work, confirm trends with periodic personal sampling under COSHH.

Used this way, mg/m³ data becomes a practical decision tool to protect people and maintain compliance.