Baseline air quality shows what “normal” looks like before works start. Without it, you cannot prove impact or choose sensible action levels. A structured baseline saves disputes, guides controls, and helps you comply with COSHH duties.

Plan the baseline

Define scope: which areas, trades, and dust types matter. Select monitors reporting the relevant fractions (PM10, PM4, PM2.5) in mg/m³. Decide logging intervals (1–5 minutes) and duration long enough to capture variation—typically at least two working days and one non-working day.

Place and document

Position instruments at breathing height in representative zones: near planned tasks, near receptors (offices, public routes), and a true background location. Record floor plans, weather, ventilation settings, occupancy, and any nearby works. Zero instruments and check flows at deployment.

Collect and analyse

Aim for stable operation with minimal movement. After collection, chart time series and summarise percentiles (median, 75th, 95th). Note daytime versus night differences and any external influences. This forms your baseline profile for each fraction and zone.

Set action levels

Combine baseline with WELs to set tiered triggers. For example, Alert at baseline 95th percentile, Action at 50% of the relevant WEL, and Stop/Review at 75%–100% of the WEL depending on risk. Link each tier to predefined responses: improve source capture, increase air changes with HEPA filtration, or pause and redesign.

Review during works

Re-baseline after major phase changes or control upgrades. Track deviations against the baseline so you can show when controls are working. When fine fractions increase, prioritise H13/H14 filtration and leakage checks; when coarse fractions rise, tighten housekeeping and material handling.

Practical takeaways

• Measure pre-works for at least two working days plus one non-working day.
• Log PM10, PM4 and PM2.5 at breathing height and document context.
• Use percentiles to define “normal” and set tiered action levels tied to controls.
• Revisit the baseline after process or layout changes.

With a clear baseline, you can defend decisions, protect people, and avoid avoidable stoppages.