Airborne Dust in a Small Unpowered Workshop: Settling Times, Breathing-Zone Concentration, and the Ventilation Rate a Simple Opening Must Provide

A small workshop without powered extraction is often assumed to clear itself: work stops, dust settles, and the air is clean again. This paper tests that assumption for a 4.0 by 3.0 by 2.5 m workshop (30.0 m3) in which Scots pine is hand sawn for 1 hour and hand sanded for 2 hours in an 8-hour day, with stated airborne generation rates of 1200 mg per hour for sawing and 3600 mg per hour for sanding. Settling in still air is computed from Stokes' law with a slip correction for six particle sizes. A 100 micrometre particle falls 1.5 m in 8.3 seconds and a 10 micrometre particle in 13.6 minutes, but a 5 micrometre particle needs 53.7 minutes, a 2.5 micrometre particle 3.47 hours, and a 1 micrometre particle 19.82 hours, so settling alone does not clear the finest fraction within a shift. In a sealed workshop the modelled breathing-zone concentration peaks at 109.62 mg/m3, the 8-hour time-weighted average is 52.12 mg/m3 against a stated limit of 5 mg/m3, and 33.33 mg/m3 remains at the end of the day after 5 quiet hours. Holding the steady sanding concentration at 5 mg/m3 requires 20.02 air changes per hour, or 600.6 m3 per hour, which a simple opening can supply only as a 0.278 m2 free area in a 1.0 m/s wind or 0.927 m2 under a weak 0.3 m/s stack-driven flow. A lower rate of 4.65 air changes per hour holds the 8-hour average for this particular schedule at the limit, but it leaves the peak at about 17 mg/m3 at 5 air changes per hour and does not hold the concentration down while sanding is under way.