Inhalable and Respirable

Airborne dust is described by size fractions that reflect how far into the lung the particles travel.

• Inhalable — the fraction that enters through nose and mouth, up to roughly 100 micrometres. Deposits mainly in the upper airways.
• Thoracic — penetrates below the larynx.
• Respirable — below about 10 micrometres, reaching the gas exchange region of the lung, where clearance is slow or absent.

The visibility problem

Respirable particles are far too small to see individually. The visible cloud from cutting concrete is mostly coarse dust that will never reach the deep lung; the dangerous fraction is invisible and remains airborne for hours after the visible cloud has settled. This is why a cleared work area can still be hazardous, and why clean-up hours later can produce significant exposure.

If you can see the dust, there is far more of the dangerous kind that you cannot.