What each fraction represents

The inhalable fraction corresponds to material that can enter the nose and mouth; the thoracic fraction to material that can pass the larynx; and the respirable fraction to material that can reach the lung’s gas-exchange region. These categories help assess exposure but do not diagnose how much dust has deposited in each person.

Size is not a rigid boundary

The conventions use efficiency curves based on aerodynamic diameter. They do not mean that every particle below one size enters while larger particles remain outside. Aerodynamic diameter incorporates airborne behaviour: two particles with the same visual dimensions may behave differently because of shape or density.

Choose before measuring

Assessing respirable crystalline silica requires the respirable fraction and silica-specific analysis. Weighing inhalable dust does not answer that question. Sampler, collection medium, flow rate and analytical method are selected together, checking that they correspond to the required fraction.

What can alter the result?

Sampler orientation, airflow, wall losses and incorrect assembly can change the collected sample. Changing the flow rate through size-selective devices may change particle selection. A commercial equipment name is insufficient: the actual configuration and its suitability for the procedure must be documented.

Interpretation example

In a hypothetical example, a plant compares an old total-dust survey with a new respirable-fraction survey and finds a lower value. That difference alone does not demonstrate improved extraction: different quantities were measured. Fraction, composition, tasks and conditions of both surveys must first be reviewed.

Make the report comparable

The report must name the fraction, contaminant analysed and method, alongside sampling volume and period. That information should be retained in every historical series. Occupational conventions should not automatically be equated with environmental indicators such as PM10 or PM2.5: purpose, sampler and assessment reference may differ.