Substances that might otherwise be considered non-hazardous or inert may cause serious harm if the particle size is small enough that the powder or dust is respirable- especially if the process causes said dust to become airborne.

Particle size range and distribution determines how much of the dust is inhalable (passes into the mouth, trachea and bronchi) and respirable (able to pass beyond this point into the lungs)

The exact particle size that is respirable varies for the substance and the person but by convention it is considered to be less than 10 microns.

When material enters the lungs and airways the body must work to get rid of it and the exposure will be prolonged. Biologically active material such as dust (containing dander, pollen etc) and sawdust can cause sensitisation, especially in those with asthma or existing allergies.

Common construction materials (concrete, brick etc) contain Respirable Crystalline Silica or RPS. This silica becomes entrained in the lungs and can lead to silicosis, COPD and cancer- serious occupation illnesses linked to the construction industry.