1.4. What is respirable crystalline silica (RCS)?
When considering dust, three dust fractions are of main concern: the inhalable, thoracic and respirable dust fractions. The respirable dust fraction corresponds to the proportion of an airborne contaminant, which penetrates to the pulmonary alveolar (gas exchange) region of the lungs. Respirable crystalline silica enters the body when dust containing a proportion of crystalline silica is inhaled. Respirable crystalline silica is the silica dust fraction which can be of concern to health when inhaled. For this reason, national occupational exposure limit values for crystalline silica apply to the respirable dust fraction.
This fraction normally represents 10 to 20% of the inhalable dust fraction, but the proportion can vary considerably. Respirable particles are tiny, measuring only a few microns (thousandths of a millimetre) in diameter. Airborne respirable dust particles are generally too small to be seen with the naked eye unless they are illuminated under a beam of intense light.
Respirable crystalline silica enters the body when dust containing a proportion of crystalline silica is inhaled. When the particle size range of the dust is sufficiently small (such that the particles fall within the respirable fraction), the dust will travel deep into the lungs. It is at this point that respirable crystalline silica can cause health effects.
Diagram showing the different parts of the lung.