respirable

Respirable is an adjective used in aerosol science and occupational hygiene to describe particles small enough to penetrate the gas exchange regions of the lungs when inhaled. In practice, respirable particles have an aerodynamic diameter of about 4 micrometers or smaller. This size distinction separates the respirable fraction from the inhalable fraction, which includes larger particles that deposit mainly in the nose and upper airways.

The concept is important because respirable particles can reach deep into the lungs and cause health effects. Exposure to respirable dust and aerosols is associated with diseases such as silicosis from crystalline silica, coal workers’ pneumoconiosis, asbestosis, lung cancer, and chronic obstructive pulmonary disease. The health impact depends on particle composition, solubility, shape, and the duration and intensity of exposure.

Measurement and regulation often focus on the respirable fraction. Air sampling uses specialized samplers and cyclones to separate particles by size, reporting results as respirable dust mass concentration (mg/m3) or respirable crystalline silica when composition is known. Regulatory limits for respirable substances exist in many jurisdictions and can vary by country and substance. For example, respirable crystalline silica limits are commonly cited around 0.05 mg/m3 as a time-weighted average in some frameworks, but exact values differ. Accurate assessment typically requires laboratory analysis, such as X-ray diffraction for silica content.

In summary, respirable describes the subset of airborne particles capable of reaching the deepest lung regions, with implications for exposure risk, measurement methods, and regulatory standards.