Respirable dust is any material with a size less than 10 mincrons that has the capability of entering deep into the lungs. These particles are unable to be filtered and flushed out by the respiratory system and are retained. This can be a dangerous issue with any material, but the two dust particles focused on primarily are coal and silica. An extended time or amount of exposure to these dusts can result in Pneumoconiosis ("black lung") or silicosis, respectively.