<prompt> The size of Respirable dust is -1-5 microns5-10 microns15 microns20 microns  <response>: - dust paicles smaller than 5 microns are directly inhaled into the lungs and are retained there. This fraction of dust is called respirable dust. - it is mainly responsible for pneumoconiosis. Reference : Parks textbook of preventive and social medicine, 23rd edition, pg no:805 p 