Respirable Dust

Dust of less than 10 microns which is capable of penetrating deep into the alveoli, (source BSEN 481). See Total Inhalable Dust and Thoracic Dust.
Found on http://www.encyclo.co.uk/local/20474

respirable dust

respirable particles Mass fraction of dust (particles) that penetrates to the unciliated airways of the lung (the alveolar region). Note: This fraction is represented by a cumulative log-normal curve having a median aerodynamic diameter of 4 µm, standard deviation 2 µm (values for humans). [
Found on http://sis.nlm.nih.gov/enviro/iupacglossary/glossaryr.html

Respirable dust

Dust particles 5 microns or less in size.
Found on http://www.coaleducation.org/glossary.htm
No exact match found