Fibrous dusts are airborne particles of inorganic or organic substances that have an elongated geometry. Fibres that have a length of > 5 µm, a diameter < 3 µm and exceed a length-to-diameter ratio of 3:1 play a special role, as only they can penetrate the deeper respiratory tract. Fibres of this geometry are also called WHO fibres.  

source https://www.dguv.de/staub-info/was-ist-staub/fasern/index.jsp