Although aerosol particles have differentiated sizes and various shapes, some of them seem to be especially interesting and important. These particles with an aspect ratio of at least three are called “fi bers”. However, in addition to isolated fi bers, assemblages of spherical particles and fi bers frequently occur in air samples. Groupings of fi bers and spherical particles, referred to as “structures”, for example, “asbestos structures”, are defi ned as fi brous bundles, clusters, and matrices. Examples of the various types of morphological structure, and the manner in which these shall be recorded, are shown in Fig. 5-1.