Derived from all-natural alpha cellulose, the fibrous ribbons create a structural matrix when mixed with epoxy for use in structural joinery.  Used in place of colloidal silica (such as Cab-o-sil), it will not billow in the air like silica, and you will need half as much to create the same consistency.  There are no health hazards associated with Cell-O-Fill and International standards consider it only a "nuisance dust."  Cell-o-fill is included in most of the CLC epoxy kits.