A combined SEM-TEM study was performed on samples of metal inert gas (MIG) stainless steel (SS) arc welding fumes to characterize fine and ultrafine particle fractions in terms of size distributions, crystal structures, and chemistry. Overall observations indicate that welding fumes consist of a complex mixture of magnetite-like nanocrystals and non-spinel particles in different phases, with distinguishing grain size distributions and chemical features. Results confirm the great health hazard posed by MIG-SS ultrafine particles and illustrate the complex array of morphological and chemical features characterizing airborne dusts in working environments. The possible implications of these features in interaction and toxicity mechanisms are discussed. (C) 2009 Elsevier Ltd. All rights reserved.