in light of recent research on the potential health problems associated with sub-micrometer aerosols, a study was conducted to determine the effect that droplet mass transfer mode, shield gas composition, and welding spatter had upon the aerosols generated from a Gas Metal Arc Welding (GMAW) Operation. The results revealed that the sub-micrometer aerosols produced during spray transfer resulted in markedly higher concentrations of nucleated particles than those produced during globular transfer. This probably resulted from a larger droplet surface area for vaporization of metallic species. The shield gas experiments results revealed that as the percentage of carbon dioxide increased the number of nucleated particles also increased. It appears that oxygen may have facilitated chemical reactions with the alloy constituents, thereby increasing the mass transfer rate from the evaporating metal droplets in the plasma. Finally, an attempt to characterize the spatter aerosol revealed a distinct particle size distribution with a mode particle diameter of 6.8 mum. This particle size distribution appeared to be independent of shield gas composition, and the particle number concentration was significantly smaller than the sub-micrometer aerosols formed during the GMAW process (i.e., two-orders of magnitude smaller when weighted by particle mass). Published by Elsevier Science Ltd.