IntroductionDuring the process of welding@ metal vapors are produced in the electric arc. As these vapors cool and solidify@ a fume is formed that may be a potential health hazard to the welder and to others working in the same area. Such fine aerosols are all irritating to the respiratory system. Yet some fumes may potentially be more dangerous than others because of the specific substances present. The purpose of this study is to provide a data base of chemical@ crystallographic@ and physical data for representative welding fume types which will aid in the understanding of the interactions of these particles with the human respiratory system. Such interactions are affected by many variables. Therefore@ a simple percent weight analysis for various elements does not provide adequate information since individual particle size and chemistry affect toxicity. For example@ a few large particles may dominate a percent by weight analysis. However@ if these particles were over 10 ??M in diameter@ they might not reach the lower respiratory system at all@ while compounds present in thousands of fine particles would penetrate to the alveoli of the lungs and could be absorbed into the blood. Particle morphology is also significant since particles with sharp edges or fibers are more irritating to the lungs than smooth@ sphere-shaped objects. Finally@ specific compounds must be identified since such factors as crystallinity@ solubility@ and oxidation state affect toxicity. Such information may influence the determination of federal standards for occupational exposure. These objectives were accomplished by using various macro and micro scale techniques. Initially@ energy dispersive X-ray analysis (EDXA) and X-ray diffraction (XRD) were used to obtain background information on bulk fume properties. The focus of this work was the analysis of the welding fume on a particle by particle basis. Automated electron beam analysis (SPEC) was used to analyze large numbers of particles@ and specially designed computer software sorted the particle data by size and chemistry. Finally@ a scanning transmission electron microscope (STEM) was used for a manual examination of a smaller number particles for size@ chemical composition@ and crystallinity. An examination of all of the data available for a fume can then be used to decide whether toxicological testing may be advisable.