Fibrogenic potential of welding fumes. 
A search of 3600 indexed pathology cases has disclosed pulmonary fibrosis in 29 welders.  Scanning electron microscopy of biopsy material revealed macrophages laden with inorganic particulates which have characteristics compatible with welding aerosols.  In order to establish a possible relationship between fibrotic reaction and welding-fume exposure, the fibrogenic potential fo some 11 different welding fumes and metallic aerosols, considered to be reference standard surrogates for the commonly used welding technologies and applications responsible for 70% of welders exposure, were screened using the Rat Peritoneal Macrophage in vitro bioassay.  Only one class of fumes, that from the manual metal arc welding of stainless steel, showed distinct fibrogenic potential.  This fume is, however, not common to more than four or five of the heretofore 90 cases of pulmonary fibrosis reported among welders.  Thus, although insoluble Cr(VI) is probably the active fibrogen in stainless steel fumes, an etiological factor common to all fibrogenic welding exposures must be sought.  It is tentatively proposed to be NO2, a potential experimental in vivo fibrogen copiously produced by certain welding processes and ubiquitous at low concentrations in the welding environment.