Results are presented, based on investigation of industrial hygiene conditions carried out by the author at vanadium production plants undergoing modernization of outdated technological processes and assimilating newer ones. The quantitative aerosol content of the atmosphere of the working areas, as well as physical properties and chemical composition of the dispersed aerosol phase, were studied. The incidence of occupational diseases and poisoning, the presence of which may be attributed to the effects of composite aerosols and of vanadium compounds, was determined by means of systematic examination of the health of the workers. Maximum permissible concentrations of iron oxide dust containing admixtures of associated metal oxides, as well as of vanadium slag and vanadium trioxide dust, are presented. Through analysis of working conditions and the state of health of the workers, certain measures for prevention of occupational disease and poisoning are recommended by the author.