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Selenium (Se) exhibits a very small safety margin between recommended daily intake and harmful dosages (e.g. elevated risk for diabetes). Volunteer exposure studies have indicated different metabolisms for organic and inorganic Se compounds. The objective of this study was to establish a comprehensive biological monitoring approach for elemental/inorganic Se exposure at workplaces and to evaluate its potential effects in men.In a case-control study we determined total Se in plasma and urine, Se speciation in urine, Glutathione peroxidase activity, prothrombin time, glucose, HbA1c and proinsulin in 18 employees of a Se processing plant and 18 age matched controls without occupational Se exposure. For the Se-exposed group we additionally measured the average shift level of total Se and of water-soluble Se-fraction in air, Se in urine at the beginning and at the end of the shift, and after at least two weeks off-work.Se plasma levels of the exposed individuals ranged between 62 and 123 µg/L and urine levels after shift ranged between 22 and 340 µg/g creatinine. Both were significantly higher than the levels in the controls. The air exposure to total Se ranged between 3. For glutathione peroxidase activity, prothrombin time, glucose, HbA1c and proinsulin there were no significant differences between the two groups.At workplaces in the selenium processing industry considerably high air concentrations of Se were detected, which exceeded the German exposure limit (MAK 20 µg/m3). In contrast, the biological tolerance limit (BAT 150 µg Se/L plasma) was not exceeded. These diverging results necessitate a differentiated look on Se at workplaces. The workplace conditions including the kind of selenium (elemental, inorganic, water-soluble, organic) should be considered. A revision of selenium threshold values seems advisable.