Nb– Niobium is found in igneous rocks at 20 ppm; shale at I I ppm; sandstone at 0.05 ppm; limestone 0.3 ppm; sea water 0.00001 ppm; land plants 0.3 ppm and marine animals 0.001 ppm.
Schroeder and Balassa considered niobium a much-neglected element from a biological viewpoint. They based this statement on their finding that levels of niobium in tissues of humans and wild, domestic, and laboratory animals were comparable to those of copper and were exceeded among the trace elements only by iron, zinc, and rubidium. They found niobium in nearly all animal and vegetable foods they analyzed, with most in the range of 0.5 to 3.0 mcg/g fresh weight. Thus, they suggested a daily human intake of equal to or greater than 600 mcg of niobium, of which approximately one-half was absorbed and excreted in the urine.
Finally, Schroeder and co-workers found that 5 mcg niobium per milliliter of drinking water, a level not much higher than their analyses showed would be normally present in food, was toxic to mice and rats. In mice, the niobium supplement increased the incidence of hepatic fatty degeneration, decreased median life span and longevity, and suppressed growth. In rats, the niobium supplement elevated copper, manganese, and especially zinc in a variety of organs, with the most marked changes occurring in heart and liver. Regardless of the true levels, the findings indicate niobium is present in living things and can affect biological mechanisms.