Nickel (Ni) is a highly abundant element with a silvery-white appearance. Nickel is frequently combined with other metals to form alloys and is essential for the catalytic activity of some plant and bacterial enzymes (including in several pathogenic and symbiotic species in humans) but has no known role in humans. Values 2.0 ng/mL and higher represent possible environmental or occupational exposure to nickel (Ni). Toxic Ni concentrations are greater or equal to 10 ng/mL. Values 2.0 ng/mL and higher represent possible environmental or occupational exposure to nickel (Ni).