In biochemistry, an ultratrace element is a chemical element that normally comprises less than one microgram per gram of a given organism (i.e. less than 0.0001% by weight), but which plays a significant role in its metabolism.
Possible ultratrace elements in humans include boron, silicon, nickel, arsenic and vanadium [1].
Other possible ultratrace elements in other organisms include bromine, cadmium, fluorine, lead, lithium, and tin. [2].