"Heavy metals is a vague term referring to metals of toxicologic significance. While historically this referred to metals of a high density/mass, such as those in the lanthanides and actinides (on the periodic table), it now encompasses a broad array of elements, including some metalloids (e.g. arsenic) and some non-metals (e.g. selenium). Some members of this class are also essential trace elements, crucial for normal human physiology, but also may exert route, location and dose-dependent toxicity."