The term "heavy metals" is loosely defined. It is related to the periodic table of elements and refers to a variety of elements with high density or metallic properties. These elements are found naturally throughout the environment and are also used by industries to manufacture a wide range of common products. Some of them, such as iron, copper, selenium, molybdenum, and zinc, are required in trace amounts by the body for normal function but can be toxic at higher levels. Significant concentrations of any of the heavy metals can be irritating or damaging to the body and can contaminate soil, air, food, and water, persisting indefinitely in the environment. Because they are a source of potential injury, the term "heavy metals" is frequently used interchangeably with the term "toxic metals."