A
heavy metal is a member of a loosely-defined subset of elements that exhibit metallic properties. It mainly includes the
transition metals, some
metalloids,
lanthanides, and
actinides. Many different definitions have been proposed—some based on
density, some on
atomic number or
atomic weight, and some on
chemical properties or
toxicity.
The term heavy metal has been called a "misinterpretation" in an
IUPAC technical report due to the contradictory definitions and its lack of a "coherent scientific basis".
There is an alternative term
toxic metal, for which no consensus of exact definition exists either. As discussed below, depending on context, heavy metal can include elements lighter than carbon and can exclude some of the heaviest metals. Heavy metals occur naturally in the ecosystem with large variations in concentration. In modern times, anthropogenic sources of heavy metals, i.e. pollution, have been introduced to the ecosystem. Waste-derived fuels are especially prone to contain heavy metals, so heavy metals are a concern in consideration of waste as fuel.
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