'Heavy Metal' Does Not Necessarily Mean 'Toxic'
THE MAGAZINE DEVOTED TO NICKEL AND ITS APPLICATIONS
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Lumping nickel in with other so-called 'heavy metals' does a great disservice to a metal which is
critical to many useful alloys By Virginia Heffernan
Nickel Magazine, July 2006 -- The lack of a clear definition of the term "heavy metal"
and its common use as a substitute for "toxic substance" continues to cause confusion in public debate.
Once used to describe a large gun, the term now has at least 38 different definitions, according to
"‘Heavy metals’ – a meaningless term?", a report of the International Union of Pure and Applied
Chemistry (IUPAC). "Heavy metal" definitions can refer to anything from density and atomic weight or number
to chemical properties or toxicity. As a result, lists of "heavy metals" may differ from one set of
regulations to another. Often the term is used without specifying the metals to which it applies.
"What is surprising is the persistence of the term and its continuing use in literature, policy and
regulations, with widely varying definitions leading to confusion of thought, failure in communication, and
considerable waste of time and money in fruitless debate," wrote J.H. Duffus, author of the IUPAC report,
published in Volume 74 of Pure and Applied Chemistry, 2002. His complaint still rings true.
There is a tendency to believe that all "heavy metals" and their compounds have toxic properties – "a false
assumption", says Bruce McKean, Director of Stewardship and Sustainable Development for the Nickel Institute.
"Virtually any substance will be toxic at a certain level," he says, "because toxicity is a function of
dose." And just because a metal is dense or "heavy" does not mean it has more toxic potential than a
relatively "light" metal."
Beryllium, for example, is a low atomic mass metal with high toxic potential and iron is a metal with and
atomic mass nearly the same as nickel that has a low potential to be toxic. Similarly, among the high atomic
mass metals, cadmium, mercury, lead and uranium have a high toxic potential where as gold, indium and niobium
have a low potential (see accompanying table). Ultimately, it is not the mass of the metal that determines
whether it is toxic or not at equivalent mass exposures.
Bioavailability, the degree of availability of a substance to be taken up by biological organisms, is also an
important consideration when metals are classified based on assessments of toxicity, according to
Duffus.
This is an important consideration since the amount of a metal absorbed into the body cannot exceed the
amount that is available for absorption. Availability is determined by the release (corrosion) of metal ions
from the surface of a metallic object. Obviously, if a child swallows a "nickel" the whole coin is not
absorbed into the bloodstream, only some of the ions that are corroded from the surface of the coin in the
stomach.
Duffus had thought that the term "heavy metal" would soon become obsolete because it no longer had any
consistent meaning, and yet four years after he wrote his paper, the term is still widely used, and there is
a lingering misguided tendency to group pure metals simply by their atomic mass.
Duffus suggested metals be classified according to their position in the periodic table, in which elements are
grouped according to chemical reactivity and, by association, behavior in the environment. A more precise
classification according to Duffus would group metallic elements based on their Lewis acidity, or net
positive charge, which determines their interaction with living systems.
Virginia Heffernan is a Toronto-based
freelance writer.
Duffus, J. H., ‘Heavy metals’ – a meaningless term?, International Union of Pure and Applied
Chemistry (IUPAC), Pure and Applied Chemistry, Vol. 74, 2002 |
