The lack of a clear definition for the term “heavy metal” and its common use as a substitute for 'toxic substance' continue to cause confusion in public debate.
Once used to describe a large gun, the term now has a least 38 different definitions that - depending on the scientific literature source - relate to anything from density and atomic weight or number to chemical properties or toxicity, according to “Heavy metals” – a meaningless term?”, a 2002 International Union of Pure and Applied Chemistry (IUPAC) report.
As a result, lists of heavy metals may differ from one set of regulations to another and the term is often used without specifying the metals included in the list.
“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.F. Duffus, author of the report published in Volume 74 of Pure and Applied Chemistry. His complaint still rings true today.
There is also a tendency to assume that all of the heavy metals and their compounds have toxic properties, a false assumption says Bruce McKean, Director of Stewardship and Sustainable Development for the Nickel Institute.
He argues that any substance will be toxic at a certain level because toxicity is a function of dose. And just because a metal is dense or “heavy” does not mean that it has more toxic potential than a relatively “light” metal.
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.
Under most traditional definitions, nickel would be considered a heavy metal because it is relatively heavy (atomic number 28) and dense (specific gravity 8.9), compared to other elements.
But there are more than 150 nickel compounds in industrial or commercial use, each with different biochemical characteristics and potential for toxicity. Grouping them together with pure nickel is misleading.
Duffus thought that the term “heavy metal” would soon become obsolete because it no longer had any consistent meaning. But four years after he wrote his paper, the term is still widely used and there continues to be a misguided tendency to group pure metals and their compounds together.
For example, sodium metal and sodium chloride are considered by the usage to be equivalent, but the pure metal can cause life-threatening damage when swallowed, while sodium chloride (salt) is an essential part of the human diet. Similarly, chromium and its alloys are safe for medical and dental applications, but chromate has been identified as a carcinogen. And the list goes on.
Duffus suggested metals be classified according to their position on the periodic table, in which elements can be grouped according to chemical reactivity and, by association, behaviour in the environment. A more precise classification would group metallic elements based on their Lewis acidity, or net positive charge, which determines their interaction with living systems.
For example, it is known that certain “soft” metals similar in size to the calcium ion are likely to cause harmful changes in membrane structure because of their affinity for phosphate groups and non-oxygen centres in membranes.
“Such a classification would permit interpretation of the biochemical basis for toxicity. It would also provide a rational basis for determining which metal ionic species or compounds are likely to be most toxic,” Duffus wrote.
