What are heavy metals? Long-standing controversy over the scientific use of the term ‘heavy metals’ – proposal of a comprehensive definition

ABSTRACT

The term ‘heavy metals’ is commonly used in the environmental literature to refer to metals and metalloids associated with environmental pollution, toxicity and adverse effects on biota. The term has been diversely defined, mostly in terms of density, relative atomic mass and atomic number. This diversity of definitions has raised questions about the nomenclature of these elements. The inclusion of the metalloid As and the nonmetal Se with heavy metals is also an important issue. Some people have called the term as meaningless, imprecise and poorly defined and have suggested abandoning the use of the term. The term itself may not be problematic but the careless and inconsistent use of terminology has led to a confusion about the meaning of the term. The use of the term may be continued but it should be defined in a more comprehensive and scientific way. Here, a more comprehensive definition of the term ‘heavy metals’ is suggested as ‘naturally occurring metals having atomic number (Z) greater than 20 and an elemental density greater than 5 g cm⁻³.’ A screening of the Periodic Table according to this definition yields 51 elements to be called ‘heavy metals.’ As and Se are excluded from heavy metals.

KEYWORDS:

• Controversy
• definition
• environmental literature
• heavy metals
• metalloids

Acknowledgments

The credit for motivation of this publication goes to Duffus, who published the pioneering and fundamental article in Pure and Applied Chemistry in 2002 and brought the issue into the notice of the scientific community. All those who contributed to the growth and resolution of the controversy on the term ‘heavy metals’ are gratefully acknowledged.

Disclosure statement

No potential conflict of interest was reported by the authors.