Environmental Pollution 129 (2004) 341–343
www.elsevier.com/locate/envpol
Invited Paper
Heavy metals—geochemical bogey men?
Mark E. Hodson*
Department of Soil Science, School of Human and Environmental Sciences, University of Reading, Reading RG6 6DW, UK
Received 30 October 2003; accepted 10 November 2003
‘‘Capsule’’: Serveral alternatives are presented for poorly defined term ‘‘heavy metals’’.
‘‘When I use a word,’’ Humpty Dumpty said in rather a scornful tone, ‘‘it means just
what I choose it to mean—neither more nor less.’’
Lewis Carroll, Alice through the looking glass.
Abstract
Heavy metals is a loose term usually used to indicate environmentally ‘‘bad’’ metals. It is poorly defined with a multitude of often
contradictory definitions based on density, atomic weight, atomic number or other properties of the elements or their compounds.
Alternative, scientifically rigorous definitions exist for subgroups of metals and should be used instead.
# 2003 Elsevier Ltd. All rights reserved.
Keywords: Heavy metals; Pollution; Environment; Terminology
1. Introduction
Heavy metals is a popular term in the environmental
literature. Web of Science, a Web based reference tool,
records 293 articles with the term ‘‘Heavy metal’’ in the
title published in 2002, 308 in 2001 and 270 in 2000.
This tails off to 107 articles in 1981 when the coverage
of Web of Science is less good. But what are heavy
metals and is the term used consistently or, as in the case
of Lewis Carroll’s Humpty Dumpty does the word
mean what we, rather than someone else, choose it to
mean at any particular moment? Use of the word heavy
metals calls to mind the case of the man who has heard
much about elephants but never seen one—he can’t
describe an elephant, but would know one when he saw
it. The difference is that whilst most of us would quite
like to see an elephant the majority view is that heavy
metals are synonymous with pollution and toxicity.
Heavy metals are regarded as bogey men, responsible
for all manner of evils in the environmental world. Thus
* Tel.: +44-118-378-6974; fax: +44-118-378-6660.
E-mail address: [email protected] (M.E. Hodson).
0269-7491/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.envpol.2003.11.003
whenever I see a paper titled ‘‘Distribution of heavy
metals in. . ...’’, ‘‘Effects of heavy metals on. . ..’’ or
‘‘Uptake of heavy metals by. . ..’’ I know that I am in for
a paper on pollution, environmental impact or anthropogenic influences. But, just like bogey men, do heavy
metals really exist?
2. What are heavy metals?
The above preamble has a serious point. In 2002,
John Duffus published an excellent article ‘‘Heavy
metals—a meaningless term?’’ in Pure and Applied
Chemistry which is an official IUPAC journal (Duffus,
2002). IUPAC is the International Union of Pure and
Applied Chemistry and amongst other things ‘‘IUPAC
has long been recognized as the world authority on
chemical nomenclature, terminology, standardized
methods for measurement, atomic weights and many
other critically evaluated data’’ (IUPAC, 2003). In
terms of chemical nomenclature, what IUPAC says
goes. In his article, Duffus (2002) acknowledged that
some form of classification beyond ‘‘metal’’ is useful to
science, rather like the group classifications ‘‘alkanes’’
and ‘‘alkynes’’ etc. in the field of organic chemistry.
342
M.E. Hodson / Environmental Pollution 129 (2004) 341–343
However, he argues heavy metals is an ill defined term
that means many different things to many different
people. There are a host of different definitions of the
term in the literature many of which are contradictory.
You may know what you mean when you talk about a
heavy metal but someone else may mean something
entirely different. At the end of his paper Duffus provides a variety of other metal/element classification
schemes that are chemically defensible and which illustrate that the heavy metal apologists’ main argument
‘‘There are no alternative terms’’ is simply untrue. What
follows is a brief summary of the Duffus paper. I would
recommend that (1) people stop using the term heavy
metal in their papers and (2) people read the article by
Duffus (2002) and then act upon it.
According to the Oxford English Dictionary the oldest
scientific use of the term heavy metal is to be found in
the 3rd Danish edition of Bjerrum’s Inorganic Chemistry
(Bjerrum, 1936) where heavy metals are described as
those metals with an elemental density above 7 g cm 3.
Before this time the term was used to mean ‘‘guns or
shot of large size’’ or ‘‘great ability’’ (Ogilvie, 1884;
Williams, 1930). From 1936 onwards heavy metals are
referred to within chemistry textbooks and articles.
However, the definition of the phrase varies wildly.
Interested readers are referred to Duffus’ 2002 article
for a whole string of definitions which are summarised
in Table 1. As Duffus points out, the various definitions
are not internally consistent or based on any obvious
logical scheme. For example, on the basis of some of the
definitions using atomic weight as a marker Mg would
be classified as a heavy metal; however some definitions
that use atomic weight as a marker specifically exclude
the alkaline earth metals and therefore would not classify Mg as a heavy metal. Similarly, definitions based on
atomic number would include Sc as a heavy metal and
yet the density of Sc is 3 g cm 3 (at an unspecified
temperature and pressure: Sharp, 1990) excluding it
from many density-based definitions.
Duffus (2002) also makes the point that even if one
were to accept heavy metals as a valid term, its use in the
literature as a synonym for ‘‘bad’’ metals is unjustified
given that toxicity is a function of the chemical properties of the element/compound and the biological properties of the organism at risk. Also as Paracelsus (1493–
1541) points out, toxicity is always dependent on dose.
Given all of the above there is no a priori reason to
assume that the presence of a heavy metal will cause
harm or is a bad thing. To illustrate this point Duffus
quotes two examples in which the same element may be
present in either a toxic or relatively non-toxic form—
tin (relatively non-toxic) and tributyltin (highly toxic to
shell fish) (WHO, 1980) and chromium in stainless steal
(relatively non-toxic) and the chromate ion that has
been associated with causing lung cancer (relatively
toxic) (WHO, 1988). Similarly much work on the in situ
remediation of heavy metal contaminated soils seeks to
convert the metals into inert, non-toxic forms (e.g.
much work has been carried out investigating the conversion of lead compounds into lead phosphates: Cotter-Howells and Caporn, 1996; Hodson et al., 2001,
2000; Laperche et al., 1997; Ma et al., 1994; Xu et al.,
1994). Incidentally the above is a good reason for not
adopting potentially toxic metal/element as a heavy
metal alternative. All metals/elements are toxic in
certain forms and in sufficiently high doses.
3. There is an alternative
Many heavy metal apologists amongst the environmental science community defend their use of the term
Table 1
Range of definitions for the term heavy metals (summarised from Duffus, 2002)
Property forming basis
of definition
Range of values characterising a ‘‘heavy metal’’
Metal density
Atomic weight
3.5–5 g cm 3, >4 g cm 3, >4.5 g cm 3, >5 g cm 3, 55 g cm 3, 55 g cm 3, >6 g cm 3, >7 g cm 3
‘‘high’’, >22.99 (Na, i.e. heavy metals are from Mg onwards), >40 (Ca, i.e. heavy metals are from Sc onwards)
Some definitions also specifically include (e.g. common transition metals) or exclude (e.g. alkaline earth metals,
lanthanides and actinides) groups of elements and also add caveats such as ‘‘can damage living things at low
concentrations’’, ‘‘cannot be processed by living organisms’’, ‘‘are toxic’’
‘‘moderate to high’’, ‘‘high’’, > 20 (Ca), the range 21 (Sc)–92 (U)
Atomic number
Other definitions
Elements in the block defined by Ti, Hf, As and Bi plus Se and Te
Very dense alloys used for radiation screening or balancing purposes
Intermetallic compound of Fe and Sn
Pb, Zn and alkaline earth metals that react with fatty acids to form soaps
Any metals that react with dithizone (C6H5N)
Elements commonly used in industry and generically toxic to animals and to aerobic and anaerobic processes
Pb, Cd, Hg and some other elements which are relatively toxic in nature. . .also sometimes refers to compounds containing these elements
M.E. Hodson / Environmental Pollution 129 (2004) 341–343
with the statement ‘‘I know it is a terrible term but
everyone knows what it means and there aren’t any
alternatives’’. Having laid the lie that everyone knows
what heavy metal means (they may do but it means
something different to every one of us), Duffus (2002)
proceeds to detail some of the alternatives for
categorising metals. The alternatives he suggests are well
established in the literature and are based on scientifically defensible chemical principles. There is absolutely
no reason why they should not be used.
The best established chemically-based classification
of metals, and all elements, is the periodic table.
Phipps (1981) discussed the biologically significant
properties of s-, p-, d- and f-block elements. d-block
elements are particularly important in enzymes, whilst
higher atomic number p-block elements bind strongly to
sulphur, often leading to a toxic effect. An alternative
classification was suggested by Lewis (1923), and
further developed by authors such as Nieboer and
Richardson (1980). This classification applies to ions
and places them into either Class A/hard, Class B/soft,
or Borderline/Intermediate class ions. The scheme is
not absolute but there is general agreement between
authors as to which class different ions belong.
Typically Class A metal ions preferably form ionic
bonds and are easily displaced and mobile, whilst Class
B form covalent bonds, show strong affinity to soft
ligands such as sulphide or sulphur donors, and
tend to accumulate in organisms with resultant
toxicity. As Duffus (2002) and Nieboer and Richardson (1980) point out, the differing toxicities of Class A
and B metal ions hints at a way of developing a
classification of metals that is based on chemical
properties and also has biological, toxicological and
environmental relevance.
These alternatives may seem unfamiliar or nonintuitive to environmental scientists used to the term
heavy metals. This is no excuse for not using them! Over
time the terms will become normal and accessible. If, on
reading this, you still feel that you will have an aversion
to using such terms, I suggest you stick to the straight
forward term ‘‘metal’’.
4. Conclusion
Heavy metals is a poor scientific term and alternatives
exist. As scientists we should use them.
343
Acknowledgements
I would like to thank John Duffus for giving freely of
his time in discussing his paper and Steve Robinson
(Soil Science) for commenting on an earlier version of
this text. The current contribution was largely written at
the Natural History Museum, London, where I am a
Scientific Associate and am provided with a desk away
from the distractions of University students.
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