Metals and metalloid elements
Metals and metalloid elements in the environment have a major impact on human, animal and plant life as seen in the recent Hungarian red mud catastrophe. Some elements like calcium, iron, zinc etc serve as essential nutrients without which life fails to thrive, whereas lead, cadmium, mercury, arsenic, antimony and bismuth are xenobiotic elements and potentially toxic if present to excess. I read with considerable concern in scientific communications, references to such elements as titanium, silver, aluminium and even arsenic listed erroneously as trace elements in human biology and plant growth (Chemistry World, Nov 2011, p11).
Whether we are speaking of mammalian or botanical systems, it is imperative that we recognise what is meant by the term ’trace element’ and to distinguish it from non-trace or xenobiotic elements which may be identified in tissues by atomic absorption spectrometry or other means and which have no defined role as enzyme co-factors, structural components or physiological modulators. I emphasise the criteria defined by Dr Eric Underwood in 1977, that trace elements should fulfil one or more essential functions in the body and where deficiencies are manifest by characteristic defects in growth, maturation and general wellbeing. These defects are mitigated by dietary repletion. Failure to thrive can occur in cases of chronic trace metal deficiencies. In human biology, we include calcium, iron, zinc, magnesium, manganese, cobalt, sodium, potassium, copper, chromium, nickel, selenium, silicon, molybdenum, vanadium and possibly tin as essential nutrients and trace elements.
Elements found in the geosphere are commonly absorbed into humans and animals in the diet and in drinking water, as was probably the case in the Hungarian disaster. It is also common knowledge that many elements accumulate in plant life and enter human food chains. This is particularly well illustrated by arsenic, which is found in surface waters, rivers and streams, and where exposed populations are at risk of skin and other cancers, and where congenital deformities are increased. Many other xenobiotic elements including titanium, aluminium, silver, gold, arsenic, and lead are commonly found in the human body through consumption in the diet, migration from bone prostheses, dental appliances, dermal application or contact, and through inhalation. Some may accumulate in bone, others like silver and gold precipitate in soft tissues in the form of argyria or chrysiasis. The physiological and toxicological implications of metals in biology provide challenging opportunities for research and environmental protection.
Alan Lansdown FRSC FRCPath
Imperial College London, UK