The notion of metallic trace elements is not yet familiar to the ears of the wider public. In scientific literature, it replaces another term used until recently, considered to be too vague, but known to those who are interested in the pollution caused by industry: heavy metals.

So what are they? In fact, the metallic trace elements are metals which are naturally present in small quantities in the soil, water and air. Yet some of these substances show an abnormally high presence in some places which is generally due to waste produced by human activity. Indeed, heavy metals are part of the composition or fabrication of objects that are current in daily life. Some examples?

- iron (chemical symbol: Fe) and its alloys are present in steel, stainless steel and all the numerous products that incorporate it;
- lead (Pb) is present in car batteries, pipes, welding, anti-corrosion paint and ammunition;
- mercury (Hg) has numerous uses including dental alloys and electric batteries;  
- chromium (Cr), for chroming other metal objects;
- copper  (Cu), omnipresent in electronic equipment and as a fungicide used for treating vines;
- zinc (Zn), for galvanizing steel and for moulded parts used in the automobile industry;
- titanium (Ti), used especially in the making of artificial limbs.

Over the course of time, the heavy metals are dispersed into the natural world in a generally uncontrolled manner. According to the elements and the context (acidity of the environment, association with other pollutants), they are more or less biodegradable and can be bio-concentrated in the food chain. The absorption of some of these substances by living organisms is not necessarily a problem. On the contrary, a lot of heavy metals are even useful in biological processes. Iron, for example, is an essential component of haemoglobin; copper and zinc are indispensable as is selenium in small doses. We are therefore talking about essential trace elements.

They meet three criteria:

-they are present in living tissue at a relatively constant concentration;
-they cause structural and physiological abnormalities if they are removed from the organism;
-they prevent or cure these problems by supplying the element in deficit.

However, these essential trace elements can also become toxic beyond a certain threshold.

On the other hand there are other trace elements, said to be non-essential, and which do not have a known positive role in terms of biological activity but which can, on the contrary, cause serious illnesses. In particular these include mercury, lead and cadmium which, one they are discarded into the environment, make their way into the food chain and can cause serious illness in the organisms they contaminate. In this way the absorption of lead can cause lead poisoning, cadmium destroys the kidneys and damages the liver, and mercury is a powerful neurotoxin.

This is why the use of some heavy metals is strictly regulated, and even forbidden in certain applications. Likewise, their dumping into the environment after use must be avoided, to the advantage of specific recycling. But these principles are unequally applied throughout the world; there is much clandestine dumping and some of it took place in abundance before science was able to establish the toxicity of the offending substances and their use became regulated. This is why a lot of heavy metals and numerous other pollutants dispersed into the ocean currents make their way into ocean environment sometimes very far from the place where they are believed to have been dumped in the first place.

Understanding the impact of these trace elements on metabolic processes constitutes a vast field of basic research and mobilises a multi-disciplinary cooperation involving numerous disciplines: biology, biochemistry, pharmacology, toxicology, genetics, epidemiology, agronomics and veterinary medicine.