The plateau region of concentration depicts optimum growth, health and reproduction. Beyond the plateau region, all metals (essential or non-essential) become toxic and eventually lethal.
The toxicity of heavy metals and their compounds, in mammalian systems, is influenced by the following factors:
1. Electrochemical character and the state of oxidation of the metal.
2. The stability, reactivity, and solubility of the metal compounds in body fluids and tissues.
3. The rate of absorption versus clearance.
4. The transport of metal compounds in blood, then distribution and retention in the tissues.
5. The ability of these heavy metals to chelate with ligands of biological macromolecules and tissue components, and the stability of these metal chelates.
6. The efficiency of the enzymatic and homeostatic mechanism which controls the absorption, excretion, distribution and retention of the toxic metal ions or compounds
The criteria for metal toxicity in mammals are growth retardation, decreased health and intellectual capability, detrimental changes in reproductive cycle with mortality of offspring, increased morbidity, and pathological changes, appearance of tumors and chronic disease symptoms and decreased longevity.
The adverse effect of a metal on biological systems could be the result of an interaction of the metal with protein, leading to denaturation, an interaction with DNA, leading to a mutation, effect upon cell membranes, effect on regularity enzymes, involved in the metabolic disposition of a variety of endogenous and exogenous chemicals or even a change in the ecological system.
Environmental contamination is largely endangered by man himself and presents both cultural and genetic hazards. Just how great the potential for genetic damage is, has become evident only in very recent years. It is also now well-established that the human species pays a large price for genetic variability.
Approximately one-third or all spontaneous abortions, or five per cent of all recognisable conceptions, carry a genetic abnormality but nature reduces the number of major genetic errors in new-borns by ten-fold or approximately one in every 200 births.
Those defects (mutants) which survive, however, impose not only an economic load upon society but also bring untold human suffering, both physical and emotional. Fortunately, one half of these are sterile, and does not pollute the genetic pool.
How can we cope with this problem? In order to inhibit the increase of mutagenic in the environment, planners in environmental health sciences should ban those chemicals which are known or suspected to harmful to be human beings and which are being produced and marketed at an increasing rate, unless their safety can be ascertained. Detrimental mutations can be easily induced in the laboratory in a variety of biological systems, attesting to the occurrence of induced genetic effects in all life forms.
“About five percent of babies in West Germany have genetic defects” says a specialist on treating the handicapped. This means that, of the average 600,000 births per year in the past 20 Years, 30,000 babies are born with disorders, says Prof. Gerhard Wendt of the Foundation for the Handicapped Child Marburg.