Elements depending upon the requirement by
human beings can be divided into two groups,
essential and non essential. Essential elements like
Zn, Cu, Fe, Ca, F etc., play an important role in the
metabolism of human beings. But even the essential
metals at high concentration, when they cross
permissible limits prove to be harmful. For example,
fluoride is an essential element for human and animal
health particularly of dental caries. But high fluoride
content in drinking water gives rise to harmful effects
in human beings and animals.