An element which is required in amounts smaller than 0.01% of the mass of the organism is called a trace element. Table 2 shows that the average person weighing 70 kg requires the following amounts of metals in the body to maintain good health.
Only the last six ions are in small enough quantities to be considered trace elements. Trace metals function mostly as catalysts for enzymatic activity in human bodies. However, all essential trace metals become toxic when their concentration becomes excessive. Usually this happens when the levels exceed by 40- to 200-fold those required for correct nutritional response.
Drinking water containing the above trace metals in very small quantities may actually reduce the possibility of deficiencies of trace elements in the diet. However, in some cases, if the metal is present in the water supply, there is a danger of overdose and toxic effect.
In addition to the metals essential for human life, water may contain toxic metals like mercury, lead, cadmium, chromium, silver, selenium, aluminum, arsenic, and barium. These metals can cause chronic or acute poisoning and should be eliminated from the drinking water if possible.