Mercury can cause acute poisoning in a large dose. Since mercury accumulates in body tissues, it can cause chronic effects to the nervous system, kidney or intestines at low doses over a long period of time. Mercury contamination of water is caused by industrial or agricultural wastes. Selenium in small amounts is beneficial to health. Levels over the primary drinking water standard of 0.01 milligrams per liter may cause nervous system disorders, skin problems and in extreme cases may be fatal.
Selenium occurs naturally in drinking water, though in trace amounts only.
It can also form in higher concentrations in some soils. Selenium is a by-product of copper refining. Amounts of silver in drinking water over the drinking water standard of 0.05 milligrams per liter may cause a permanent blue-gray discoloration of eyes, skin and mucous membranes. Silver contamination of water can be caused by disposal of industrial waste, including waste from metal plating and photographic processing industries. The secondary drinking water standard for zinc is 5 milligrams per liter.