Some trace elements (minerals) are essential but can also be hazardous

True. The trace elements copper, iodine, fluorine, selenium and zinc are all essential nutrients but they can also be hazardous to health. Trace elements are generally minerals which are essential to life, but only in tiny amounts. At the most, only a few milligrams or even micrograms are required. Trace elements like copper, zinc and selenium form part of enzymes which facilitate chemical reactions in the body. Sometimes they have a different role like that of iodine as part of thyroid hormones, or fluorine as part of bone. The normal biological relationships that exist in a living organism can be disrupted by changing the balance of these essential constituents. For example, if too much zinc is consumed, the amount of copper digested will be lowered (the two compete with each other), so that a consequence of an excessive intake of zinc is copper deficiency. Iodine can, if insufficient is consumed, result in depressed thyroid function and an overworked and enlarged thyroid gland (goitre), but large amounts can actually moderate excessive thyroid activity. The amount of selenium required depends on how much vitamin E is available. Thus, imbalances can give varied responses. The effect can be either beneficial or harmful to well-being.

Any substance can be hazardous if we are exposed to enough of it. Substances which have small differences between an innocuous or beneficial amount and the amount which is hazardous are especially dangerous. Fluoride has a particularly narrow margin of safety. Too little fluoride intake predisposes to tooth decay and too much leads to tooth discolouration and brittleness. About 1 part per million (1 ppm = 1 microgram per millilitre) in drinking water will protect against tooth decay, whereas about 2 to 3 parts per million can cause light mottling of the teeth. It is of interest that fish and seafood contain 5 to 10 ppm of fluoride (we eat less fluoride in fish than we drink in water) and tea 0.5 to 2 ppm.

With copper, the difference between a safe level and a hazardous amount is greater. We can tolerate at least 20 times the usual consumption levels before our health is likely to be affected.

It is wise to bear in mind that while a little may be good for you, more is not always better.