Definition of Poison:
A toxic substance; however may have medicinal properties if used in sufficiently small (drop) doses [ORWJr].
Consider the words of the famous 16th century alchemist Paracelsus, sometimes called the father of toxicology:
"Alle Dinge sind Gift, und nichts ohne Gift; allein die Dosis macht, daß ein Ding kein Gift ist [Paracelsus]."Or, more commonly: "The dose makes the poison" That is to say, substances considered toxic are harmless in small doses, and conversely an ordinarily harmless substance can be deadly if over-consumed [ORWJr].
"All things are poison, and nothing is without poison; only the dose permits something not to be poisonous."Consider for example the deadly consequences of over consumption of water: on January 12, 2007, a [radio show contestant] named Jennifer Strange, 28, died of water intoxication hours after taking part in the "Hold Your Wee for a Wii" contest [KDND2007].
Keep in mind that nature has trace amounts of many toxins, even in areas where industry has not intruded. You can run but you cannot hide from TRACE amounts of toxins. The big question is whether the traces are NECESSARY NUTRIENTS, TOXIC POLLUTANTS, or MEANINGLESS. All of the necessary minerals in the body (Iron, copper, calcium, magnesium, selenium, iodine, sodium, potassium, etc) are toxic if consumed in too high of an amount. Unfortunately, medical science does not know it all (yet) [ORWJr].
Just because our most sensitive instruments today can measure the presence of traces of toxins in the water, does not necessarily mean that the water is dangerous. Generally speaking, [homeopathy notwithstanding], toxicity rises exponentially as the dose is increased, and conversely, falls exponentially as the dose is reduced [ORWJr].
In fact, there are trace elements, such as selenium, that we now know are essential to life, that were once only viewed as toxins at higher doses There is even some evidence that arsenic is also a NECESSARY nutrient at appropriate low doses. The appropriate level has not yet been established [ORWJr].
Additional factors can also affect what dose of a substance is needed to become toxic. One additional factor is the nutritional status of the person. The liver is the primary organ for elimination of toxins (and metabolic waste) in the body. The liver needs energy (ATP), vitamins, minerals, and protein to carry out its "Phase I and Phase II" processing of toxins in order to eliminate them. If a person is malnourished, then the liver does not function well to eliminate the toxins. Furthermore, the liver does this processing using a large number of enzymes called cytochromes - a different enzyme for each "kind" of toxin being processed (I am simplifying this and omitting detail). There is genetic variation in the efficiency of these various enzymes, so not all people process the various toxins as efficiently as others. So substance A might be more toxic for one person because that person's enzyme to process A is not as efficient as most people's; at the same time, substance B might be less toxic for that same person because that person's enzyme to process substance B is MORE efficient than most people's. Substances A and B can also be drugs - which explains why some drugs cause side effects in some people, but not others [ORWJr].
This is all very fascinating, but there is a lot more to be learned about this area of individualized medical care [ORWJr].