Many substances are toxic in high concentrations but essential for life in small concentrations. Iron, copper, chromium, and selenium, and many more, are all in this category. There has recently been concern about trace levels of arsenic in drinking water. Is there evidence that these levels are actually toxic, or is the "safe" level obtained from extrapolation from short exposure to high levels used by the EPA misleading? Note that arsenic is under nitrogen and phosphorus in the periodic table (and hence similar in many properties), and that bacteria have been identified which can use arsenic in lieu of phosphorus, and some studies show that chickens require trace amounts of arsenic for optimum growth.