Continued progress in the theory and practice of trace element analytical chemistry has made possible significant advances in investigating the role and fate of trace elements in biological systems. Public health commissions and environmental protection agencies have subsequently established requirements for intakes of and exposures to trace elements both from the nutritional (copper-zinc) and from the toxicological (cadmium-mercury) perspectives. Some trace elements demonstrate the properties of both categories, and consequently give rise to questions about the toxicity of essential dietary minerals. Selenium and chromium are typical examples of this toxicity-essentiality paradox. The systemic intoxication by and/or nutritional importance ...