Trace elements are important for life. However, adverse health effects might occur either after deficient intake or upon excessive exposure. Route of exposure and intake of protein, zinc, calcium and iron, and metal species influence toxicity and essentiality of metals. Metabolism and kinetics of trace elements involve metallothionein, consisting of four major forms I-IV of βΌ6,500 Da proteins. Changes in the geo-environment and environmental exposure from human activities will be due to, e.g., recycling of and leakage from waste dumps energy system including materials from semi-and superconductors where attention should be paid to gallium and indium. Acidification in the geo-environment changes availability and mobility of trace elements exemplified by arsenic, mercury, cadmium, copper, selenium, and aluminum. In the geo-environment, chemical reactions such as methylation and demethylation of mercury, e.g., take place. Aluminum becomes less firmly bound and increased mobility increases exposure of the human being. Selenium, on the contrary, shows less bioavailability in an acidic environment. Cadmium increases in food and crops, causing increased exposure to humans. This geo-environment recognizes pregnant women, children, people with poor nutritional and low iron status as vulnerable groups. In relation to trace elements, metallothionein (MT) might serve as an indicator of environmental exposure and biochemical indicator of exposure. Increased tissue concentration of metallothionein indicates for MT-I and -II increased exposure to trace elements. MT-III, not inducible, might play an important role in zinc metabolism and elements causing neurotoxicity. The mechanistic model for cadmium and metallothionein illustrates toxic action of cadmium in the cell and how adverse health effects develop.