Selenium is a fascinating element. Very necessary for healthy life, UpTo to a level, then it may be toxic. Different areas of the planet have more present in th

Selenium is a fascinating element. Very necessary for healthy life, UpTo to a level, then it may be toxic. Different areas of the planet have more present in the soil. So on some places, it is added as a supplement to feed(especially animals - horses cows), but if that feed is given to an animal that doesn't need the supplement, it may kill it. (See polo pony kerfuffle.) There are various states of selenium which makes analysis, and chemistry complex. Se^-2,0,+2,+4, and even +6. Similar(but not) to sulphur and in strange ways to iron. Certain oxidation states are more reactive(obviously) and therefore are more toxic. But when dealing with environmental issues, finding which state of selenium is present, is extremely difficult, and requires very complex laboratory procedures. Total selenium does not answer most questions. Chemistry can be fun!