There is a recent paper suggesting it may in fact be essential as a trace element: https://www.ncbi.nlm.nih.gov/pubmed/24906154 https://www.ncbi.nlm.nih.gov/pub

There is a recent paper suggesting it may in fact be essential as a trace element: https://www.ncbi.nlm.nih.gov/pubmed/24906154 https://www.ncbi.nlm.nih.gov/pubmed/24906154 Although exactly where modern diet/exposure lies on the necessary/harm spectrum is substantially different question.