Trace elements mainly include three categories: 1) essential trace elements, including iodine, zinc, selenium, copper, molybdenum, chromium, cobalt and iron, which are essential for maintaining the normal biochemical metabolism of the body; 2) possible essential trace elements, including manganese, silicon, nickel, boron and vanadium, which are five possible essential trace elements; 3) potentially toxic trace elements, but at low doses, which are functional for the body. Trace elements that are potentially toxic but functional to the body at low doses include fluorine, lead, cadmium, mercury, arsenic, aluminum, lithium, and tin, making a total of eight.