- Arsenic (As) is a metalloid widely recognized as highly toxic and carcinogenic. - Chromium (Cr) has both essential and toxic forms; Hexavalent chromium (Cr(VI)) is highly toxic and carcinogenic. - Mercury (Hg) is a heavy metal that is highly toxic, particularly in its organic form (methylmercury), affecting the nervous system. - Copper (Cu) is an essential trace element necessary for many biological processes. While excessive intake can cause toxicity, it is less likely to be classified as highly toxic compared to arsenic, mercury, and certain forms of chromium in common environmental exposures, as the body has mechanisms to regulate copper levels and it is required for life.