It is one of the most toxic substances known to humanity, particularly when comined with other atoms and molecules, such as the methyl or chloride forms. These have far greater biological penetration.
Brain tissue is particularly at risk.
Despite its poisonous qualities, indiustrial applications of mercury have grown dramatically. There are really three issues where mercury is concerned. The first is the tendency of mercury to concentrate electrical fields. This is an ordinary biophysical effect and explains why people with too many dental fillings cannot work in areas where large field currents are developed. The second issue is hypersensitivity.