Heavy metals 

The heavy post-transition metals like Cd, Hg and Pb are toxic due to the very strong complexing capability of 'soft' cations like Hg²⁺. They comprise low concentrations in natural waters since they form insoluble sulfides. Compounds which are either more soluble in water or volatile pose an environmental danger. Of these elements, lead has been the most extensively used, in paints, in pipes for drinking water, (in the form of tetraethyl lead Pb(C₂H₅)₄ like a gasoline additive to enhance combustion. Like the toxic hazards have been very visibly identified, these uses have been phased out.

Mercury also had several applications, that are including in hat-making (in which the symptoms of mercury poisoning priveded rise to the saying 'mad as a hatter') but its industrial use (example for NaCl electrolysis) has also refuses. Cases of acute mercury poisoning contain resulted from eating fish from water polluted through industrial Hg compounds. A number of organisms convert inorganic compounds into ones consisting of [CH₃Hg]⁺, that are particularly toxic like they pass more easily via the nonpolar constituents of cell membranes. It is probable that methylcobalamin is concerned in this transformation.