Environmental pollution by metals with a high relative atomic mass, such as lead and mercury. These metals derive from a number of sources, including industrial effluents and leaching of metal ions from the soil into lakes and rivers by acid rain. They are easily incorporated into biological molecules and exert their toxic effects by displacing essential metals of a lower binding power in biologically active molecules or by acting as noncompetitive inhibitors of enzymes (see inhibition). However, some plants, known as hyperaccumulators, can tolerate high concentrations of heavy metals and accumulate them in tissues. See also ecotoxicology.