Heavy metals

The criteria used, and whether metalloids are included, vary depending on the author and context and has been argued that the term "heavy metal" should be avoided.

Some heavy metals are either essential nutrients (typically iron, cobalt, copper and zinc), or relatively harmless (such as ruthenium, silver and indium), but can be toxic in larger amounts or certain forms.

Potential sources of heavy metal poisoning include mining, tailings, smelting, industrial waste, agricultural runoff, occupational exposure, paints and treated timber.

They are used in, for example, golf clubs, cars, antiseptics, self-cleaning ovens, plastics, solar panels, mobile phones, and particle accelerators.

The International Union of Pure and Applied Chemistry (IUPAC), which standardizes nomenclature, says "the term heavy metals is both meaningless and misleading".

[2] The IUPAC report focuses on the legal and toxicological implications of describing "heavy metals" as toxins when there is no scientific evidence to support a connection.

[22] Six elements near the end of periods (rows) 4 to 7 sometimes considered metalloids are treated here as metals: they are germanium (Ge), arsenic (As), selenium (Se), antimony (Sb), tellurium (Te), and astatine (At).

These are iron and copper (oxygen and electron transport); cobalt (complex syntheses and cell metabolism); vanadium and manganese (enzyme regulation or functioning); chromium (glucose utilisation); nickel (cell growth); arsenic (metabolic growth in some animals and possibly in humans) and selenium (antioxidant functioning and hormone production).

Gallium, germanium (a metalloid), indium, and most lanthanides can stimulate metabolism, and titanium promotes growth in plants[52] (though it is not always considered a heavy metal).

Chromium, arsenic, cadmium, mercury, and lead have the greatest potential to cause harm on account of their extensive use, the toxicity of some of their combined or elemental forms, and their widespread distribution in the environment.

[55] These five elements have a strong affinity for sulfur; in the human body they usually bind, via thiol groups (–SH), to enzymes responsible for controlling the speed of metabolic reactions.

[56] Chromium (in its hexavalent form) and arsenic are carcinogens; cadmium causes a degenerative bone disease; and mercury and lead damage the central nervous system.

[61] Other heavy metals noted for their potentially hazardous nature, usually as toxic environmental pollutants, include manganese (central nervous system damage);[62] cobalt and nickel (carcinogens);[63] copper,[64] zinc,[65] selenium[66] and silver[67] (endocrine disruption, congenital disorders, or general toxic effects in fish, plants, birds, or other aquatic organisms); tin, as organotin (central nervous system damage);[68] antimony (a suspected carcinogen);[69] and thallium (central nervous system damage).

Kidney failure and fatalities have been recorded arising from the ingestion of germanium dietary supplements (~15 to 300 g in total consumed over a period of two months to three years).

[75] Heavy metals can degrade air, water, and soil quality, and subsequently cause health issues in plants, animals, and people, when they become concentrated as a result of industrial activities.

[76][77] Common sources of heavy metals in this context include vehicle emissions;[78] motor oil;[79] fertilisers;[80] glassworking;[81] incinerators;[82] treated timber;[83] aging water supply infrastructure;[84] and microplastics floating in the world's oceans.

[85] Recent examples of heavy metal contamination and health risks include the occurrence of Minamata disease, in Japan (1932–1968; lawsuits ongoing as of 2016);[86] the Bento Rodrigues dam disaster in Brazil,[87] high levels of lead in drinking water supplied to the residents of Flint, Michigan, in the north-east of the United States[88] and 2015 Hong Kong heavy metal in drinking water incidents.

[89] Despite their overall scarcity, heavy metals can become concentrated in economically extractable quantities as a result of mountain building, erosion, or other geological processes.

[107][108] Recent speculation suggests that uranium (and thorium) in the core may generate a substantial amount of the heat that drives plate tectonics and (ultimately) sustains the Earth's magnetic field.

Examples of such atomic properties include: partly filled d- or f- orbitals (in many of the transition, lanthanide, and actinide heavy metals) that enable the formation of coloured compounds;[120] the capacity of heavy metal ions (such as platinum,[121] cerium[122] or bismuth[123]) to exist in different oxidation states and are used in catalysts;[124] strong exchange interactions in 3d or 4f orbitals (in iron, cobalt, and nickel, or the lanthanide heavy metals) that give rise to magnetic effects;[125] and high atomic numbers and electron densities that underpin their nuclear science applications.

[127] Some uses of heavy metals, including in sport, mechanical engineering, military ordnance, and nuclear science, take advantage of their relatively high densities.

[133] In track and field sport, steel balls used in the hammer throw and shot put events are filled with lead in order to attain the minimum weight required under international rules.

[145] In the 1970s, tantalum was found to be more effective than copper in shaped charge and explosively formed anti-armour weapons on account of its higher density, allowing greater force concentration, and better deformability.

[146] Less-toxic heavy metals, such as copper, tin, tungsten, and bismuth, and probably manganese (as well as boron, a metalloid), have replaced lead and antimony in the green bullets used by some armies and in some recreational shooting munitions.

[150] Platinum, osmium, copper, ruthenium, and other heavy metals, including arsenic, are used in anti-cancer treatments, or have shown potential.

[152] Copper, zinc, silver, gold, or mercury are used in antiseptic formulations;[153] small amounts of some heavy metals are used to control algal growth in, for example, cooling towers.

[154] Depending on their intended use as fertilisers or biocides, agrochemicals may contain heavy metals such as chromium, cobalt, nickel, copper, zinc, arsenic, cadmium, mercury, or lead.

[157] The colours of glass, ceramic glazes, paints, pigments, and plastics are commonly produced by the inclusion of heavy metals (or their compounds) such as chromium, manganese, cobalt, copper, zinc, zirconium, molybdenum, silver, tin, praseodymium, neodymium, erbium, tungsten, iridium, gold, lead, or uranium.

[165] Magnets are often made of heavy metals such as manganese, iron, cobalt, nickel, niobium, bismuth, praseodymium, neodymium, gadolinium, and dysprosium.

Niche uses of heavy metals with high atomic numbers occur in diagnostic imaging, electron microscopy, and nuclear science.