Heavy metal (chemistry)
A heavy metal is a member of a loosely defined subset of elements that exhibit metallic properties. It mainly includes the transition metals, some metalloids, lanthanides, and actinides. Many different definitions have been proposed—some based on density, some on atomic number or atomic weight, and some on chemical properties or toxicity. The term heavy metal has been called a "misinterpretation" in an IUPAC technical report due to the contradictory definitions and its lack of a "coherent scientific basis". There is an alternative term toxic metal, for which no consensus of exact definition exists either. As discussed below, depending on context, heavy metal can include elements lighter than carbon and can exclude some of the heaviest metals. Heavy metals occur naturally in the ecosystem with large variations in concentration. In modern times, anthropogenic sources of heavy metals, i.e. pollution, have been introduced to the ecosystem.
Relationship to living organisms
Living organisms require varying amounts of "heavy metals". Iron, cobalt, copper, manganese, molybdenum, and zinc are required by humans. Excessive levels can be damaging to the organism. Other heavy metals such as mercury, plutonium, and lead are toxic metals and their accumulation over time in the bodies of animals can cause serious illness. Certain elements that are normally toxic are, for certain organisms or under certain conditions, beneficial. Examples include vanadium, tungsten, and even cadmium.
Heavy metal toxicity can result in damaged or reduced mental and central nervous function, lower energy levels, and damage to blood composition, lungs, kidneys, liver, and other vital organs. Long-term exposure may result in slowly progressing physical, muscular, and neurological degenerative processes that mimic Alzheimer's disease, Parkinson's disease, muscular dystrophy, and multiple sclerosis. Allergies are not uncommon, and repeated long-term contact with some metals (or their compounds) may cause cancer.
Heavy metals have an effect on the stability of colloids. Prokaryotic and eukaryotic cells are colloids, too. Colloids are sensitive to change of ion or heavy metal concentration and it leads to violation of colloid stability and subsequent disintegration. 
Heavy metal pollution
Motivations for controlling heavy metal concentrations in gas streams are diverse. Some of them are dangerous to health or to the environment (e.g. mercury, cadmium, lead, chromium), some may cause corrosion (e.g., lead), some are harmful in other ways (e.g. arsenic may pollute catalysts). Within the European community the eleven elements of highest concern are arsenic, cadmium, cobalt, chromium, copper, mercury, manganese, nickel, lead, tin, and thallium, the emissions of which are regulated in waste incinerators. Some of these elements are actually necessary for humans in minute amounts (cobalt, copper, chromium, manganese, nickel) while others are carcinogenic or toxic, affecting, among others, the central nervous system (manganese, mercury, lead, arsenic), the kidneys or liver (mercury, lead, cadmium, copper) or skin, bones, or teeth (nickel, cadmium, copper, chromium).
Heavy metal pollution can arise from many sources but most commonly arises from the purification of metals, e.g., the smelting of copper and the preparation of nuclear fuels. Electroplating is the primary source of chromium and cadmium. Cadmium, lead and zinc are released in tiny particulates as dust from rubber tires on road surfaces; the small size allows these toxic metals to rise on the wind to be inhaled, or transported onto topsoil or edible plants.
Through precipitation of their compounds or by ion exchange into soils and muds, heavy metal pollutants can localize and lay dormant, which can have severe effects on the environment. Unlike organic pollutants, heavy metals do not decay and thus pose a different kind of challenge for remediation. Plants, mushrooms, or microrganisms are occasionally successfully used to remove some heavy metals such as mercury. Plants which exhibit hyper accumulation can be used to remove heavy metals from soils by concentrating them in their bio matter. Some treatment of mining tailings has occurred where the vegetation is then incinerated to recover the heavy metals. In Electrokinetic extraction, heavy metals are removed from the soil using an electric field to induce electroosmosis to move the heavy metals. 
One of the largest problems associated with the persistence of heavy metals is the potential for bioaccumulation and biomagnification causing heavier exposure for some organisms than is present in the environment alone. Coastal fish (such as the smooth toadfish) and seabirds (such as the Atlantic Puffin) are often monitored for the presence of such contaminants. The fungus Aspergillus niger plays a role in the solubilization of heavy metal sulfides. 
In medical usage, heavy metals are loosely defined and include all toxic metals irrespective of their atomic weight: "heavy metal poisoning" can possibly include excessive amounts of iron, manganese, aluminium, mercury, cadmium, beryllium or such a semimetal as arsenic. This definition excludes bismuth, the heaviest of approximately stable elements, because of its low toxicity.
Heavy metals are valuable contrast agents for medical and scientific applications. They absorb X-rays and thus are valuable in X-ray imaging, as with barium meal. In a transmission electron microscope, heavy metals can be used to create specific contrast and expose fine detail.
Heavy metals continue to be used in some pigments, for instance lead(II) chromate. Although organic dyes can have very high absorbances, they can be eventually bleached on exposure to sunlight, thus sunlight-stable inorganic heavy metal pigments have their uses.
Heavy metals in a hazardous materials (or "hazmat") setting are for the most part classified in "Misc." on the UN model hazard class, but they are sometimes labeled as a poison when being transported.
Burnup of nuclear fuel is expressed in gigawatt-days per metric ton of heavy metal, where heavy metal means actinides like thorium, uranium, plutonium, etc., including both fissile and fertile material. It does not include elements such as oxygen that may be bonded to the fuel metals, or cladding materials such as zirconium, which might be considered heavy metals by some other standards.
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