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Chromium is a chemical element; it has symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.
Chromium is valued for its high corrosion resistance and hardness. A major development in steel production was the discovery that steel could be made highly resistant to corrosion and discoloration by adding metallic chromium to form stainless steel. Stainless steel and chrome plating (electroplating with chromium) together comprise 85% of the commercial use. Chromium is also greatly valued as a metal that is able to be highly polishing while resisting . Polished chromium reflects almost 70% of the visible spectrum, and almost 90% of infrared. The name of the element is derived from the Ancient Greek word χρῶμα, chrōma, meaning color, χρῶμα , Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus because many chromium compounds are intensely colored.
Industrial production of chromium proceeds from chromite ore (mostly FeCr2O4) to produce ferrochromium, an iron-chromium alloy, by means of aluminothermic or silicothermic reactions. Ferrochromium is then used to produce alloys such as stainless steel. Pure chromium metal is produced by a different process: roasting and leaching of chromite to separate it from iron, followed by reduction with carbon and then aluminium.
Trivalent chromium (Cr(III)) occurs naturally in many foods and is sold as a dietary supplement, although there is insufficient evidence that dietary chromium provides nutritional benefit to people. In 2014, the European Food Safety Authority concluded that research on dietary chromium did not justify it to be recognized as an essential nutrient.
While chromium metal and Cr(III) ions are considered non-toxic, chromate and its derivatives, often called "hexavalent chromium", is toxic and carcinogenic. According to the European Chemicals Agency (ECHA), chromium trioxide that is used in industrial electroplating processes is a "substance of very high concern" (SVHC).
Chromium is the first element in the 3d series where the 3d electrons start to sink into the core; they thus contribute less to metallic bonding, and hence the melting and boiling points and the enthalpy of atomisation of chromium are lower than those of the preceding element vanadium. Chromium(VI) is a strong oxidising agent in contrast to the molybdenum(VI) and tungsten(VI) oxides.Greenwood and Earnshaw, pp. 1004–5
Chromium has a melting point of 1907 °C (3465 °F), which is relatively low compared to the majority of transition metals. However, it still has the second highest melting point out of all the period 4 elements, being topped by vanadium by 3 °C (5 °F) at 1910 °C (3470 °F). The boiling point of 2671 °C (4840 °F), however, is comparatively lower, having the fourth lowest boiling point out of the Period 4 alone behind copper, manganese and zinc.The melting/boiling point of transition metals are usually higher compared to the alkali metals, alkaline earth metals, and nonmetals, which is why the range of elements compared to chromium differed between comparisons The electrical resistivity of chromium at 20 °C is 125 ohm-.
Chromium has a high specular reflection in comparison to other transition metals. In infrared, at 425 micrometre, chromium has a maximum reflectance of about 72%, reducing to a minimum of 62% at 750 μm before rising again to 90% at 4000 μm. When chromium is used in stainless steel alloys and polishing, the specular reflection decreases with the inclusion of additional metals, yet is still high in comparison with other alloys. Between 40% and 60% of the visible spectrum is reflected from polished stainless steel. The explanation on why chromium displays such a high turnout of reflected photon waves in general, especially the 90% in infrared, can be attributed to chromium's magnetic properties. Chromium has unique magnetic properties; it is the only elemental solid that shows antiferromagnetic ordering at room temperature and below. Above 38 °C, its magnetic ordering becomes paramagnetic. The antiferromagnetic properties, which cause the chromium atoms to temporarily ionization and bond with themselves, are present because the body-centric cubic's magnetic properties are disproportionate to the lattice periodicity. This is due to the magnetic moments at the cube's corners and the unequal, but antiparallel, cube centers. From here, the frequency-dependent relative permittivity of chromium, deriving from Maxwell's equations and chromium's antiferromagnetism, leaves chromium with a high infrared and visible light reflectance.
The surface chromia scale, is adherent to the metal. In contrast, iron forms a more porous oxide which is weak and flakes easily and exposes fresh metal to the air, causing continued . At room temperature, the chromia scale is a few atomic layers thick, growing in thickness by outward diffusion of metal ions across the scale. Above 950 °C volatile chromium trioxide forms from the chromia scale, limiting the scale thickness and oxidation protection.
Chromium, unlike iron and nickel, does not suffer from hydrogen embrittlement. However, it does suffer from nitrogen embrittlement, reacting with nitrogen from air and forming brittle nitrides at the high temperatures necessary to work the metal parts.National Research Council (U.S.). Committee on Coatings (1970). 9780309017695, National Academy of Sciences. . ISBN 9780309017695
53Cr is the radiogenic decay product of 53manganese (half-life 3.74 million years). Chromium isotopes are typically collocated (and compounded) with manganese isotopes. This circumstance is useful in isotope geology. Manganese-chromium isotope ratios reinforce the evidence from 26Al and 107Palladium concerning the early history of the Solar System. Variations in 53Cr/52Cr and Mn/Cr ratios from several meteorites indicate an initial 53Mn/55Mn ratio that suggests Mn-Cr isotopic composition must result from in-situ decay of 53Mn in differentiated planetary bodies. Hence 53Cr provides additional evidence for nucleosynthesis processes immediately before coalescence of the Solar System. 53Cr has been posited as a proxy for atmospheric oxygen concentration.
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Chromium(III) tends to form octahedral complexes. Commercially available chromium(III) chloride hydrate is the dark green complex CrCl2(H2O)4Cl. Closely related compounds are the pale green CrCl(H2O)5Cl2 and violet Cr(H2O)6Cl3. If anhydrous violet chromium(III) chloride is dissolved in water, the violet solution turns green after some time as the chloride in the inner coordination sphere is replaced by water. This kind of reaction is also observed with solutions of chrome alum and other water-soluble chromium(III) salts. A tetrahedral coordination of chromium(III) has been reported for the Cr-centered Keggin structure anion α-CrW12O405–.
Chromium(III) hydroxide (Cr(OH)3) is amphoterism, dissolving in acidic solutions to form Cr(H2O)63+, and in basic solutions to form . It is dehydrated by heating to form the green chromium(III) oxide (Cr2O3), a stable oxide with a crystal structure identical to that of corundum.
Sodium chromate is produced industrially by the oxidative roasting of chromite ore with sodium carbonate. The change in equilibrium is visible by a change from yellow (chromate) to orange (dichromate), such as when an acid is added to a neutral solution of potassium chromate. At yet lower pH values, further condensation to more complex of chromium is possible.
Both the chromate and dichromate anions are strong oxidizing reagents at low pH:
They are, however, only moderately oxidizing at high pH:
Chromium(VI) compounds in solution can be detected by adding an acidic hydrogen peroxide solution. The unstable dark blue chromium(VI) peroxide (CrO5) is formed, which can be stabilized as an ether adduct .
Chromic acid has the hypothetical formula . It is a vaguely described chemical, despite many well-defined chromates and dichromates being known. The dark red chromium(VI) oxide , the acid anhydride of chromic acid, is sold industrially as "chromic acid". It can be produced by mixing sulfuric acid with dichromate and is a strong oxidizing agent.
Compounds of chromium(IV) are slightly more common than those of chromium(V). The tetrahalides, CrF4, CrCl4, and CrBr4, can be produced by treating the trihalides () with the corresponding halogen at elevated temperatures. Such compounds are susceptible to disproportionation reactions and are not stable in water. Organic compounds containing Cr(IV) state such as chromium tetra t-butoxide are also known.
Most chromium(I) compounds are obtained solely by oxidation of electron-rich, octahedral chromium(0) complexes. Other chromium(I) complexes contain cyclopentadienyl ligands. As verified by X-ray diffraction, a Cr-Cr quintuple bond (length 183.51(4) pm) has also been described. Extremely bulky monodentate ligands stabilize this compound by shielding the quintuple bond from further reactions.
About two-fifths of the chromite ores and concentrates in the world are produced in South Africa, about a third in Kazakhstan, while India, Russia, and Turkey are also substantial producers. Untapped chromite deposits are plentiful, but geographically concentrated in Kazakhstan and southern Africa. Although rare, deposits of Native metal chromium exist. Chromium (with location data), Mindat. The Udachnaya Pipe in Russia produces samples of the native metal. This mine is a kimberlite pipe, rich in , and the Redox helped produce both elemental chromium and diamonds. Chromium from Udachnaya-Vostochnaya pipe, Daldyn, Daldyn-Alakit kimberlite field, Saha Republic (Sakha Republic; Yakutia), Eastern-Siberian Region, Russia , Mindat.
The relation between Cr(III) and Cr(VI) strongly depends on pH and oxidative properties of the location. In most cases, Cr(III) is the dominating species, but in some areas, the ground water can contain up to 39 μg/L of total chromium, of which 30 μg/L is Cr(VI).
From the 1970s, until 2019, it was widely believed that the technique of chromium plating to prevent metal corrosion had been invented in ancient China. This technique was thought to have been used, for example, to protect bronze artifacts such as arrowheads and sword blades buried as grave goods in the mausoleum of the First Emperor of Qin. However, a detailed scientific investigation in 2019 revealed that the chromium found on these artifacts originated naturally from the lacquer applied to them. Scientists now believe that the excellent preservation of the artifacts was not due to intentional chromium plating, but rather to the burial environment: the soil was fine-grained and alkaline, which limited aeration and the growth of organic matter, thereby creating optimal conditions for metal preservation.
Chromium minerals as pigments came to the attention of the west in the eighteenth century. On 26 July 1761, Johann Gottlob Lehmann found an orange-red mineral in the Beryozovskoye mines in the Ural Mountains which he named Siberian red lead. Though misidentified as a lead compound with selenium and iron components, the mineral was in fact crocoite with a formula of PbCrO4. In 1770, Peter Simon Pallas visited the same site as Lehmann and found a red lead mineral that was discovered to possess useful properties as a pigment in . After Pallas, the use of Siberian red lead as a paint pigment began to develop rapidly throughout the region. Crocoite would be the principal source of chromium in pigments until the discovery of chromite many years later.
In 1794, Louis Nicolas Vauquelin received samples of crocoite ore. He produced chromium trioxide (CrO3) by mixing crocoite with hydrochloric acid. In 1797, Vauquelin discovered that he could isolate metallic chromium by heating the oxide in a charcoal oven, for which he is credited as the one who truly discovered the element. Vauquelin was also able to detect traces of chromium in precious , such as ruby and emerald.
During the nineteenth century, chromium was primarily used not only as a component of paints, but in tanning salts as well. For quite some time, the crocoite found in Russia was the main source for such tanning materials. In 1827, a larger chromite deposit was discovered near Baltimore, United States, which quickly met the demand for tanning salts much more adequately than the crocoite that had been used previously. This made the United States the largest producer of chromium products until the year 1848, when larger deposits of chromite were uncovered near the city of Bursa, Turkey. With the development of metallurgy and chemical industries in the Western world, the need for chromium increased.
Chromium is also famous for its reflective, metallic luster when polished. It is used as a protective and decorative coating on car parts, plumbing fixtures, furniture parts and many other items, usually applied by electroplating. Chromium was used for electroplating as early as 1848, but this use only became widespread with the development of an improved process in 1924.
The largest producers of chromium ore in 2013 have been South Africa (48%), Kazakhstan (13%), Turkey (11%), and India (10%), with several other countries producing the rest of about 18% of the world production.
The two main products of chromium ore refining are ferrochromium and metallic chromium. For those products the ore smelter process differs considerably. For the production of ferrochromium, the chromite ore (FeCr2O4) is reduced in large scale in electric arc furnace or in smaller smelters with either aluminium or silicon in an aluminothermic reaction.
For the production of pure chromium, the iron must be separated from the chromium in a two step roasting and leaching process. The chromite ore is heated with a mixture of calcium carbonate and sodium carbonate in the presence of air. The chromium is oxidized to the hexavalent form, while the iron forms the stable Fe2O3. The subsequent leaching at higher elevated temperatures dissolves the chromates and leaves the insoluble iron oxide. The chromate is converted by sulfuric acid into the dichromate.
The dichromate is converted to the chromium(III) oxide by reduction with carbon and then reduced in an aluminothermic reaction to chromium.
The high hardness and corrosion resistance of unalloyed chromium makes it a reliable metal for surface coating; it is still the most popular metal for sheet coating, with its above-average durability, compared to other coating metals. A layer of chromium is deposited on pretreated metallic surfaces by electroplating techniques. There are two deposition methods: thin, and thick. Thin deposition involves a layer of chromium below 1 μm thickness deposited by chrome plating, and is used for decorative surfaces. Thicker chromium layers are deposited if wear-resistant surfaces are needed. Both methods use acidic chromate or dichromate solutions. To prevent the energy-consuming change in oxidation state, the use of chromium(III) sulfate is under development; for most applications of chromium, the previously established process is used.
In the chromate conversion coating process, the strong oxidative properties of chromates are used to deposit a protective oxide layer on metals like aluminium, zinc, and cadmium. This passivation and the self-healing properties of the chromate stored in the chromate conversion coating, which is able to migrate to local defects, are the benefits of this coating method.Edwards (1997). 9780904477160, Finishing Publications Ltd. and ASMy International. ISBN 9780904477160 Because of environmental and health regulations on chromates, alternative coating methods are under development.
Chromic acid anodizing (or Type I anodizing) of aluminium is another electrochemical process that does not lead to the deposition of chromium, but uses chromic acid as an electrolyte in the solution. During anodization, an oxide layer is formed on the aluminium. The use of chromic acid, instead of the normally used sulfuric acid, leads to a slight difference of these oxide layers. The high toxicity of Cr(VI) compounds, used in the established chromium electroplating process, and the strengthening of safety and environmental regulations demand a search for substitutes for chromium, or at least a change to less toxic chromium(III) compounds.
Chromium oxides are also used as a green pigment in the field of glassmaking and also as a glaze for ceramics.Gerd Anger et al. "Chromium Compounds" Ullmann's Encyclopedia of Industrial Chemistry 2005, Wiley-VCH, Weinheim. Green chromium oxide is extremely lightfastness and as such is used in cladding coatings. It is also the main ingredient in infrared reflecting paints, used by the armed forces to paint vehicles and to give them the same infrared reflectance as green leaves.
Chromium(VI) salts are used for the preservation of wood. For example, chromated copper arsenate (CCA) is used in timber treatment to protect wood from decay fungi, wood-attacking insects, including termites, and marine borers. The formulations contain chromium based on the oxide CrO3 between 35.3% and 65.5%. In the United States, 65,300 metric tons of CCA solution were used in 1996.
Chromium(III) salts, especially chrome alum and chromium(III) sulfate, are used in the tanning of leather. The chromium(III) stabilizes the leather by cross linking the collagen fibers. Chromium tanned leather can contain 4–5% of chromium, which is tightly bound to the proteins. Although the form of chromium used for tanning is not the toxic hexavalent variety, there remains interest in management of chromium in the tanning industry. Recovery and reuse, direct/indirect recycling, and "chrome-less" or "chrome-free" tanning are practiced to better manage chromium usage.
The high heat resistivity and high melting point makes chromite and chromium(III) oxide a material for high temperature refractory applications, like , cement , molds for the firing of and as foundry sands for the casting of metals. In these applications, the refractory materials are made from mixtures of chromite and magnesite. The use is declining because of the environmental regulations due to the possibility of the formation of chromium(VI).
Several chromium compounds are used as for processing hydrocarbons. For example, the Phillips catalyst, prepared from chromium oxides, is used for the production of about half the world's polyethylene. Fe-Cr mixed oxides are employed as high-temperature catalysts for the water gas shift reaction. (1989). 9780723408574, Taylor & Francis. ISBN 9780723408574 Copper chromite is a useful hydrogenation catalyst.
In contrast, hexavalent chromium (Cr(VI) or Cr6+) is highly toxic and . Ingestion of chromium(VI) in water has been linked to stomach tumors, and it may also cause allergic contact dermatitis.
"Chromium deficiency", involving a lack of Cr(III) in the body, or perhaps some complex of it, such as glucose tolerance factor, is not accepted as a medical condition, as it has no symptoms and healthy people do not require chromium supplementation. Some studies suggest that the biologically active form of chromium(III) is transported in the body via an oligopeptide called low-molecular-weight chromium-binding substance (chromodulin), which might play a role in the insulin signaling pathway.
The chromium content of common foods is generally low (1–13 micrograms per serving). The chromium content of food varies widely, due to differences in soil mineral content, growing season, plant cultivar, and contamination during processing. Chromium (and nickel) leach into food cooked in stainless steel, with the effect being largest when the cookware is new. Acidic foods that are cooked for many hours also exacerbate this effect.
The U.S. National Academy of Medicine (NAM) updated the Estimated Average Requirements (EARs) and the Recommended Dietary Allowances (RDAs) for chromium in 2001. For chromium, there was insufficient information to set EARs and RDAs, so its needs are described as estimates for Adequate Intake (AI). From a 2001 assessment, AI of chromium for women ages 14 through 50 is 25 μg/day, and the AI for women ages 50 and above is 20 μg/day. The AIs for women who are pregnant are 30 μg/day, and for women who are lactating, the set AI is 45 μg/day. The AI for men ages 14 through 50 is 35 μg/day, and the AI for men ages 50 and above is 30 μg/day. For children ages 1 through 13, the AI increases with age from 0.2 μg/day up to 25 μg/day. As for safety, the NAM sets Tolerable Upper Intake Levels (ULs) for vitamins and minerals when the evidence is sufficient. In the case of chromium, there is not yet enough information, hence no UL has been established. Collectively, the EARs, RDAs, AIs, and ULs are the parameters for the nutrition recommendation system known as Dietary Reference Intake (DRI).
Australia and New Zealand consider chromium to be an essential nutrient, with an AI of 35 μg/day for men, 25 μg/day for women, 30 μg/day for women who are pregnant, and 45 μg/day for women who are lactating. A UL has not been set due to the lack of sufficient data. India considers chromium to be an essential nutrient, with an adult recommended intake of 33 μg/day. Japan also considers chromium to be an essential nutrient, with an AI of 10 μg/day for adults, including women who are pregnant or lactating. A UL has not been set.
The EFSA does not consider chromium to be an essential nutrient.
After evaluation of research on the potential nutritional value of chromium, the European Food Safety Authority concluded that there was no evidence of benefit by dietary chromium in healthy people, thereby declining to establish recommendations in Europe for dietary intake of chromium.
In other parts of the petition, the FDA rejected claims for chromium picolinate and cardiovascular disease, retinopathy or kidney disease caused by abnormally high blood sugar levels.
In 2010, chromium(III) picolinate was approved by Health Canada to be used in dietary supplements. Approved labeling statements include: a factor in the maintenance of good health, provides support for healthy glucose metabolism, helps the body to metabolize carbohydrates and helps the body to metabolize fats. The European Food Safety Authority approved claims in 2010 that chromium contributed to normal macronutrient metabolism and maintenance of normal blood glucose concentration, but rejected claims for maintenance or achievement of a normal body weight, or reduction of tiredness or fatigue. Scientific Opinion on the substantiation of health claims related to chromium and contribution to normal macronutrient metabolism (ID 260, 401, 4665, 4666, 4667), maintenance of normal blood glucose concentrations (ID 262, 4667), contribution to the maintenance or achievement of a normal body weight (ID 339, 4665, 4666), and reduction of tiredness and fatigue (ID 261) pursuant to Article 13(1) of Regulation (EC) No 1924/2006 European Food Safety Authority EFSA J 2010;8(10)1732.
However, in a 2014 reassessment of studies to determine whether a Dietary Reference Intake value could be established for chromium, EFSA stated:
Acute and chronic exposure to chromium(VI) affects fish behavior, physiology, reproduction and survival. Hyperactivity and erratic swimming have been reported in contaminated environments. Egg hatching and fingerling survival are affected. In adult fish there are reports of histopathological damage to liver, kidney, muscle, intestines, and gills. Mechanisms include mutagenic gene damage and disruptions of enzyme functions.
There is evidence that fish may not require chromium, but benefit from a measured amount in diet. In one study, juvenile fish gained weight on a zero chromium diet, but the addition of 500 μg of chromium in the form of chromium chloride or other supplement types, per kilogram of food (dry weight), increased weight gain. At 2,000 μg/kg the weight gain was no better than with the zero chromium diet, and there were increased DNA strand breaks.
The carcinogenity of chromate dust has been known for a long time, and in 1890 the first publication described the elevated cancer risk of workers in a chromate dye company. Three mechanisms have been proposed to describe the genotoxicity of chromium(VI). The first mechanism includes highly reactive and other reactive radicals which are by products of the reduction of chromium(VI) to chromium(III). The second process includes the direct binding of chromium(V), produced by reduction in the cell, and chromium(IV) compounds to the DNA. The last mechanism attributed the genotoxicity to the binding to the DNA of the end product of the chromium(III) reduction.
Chromium salts (chromates) are also the cause of allergic reactions in some people. Chromates are often used to manufacture, amongst other things, leather products, paints, cement, mortar and anti-corrosives. Contact with products containing chromates can lead to allergic contact dermatitis and irritant dermatitis, resulting in ulceration of the skin, sometimes referred to as "chrome ulcers". This condition is often found in workers that have been exposed to strong chromate solutions in electroplating, tanning and chrome-producing manufacturers.
In 2010, the Environmental Working Group studied the drinking water in 35 American cities in the first nationwide study. The study found measurable hexavalent chromium in the tap water of 31 of the cities sampled, with Norman, Oklahoma, at the top of list; 25 cities had levels that exceeded California's proposed limit.
The more toxic hexavalent chromium form can be reduced to the less soluble trivalent oxidation state in soils by organic matter, ferrous iron, sulfides, and other reducing agents, with the rates of such reduction being faster under more acidic conditions than under more alkaline ones. In contrast, trivalent chromium can be oxidized to hexavalent chromium in soils by manganese oxides, such as Mn(III) and Mn(IV) compounds. Since the solubility and toxicity of chromium (VI) are greater than those of chromium (III), the oxidation-reduction conversions between the two oxidation states have implications for movement and bioavailability of chromium in soils, groundwater, and plants.
Dietary recommendations
There is disagreement on chromium's status as an essential nutrient. Governmental departments from Australia, New Zealand, India, and Japan consider chromium as essential, while the United States and European Food Safety Authority of the European Union do not.
Labeling
For U.S. food and dietary supplement labeling purposes, the amount of the substance in a serving is expressed as a percent of the Daily Value (%DV). For chromium labeling purposes, 100% of the Daily Value was 120 μg. As of 27 May 2016, the percentage of daily value was revised to 35 μg to bring the chromium intake into a consensus with the official Recommended Dietary Allowance. A table of the old and new adult daily values in the United States is provided at Reference Daily Intake.
Food sources
Food composition databases such as those maintained by the U.S. Department of Agriculture do not contain information on the chromium content of foods. A wide variety of animal and vegetable foods contain chromium. Content per serving is influenced by the chromium content of the soil in which the plants are grown, by foodstuffs fed to animals, and by processing methods, as chromium is leached into foods if processed or cooked in stainless steel equipment.
One diet analysis study conducted in Mexico reported an average daily chromium intake of 30 micrograms. An estimated 31% of adults in the United States consume multi-vitamin/mineral dietary supplements, which often contain 25 to 60 micrograms of chromium.
Supplementation
Chromium is an ingredient in total parenteral nutrition (TPN), because deficiency can occur after months of intravenous feeding with chromium-free TPN. It is also added to nutritional products for . Although the mechanism of action in biological roles for chromium is unclear, in the United States chromium-containing products are sold as non-prescription dietary supplements in amounts ranging from 50 to 1,000 μg. Lower amounts of chromium are also often incorporated into multi-vitamin/mineral supplements consumed by an estimated 31% of adults in the United States. Chemical compounds used in dietary supplements include chromium chloride, chromium citrate, chromium(III) picolinate, chromium(III) polynicotinate, and other chemical compositions. The benefit of supplements has not been proven.
Initiation of research on glucose
The notion of chromium as a potential regulator of glucose metabolism began in the 1950s when scientists performed a series of experiments controlling the diet of rats. The experimenters subjected the rats to a chromium deficient diet, and witnessed an inability to respond effectively to increased levels of blood glucose. A chromium-rich yeast was provided in the diet, enabling the rats to effectively metabolize glucose, and so giving evidence that chromium may have a role in glucose management.
Approved and disapproved health claims
In 2005, the U.S. Food and Drug Administration had approved a qualified health claim for chromium picolinate with a requirement for specific label wording:
target="_blank" rel="nofollow"> FDA Qualified Health Claims: Letters of Enforcement Discretion, Letters of Denial U.S. Food and Drug Administration, Docket #2004Q-0144 (August 2005). As of March 2024, this ruling on chromium remains in effect.
Diabetes
Given the evidence for chromium deficiency causing problems with glucose management in the context of intravenous nutrition products formulated without chromium, research interest turned to whether chromium supplementation would benefit people who have type 2 diabetes but are not chromium deficient. Looking at the results from four meta-analyses, one reported a statistically significant decrease in fasting plasma glucose levels and a non-significant trend in lower hemoglobin A1C. A second reported the same, a third reported significant decreases for both measures, while a fourth reported no benefit for either. A review published in 2016 listed 53 randomized clinical trials that were included in one or more of six meta-analysis. It concluded that whereas there may be modest decreases in fasting blood glucose and/or HbA1C that achieve statistical significance in some of these meta-analyses, few of the trials achieved decreases large enough to be expected to be relevant to clinical outcome.
Body weight
Two systematic reviews looked at chromium supplements as a mean of managing body weight in overweight and obese people. One, limited to chromium picolinate, a common supplement ingredient, reported a statistically significant −1.1 kg (2.4 lb) weight loss in trials longer than 12 weeks. The other included all chromium compounds and reported a statistically significant −0.50 kg (1.1 lb) weight change. Change in percent body fat did not reach statistical significance. Authors of both reviews considered the clinical relevance of this modest weight loss as uncertain/unreliable. The European Food Safety Authority reviewed the literature and concluded that there was insufficient evidence to support a claim.
Sports
Chromium is promoted as a sports performance dietary supplement, based on the theory that it potentiates insulin activity, with anticipated results of increased muscle mass, and faster recovery of glycogen storage during post-exercise recovery. A review of clinical trials reported that chromium supplementation did not improve exercise performance or increase muscle strength. The International Olympic Committee reviewed dietary supplements for high-performance athletes in 2018 and concluded there was no need to increase chromium intake for athletes, nor support for claims of losing body fat.
Fresh-water fish
Irrigation water standards for chromium are 0.1 mg/L, but some rivers in Bangladesh are more than five times that amount. The standard for fish for human consumption is less than 1 mg/kg, but many tested samples were more than five times that amount. Chromium, especially hexavalent chromium, is highly toxic to fish because it is easily absorbed across the gills, readily enters blood circulation, crosses cell membranes and bioconcentrates up the food chain. In contrast, the toxicity of trivalent chromium is very low, attributed to poor membrane permeability and little biomagnification.
Precautions
Water-insoluble chromium(III) compounds and chromium metal are not considered a health hazard, while the toxicity and carcinogenic properties of chromium(VI) have been known for a long time. Because of the specific transport mechanisms, only limited amounts of chromium(III) enter the cells. Acute oral toxicity ranges between 50 and 150 mg/kg. A 2008 review suggested that moderate uptake of chromium(III) through dietary supplements poses no genetic-toxic risk. In the US, the Occupational Safety and Health Administration (OSHA) has designated an air permissible exposure limit (PEL) in the workplace as a time-weighted average (TWA) of 1 mg/m3. The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of 0.5 mg/m3, time-weighted average. The IDLH (immediately dangerous to life and health) value is 250 mg/m3.
Chromium(VI) toxicity
The acute mouth toxicity for chromium(VI) ranges between 1.5 and 3.3 mg/kg. In the body, chromium(VI) is reduced by several mechanisms to chromium(III) already in the blood before it enters the cells. The chromium(III) is excreted from the body, whereas the chromate ion is transferred into the cell by a transport mechanism, by which also sulfate and phosphate ions enter the cell. The acute toxicity of chromium(VI) is due to its strong Redox properties. After it reaches the blood stream, it damages the kidneys, the liver and blood cells through oxidation reactions. Hemolysis, renal, and liver failure result. Aggressive dialysis can be therapeutic.
Environmental issues
Because chromium compounds were used in , , and leather tanning compounds, these compounds are often found in soil and groundwater at active and abandoned industrial sites, needing environmental cleanup and remediation. Primer paint containing hexavalent chromium is still widely used for aerospace and automobile refinishing applications.
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General bibliography
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