# The Double Face of Metals: The Intriguing Case of Chromium
###### Abstract  
Chromium (Cr) is a common element in the Earth's crust. It may exist in different oxidation states, Cr(0), Cr(III) and Cr(VI), with Cr(III) and Cr(VI) being relatively stable and largely predominant. Chromium's peculiarity is that its behavior relies on its valence state. Cr(III) is a trace element in humans and plays a major role in glucose and fat metabolism. The beneficial effects of Cr(III) in obesity and types 2 diabetes are known. It has been long considered an essential element, but now it has been reclassified as a nutritional supplement. On the other hand, Cr(VI) is a human carcinogen and exposure to it occurs both in occupational and environmental contexts. It induces also epigenetic effects on DNA, histone tails and microRNA; its toxicity seems to be related to its higher mobility in soil and swifter penetration through cell membranes than Cr(III). The microorganisms _Acinetobacter_ sp. Cr1 and _Pseudomonas_ sp. Cr13 have been suggested as a promising agent for bioremediation of Cr(VI). This review intends to underline the important role of Cr(III) for human health and the dangerousness of Cr(VI) as a toxic element. The dual and opposing roles of this metal make it particularly interesting. An overview of the recent literature is reported in support. chromium; essential nutrient; toxic element; epigenetics; remediation +
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diseases. The deficiency of this metal can cause blood sugar spikes, elevated cholesterol levels and blood pressure. It may also have other consequences, such as lower resistance to infections, atherosclerosis, hormonal imbalance, nervous disorders, fatigue, etc., [7,8]. Cr(III) occurs in several foods and supplementation products. It is largely contained in baker's yeast, red meat, liver, whole grains, red beets; however, it is hardly assimilated (only 3% is retained by body). The most common supplementation products include chromium-picolinate (CrPic), chromium-histidine (CrHis), chromium-dinicocysteinate (CRDC), and niacin-bound chromium (NBC) [9]. Several studies have been developed to evaluate the safety and efficacy of these supplements as insulin-sensitizing agents useful for prevention and treatment of type 2 diabetes (T2 DM) and obesity [10, 11, 12, 13, 14]. However, excessive levels of chromium can determine pathological states. In fact, even if Cr(III) compounds are not able to cross cell membranes, they may accumulate around cells causing alterations in cell functions and damaging the cell-membranes. In fact, long time exposure to Cr(III) may lead to skin allergies and cancer [15]. [PERSON] et al. (2008) reported also that the accumulation of Cr(III)-based dietary supplements, such as CrPic could induce genotoxic effects [16]. In the supplements Cr(III) may undergo a series of chemical transformations in biological media. Products deriving from partial hydrolysis of nutritional supplements containing Cr(III) may give reactive species while generation of highly reactive Cr(VI/V/IV) species and organic radicals may derive from reactions of Cr(III) with biological oxidants. However, it must be considered that Cr(III) compounds possess low bioavailability [17]. [PERSON] et al. (2008) reported that the genotoxic effects in vivo are uncommon but they can occur for elevated physiological intake levels of Cr(III) supplements [18]. While Cr(III) is an essential trace element, found in nature in rocks and soil, readily absorbed by plants, hexavalent chromium is mainly an industrial contaminant and is also produced from anthropogenic activities. Cr(VI) compounds present several applications and are vastly used as pigments for textile tints, paints, inks, plastics, corrosion preventing agents, leather tanning agents and wood preservatives [19]. Hexavalent chromium is mainly used in tanneries or industries dealing with metalworking, stainless steel welding, chromate production and the manufacture of chromium pigments [20]. The release of Cr(VI) into the air is mainly due to these industrial processes.