**9. Chromium (Cr)**

Chromium is one of the elements of the periodic table with the symbol Cr and atomic number 24. Chrome or Chromium is a hard, shiny metallic gray metal with high polishability and high boiling point and remarkable resistance to rust and tarnishing. Chromium oxide was used to coat metal weapons in the Chinese Empire for more than 2000 years. Chromium was discovered as an element in 1761 and was first used as a pigment [113]. In 1797, chrome metal was separated from its ore for the first time. Since then, almost all the chromium in the world is obtained from the ore chromite. The value of this metal is mostly due to its high resistance to rust and erosion, especially when it was discovered that adding chromium to steel has a significant effect in preventing corrosion and tarnishing of steel. Today, about 85% of the world's chrome consumption is used to make stainless steel, of which at least 10% of its volume is chrome, and chrome plating is also used [91]. Chromium metal is used in metallurgy to resist corrosion and in final polishing, as a component in alloys, for example in stainless steel, in chrome plating, in anodized aluminum, as a catalyst. Chromite is used to make molds for baking bricks. Chromium salts cause the glass to turn green, and chromates and oxides are used in hair color and ordinary colors [114, 115]. Chromium is used to harden steel and this element is used to make stainless steel products, these compounds have useful applications. This element is used for covering hard surfaces and for decorating and preventing corrosion and rusting. Chrome is glass-shaped and emerald green in color and widely used [76].

Surveys show that among heavy metals in the past, chromium was less important in agriculture and environment. While it can have severe destructive effects on plants and the environment, soil and water contamination with chromium is one of the major environmental concerns in recent decades [116]. Chromium has harmful effects on plant physiological processes such as photosynthesis, water relations and mineral nutrition, germination, growth and development of roots and leaves. The metabolic changes made by chromium in plants are either directly on enzymes and plant metabolites or through the creation of active oxygen species that cause oxidative stress [117, 118].

*Heavy Metals in Surface Soils and Crops DOI: http://dx.doi.org/10.5772/intechopen.108824*

Chromium metal and chromium III compounds are usually not hazardous to health, but chromium VI compounds are toxic if swallowed. The amount of almost half a teaspoon of toxic chromium VI compounds is lethal, and it has been proven that non-lethal amounts of chromium VI are carcinogenic. Most chromium VI compounds are harmful to eyes, skin and mucous tissues. Permanent contact with these compounds can cause permanent damage to the eyes, except for cases where complete treatment is done [119, 120]. In 1958, the World Health Organization suggested the maximum allowable consumption of chromium VI from the health aspect of 0.05 mg per liter of drinking water. This proposal was reviewed many times and the announced amount did not change during this time [121, 122].

The amount of chromium in drinking water is very low, but it is possible that contaminated water contains some chromium IV and chromium VI, which are considered dangerous types of chromium. If the amount of chromium III consumed by humans is higher than usual, it affects human health and, for example, causes skin itching. Chromium IV has many effects on human health. There is usually chrome in leather products. This combination causes severe allergy such as skin itching in people. By breathing chromium IV, the nose is stimulated and nosebleeds occur. Other diseases caused by Chromium IV include skin itching, stomach ulcers, respiratory system problems, weakening of the body's immune system, kidney and liver damage, genetic material changes, lung cancer and death.

### **10. Cadmium (Cd)**

Cadmium is an industrial and environmental pollutant that affects a number of human body organs. Cadmium is a group IIB metal with an atomic weight of 112.41 [79, 105]. In general, exposure to cadmium occurs mainly through two sources. The first food route is through water and food contaminated with cadmium, especially vegetable leaves, seeds, grains, fruits and fish [123]. The second source is through the inhalation of cadmium particles in industrial or daily activities, among which the inhalation of cigarette smoke is considered as a very dangerous source, because cadmium is easily absorbed by the lungs [124–126].

Forest fires and volcanoes, human activities such as industrial waste leachates, production of synthetic phosphate fertilizers are important sources of cadmium emissions. The main use of this element is as a stabilizer and pigment in plastic and electrolysis industries, but its main part is used in soldering and as an alloy in nickel and cadmium batteries.. This element is used in the industry as an anti-friction, catalyst, anti-rust agent or in the composition of alloys. Cadmium is also used in rod protection semiconductors in nuclear reactors, metal plating, ceramic making, PVC factories and plastic industries, battery production, fungicide compounds, engine oil, rubber making and photography [76].

The amount of cadmium absorption in foods is caused by the way animals are fed, kidneys and liver are suitable places for cadmium accumulation. Sea shells also have a high concentration of cadmium. Absorption of cadmium through the skin is very limited [21]. The biological half-life of cadmium in humans, in soft tissues and bones, is 10 to 30 years. The speed of cadmium methylation is much lower compared to mercury, arsenic, and lead, and only two bacteria named Pseudomonas sp. and Staphylococcus aureus are able to methylate cadmium in water environments [121]. A serious disease caused by it in humans is a disease called itai itai (rheumatism disease or painful skeletal deformity). The main effects of cadmium toxicity are on the

lungs, kidneys, and bones. Acute effects caused by its inhalation include bronchitis, pneumonia and liver poisoning. Chronic inhalation of cadmium compounds, in the form of vapors or dust, causes pulmonary edema, in which the small air sacs enlarge and are eventually destroyed due to the reduced lung volume. Both chronic inhalation and absorption of cadmium through the mouth affect kidney secretions, which is the first stage of protein excretion by the proximal tubules of the kidney. Acute poisoning with cadmium may cause the death of animals and birds and cause severe poisoning in aquatic animals. Absorption of cadmium from the lungs is more effective than the intestine, and 50% of the cadmium inhaled through cigarette smoke may be absorbed. On average, the concentration of cadmium in the blood of smokers is 4–5 times and in the kidneys 2–3 times more than that of non-smokers. It seems that cadmium reduces the body's defense resistance, especially the host's resistance against bacteria and viruses. Cadmium may cause demineralization of the skeleton and increase bone fragility and the risk of fracture [91].
