*4.3.3 Health hazards*

Fish consumption and dental amalgam are the major sources of mercury exposure in humans. Inhalation of mercury vapor is an other important pathway of exposure. Due to their high lipophilicity, elemental mercury and methylmercury

are highly absorbed in the tissues. Inorganic mercury salts are not lipid soluble; therefore, they do not cross the placental and blood–brain barriers. Absorption of methylmercury from the gastrointestinal tract in humans exceeds 90%, whereas absorption of inorganic mercury salts is less than 10% [7]. Once absorbed in the body, the inhaled mercury vapor is quickly diffused into the blood and distributed into all of the organs [6]. Then it is accumulated in the brain and the kidney and excreted through urine and feces. As regards methylmercury, this organic compound is accumulated in the liver and kidney after consumption and slowly converted to inorganic mercury by microflora in the intestines [31].

The toxicity of mercury, especially methylmercury, derives from its ability to interact with nucleophiles due to its electrophilic nature. During interactions, the catalytic, binding, and transport functions of nucleophiles are impaired. One essential mechanism related to the toxicity of mercury is the damage to mitochondria that enhances the generation of free radicals [17]. The major clinical features of mercury acute and chronic exposures are insomnia, weight loss, vomiting, diarrhea, cough, dyspnea, fever, tremors, gingivitis, erythrism, delusions, hallucinations, acrodynia disease, congenital malformation, renal tubular dysfunction, neurologic disorders, paralysis and death [17, 32].

#### **4.4 Other essential metals**

Most essential metals act as catalysts for enzymes and can become toxic at high levels. In tissues and fluids, metals very often form complexes with organic compounds such as amino acids, proteins and peptides. **Table 1** displays the main health effects due to deficiency or excess of five essential metals (iron, zinc, copper, iodine and selenium) [33–38].

### *4.4.1 Iron*

Iron (Fe) is a vital nutrient as it is a cofactor of a wide variety of cell functions. In the human body, iron is found as heme compounds (hemoglobin or myoglobin), heme enzymes, or nonheme compounds (flavin-iron enzymes, transferring, and ferritin). The body requires iron for oxygen transport, respiration, the tricarboxylic acid cycle, lipid metabolism, gene regulation and DNA synthesis [39, 40]. Inorganic iron and heme iron are the two forms of dietary iron. Good sources of heme iron include meat, poultry and fish whereas nonheme iron is obtained from cereals, pulses, legumes, fruits and vegetables. Iron absorption is enhanced by vitamin C, ascorbic acid and meat consumption, while phytates, calcium and some dietary fibres inhibit the nutrient absorption [41, 42].

#### *4.4.2 Zinc*

Zinc (Zn) is an essential nutrient found in all human tissue. It is a structural compound of nearly 300 enzymes, important for the metabolism of macromolecules and that of nucleic acids. It has been estimated that the adult human body contains approximately 2 g of zinc. Lean red meat is an important dietary source of zinc. In addition, its zinc is present in a highly available form. However, fats, oils, sugar and alcohol have a very low zinc level [34, 43].

#### *4.4.3 Copper*

Copper (Cu) is essential for the activity of several metalloproteins and enzymes. It plays a crucial role in the regulation of the gene expression. Copper is required for


#### *Heavy Metals in the Environment and Health Impact DOI: http://dx.doi.org/10.5772/intechopen.97204*

**Table 1.**

*Main health effects of deficiency and excess of essential heavy metals.*

growth, defense, strength, blood cell production, iron transport and metabolism. The total body content of copper has been estimated to be 80 mg, with a range of 50–120 mg. Dietary sources rich in copper include seafood, organ meats, legumes and nuts. Refined cereals, sugar and dairy products are poor in the nutrient [35, 44].
