**5. Lead (Pb)**

Lead is one of the most common and useful metals known to humans and can be detected in all environments and biological systems. The amount of lead in the environment has increased more than 1000 times during the last three centuries, which is the result of human activities, and between 1950 and 2000, the greatest increase in lead levels was observed [77, 78]. Lead is from the fourth group of the periodic table and has an atomic weight of 207.2. Lead is a gray heavy metal and is usually combined with two or more other elements [79]. Lead reaches aquatic ecosystems due to surface soil erosion and atmospheric sediments. The concentration of lead in the deep oceans is about 0.01–0.02 g/L, but it is about 0.3 g/L in the surface waters of the oceans [80, 81].

In general, lead is absorbed from food and air. Workers in smelting, casting and steel industries, battery manufacturing, plastic factories and printing industries are exposed to lead [82]. Lead is the most abundant and widely used heavy metal and its dispersion in the environment is wider [83]. It is easy to work with lead due to its low melting point, it can be easily made into various shapes. Due to the applications that have caused the uncontrollable dispersion of this element, its environmental concentration is increasing in most countries. Lead metal in car battery, ceramics, inside cans, cigarette ash, car exhaust fumes, leaded gasoline, hair dyes, insecticides, mascara, snow, soldering alloy, putties, paints, city water and or there are wells, alcoholic drinks, pipes [76].

Children are sensitive to the effects of lead, which is considered a primary environmental hazard. Metal poisoning in children causes sensitivity in the developing nervous system, which is due to the sensitivity to lead metal toxicity [84, 85]. Lead is classified in group 2B of IARC carcinogenic compounds, and its toxic effects in the body include the occurrence of disorders in four places, i.e. digestive system, central nervous system, peripheral nerves and hematopoietic system [86]. Lead may enter the human body through the intestine. It is also absorbed through the lungs, inhalation and skin or by direct ingestion and drinking [87]. Lead accumulates in high concentrations in bones, teeth, liver, lungs, kidneys, brain and spleen and passes through the blood–brain barrier and the fetal placenta. The symptoms of diseases caused by lead are completely different and unrecognizable in the first place. In the early stages, impatience, anorexia and lack of movement can be mentioned. Weight loss and blood loss are other symptoms of lead poisoning. Among women, monthly periods do not occur or are postponed. General changes also occur in the form of blood. Red blood cells change shape. As a result of lead deposition in the mouth, gums and teeth turn gray. This gray color can remain for some time even after the poisoning is removed [85, 88]. The biological halflife of lead may be much longer in children than in adults. Lead has a half-life of 35 days in blood, 40 days in soft tissue, and 20–30 years in bones [89]. The main route of excretion of absorbed lead is the urinary tract, which is usually done by glomerular filtration in the kidney. It can also be excreted through the digestive system through bile [90].

## **6. Zinc (Zn)**

Zinc metal has been used for centuries due to its low boiling point. Like other metals, zinc reacts slowly. Zinc is a blue-white or silvery metal found in the earth's crust. The abundance of zinc in nature changes depending on the place and season [91]. It combines with oxygen and other non-metals and reacts with dilute acid and releases hydrogen gas. Zinc is the fourth most common and used metal, after iron, aluminum and copper, it is the most produced metal. Features such as corrosion resistance, reactivity with iron and electrochemical properties of zinc metal have led to its use as a suitable coating against corrosion or galvanization. Galvanized steel is used in construction, power, construction of urban amenities (such as benches and tables), agriculture and transportation. This metal is used in the production of various alloys such as brass, aluminum alloys, and magnesium alloys, which are used in the construction industry and electric cars [92].

Also, this metal can enter the environment through textile and cotton, battery, rubber, paint, cosmetic, fertilizer and medical industries. In addition to metal smelting industries, the impact of acid rain on construction materials containing zinc are the main sources of this element entering the environment [76].

Zinc is one of the important components of some important biomolecules in the human body. There are more than three hundred important enzymes in the body, zinc is one of these enzymes [93]. These enzymes play an important role in maintaining body function and natural health, and some of these enzymes also play a role in the process of gene expression [94]. Zinc plays a role in regulating the synthesis of important biological molecules such as blood sugar balance, insulin hormone, glucose transport, body metabolism and its availability in the physiological system. The zinc element plays a very vital role in enzymes, so that if zinc is removed from their composition, the activity of the enzyme in question stops [95, 96].

Zinc is one of the rare elements of the body, poisoning with it leaves many effects in different organs of the body. Its acute toxic effect, which manifests itself in the form of fever, has been fully proven, but regarding the complications of chronic contact with this metal, various studies have put forward many opinions [97]. Some of the adverse effects of zinc accumulation in the body are: poisoning, fever, confusion, nausea, vomiting and diarrhea after consuming acidic drinks or foods that are prepared and stored in galvanized containers. Zinc is considered a low-risk element, but its toxicity increases in large quantities in the presence of arsenic, lead, cadmium, and antimony. Fever caused by zinc poisoning has symptoms of cold, fever and nausea. Zinc chloride vapor causes the lungs to dry [91].
