**2.2 Lead and lead compounds**

Lead and its derivatives are industrial toxic substances that have been widely used by humans for centuries and therefore play an important role in environmental pollution. Lead is a metal found in nature. It is a bluish gray metal that is rarely found in the earth's crust. It occurs mainly as lead ore, and the most important ore is lead sulfide (galena). It is found in a variety of compounds such as lead, lead acetate, lead chloride, lead chromate, lead nitrate, and lead oxide. While pure lead is insoluble in water, the solubility of lead compounds in water varies. The chemical symbol of lead is Pb and its atomic weight is 207.2 g/mol. Its vapor pressure is 1.77 mm Hg at 1000°C [18]. In nature, pure metal or its oxides, inorganic salts such as carbonate, sulfate, nitrate and acetate, and organic compounds such as lead tetraethyl exist. The primary use of lead is in the manufacture of batteries. In addition, it is widely used in in the production of paint, alloys and metal products (lead-antimony and lead-tin) in industry, electronic conductors, rubber, sheet lead, waterproof plates (roofing) or sound and radiation insulation, linoleum, solder, pipes, ammunition and toys. Its use for ceramic glazes, paint and pipe soldering has

decreased significantly. In addition, it is used in the composition of some ointments (lead acetate), diluted lead acetate (as abortive) is used infrequently in treatment under the name white water and as an insecticide (lead arsenate) in agriculture. Tetraethyl lead was used as a gasoline additive to increase the octane rating, before the EPA banned its use in gasoline in 1996 in the USA. Leaded gasoline is still used for propeller planes and some race cars [1, 19]. Metallic lead is a bluish, gray-silver soft metal. It melts at 327°C, is quite dense and can easily stretch (can take the form of a wire). It is not affected by dilute hydrochloric acid and sulfuric acid. It forms concentrated sulfuric acid and hot lead sulfate. Lead melts even in cold temperatures, forming lead nitrate and nitrous vapors. Organic acids (acetic acid, citric acid, etc.) affect lead slowly and cause the formation of their respective salts. It emits steam at temperatures above 500°C and harmful metallic dust. Lead is slightly soluble in water and may be the source of toxicity in some drinking water. Lead oxides usually form from water-insoluble compounds. PbO "oxide" (yellow masikot) forms yellow-orange lead oxide (lilac) when heated and is used for glass, enamel, accumulators, etc. in industry. PbO2 "bioxide" is the red oxide of lead and is an energetic oxidizer. Pb3O4 "minium" (cinnabar, plombi orthoplumbus) is a red-colored compound added to antirust paints and used in the varnish, enamel, crystal and battery industries. It is formed by heating lead oxide in air flow at 500°C. Lead sulfide is the substance that forms the mineral galena in its natural state. It is insoluble in water. It is found in the combination of yellow-colored varnishes used in ceramics and pottery, and color changes to dark brown when manganese oxide is added or green when copper oxide is added. These varnishes continue to be used in many countries today. It can be dissolved with organic acids. Food or acidic liquids (vinegar, wine, lemon juice, etc.) should not be placed in containers painted with this type of varnish. Lead hydrocarbonate is mixed with linen oil and used to obtain white paint. However, these dyes darken with the effect of hydrogen sulfide in the air and also have toxic effects. Although its use is prohibited in many countries, residues in old settlements can cause some poisoning cases. Arsenate lead was formerly used as insecticide in agriculture, including in Turkey, before being banned in many countries because of its toxicity. However, some residues from past use can be found in the soil due to its durability. Lead chromate is also called "chrome yellow" and can be used to obtain yellow dye. Lead stearate is used to reinforce some plastic materials. The basic iodide and acetate of lead -lead iodide and lead acetate (soluble in water)- are used in dermatology to benefit from solubilizing effect in the form of a solution. Basic lead acetate is a compound made by neutralizing lead acetate with lead oxide (PbO). Lead naphthenate is used as an additive in some moisturizing drugs. Alcoholic derivatives of tetraethyl-lead and tetramethyl-lead are organic derivatives of lead that are added to gasoline as an antidetonante (0.5%). Often ethylene dibromide and ethylene dichloride compounds are added to engines to prevent metallic lead build-up. Exhaust fumes from cars operating with leaded gasoline contain lead halides and oxides as well as a small amount of undissolved lead-alcohol (1–10%). In many countries, the proportion of lead in gasoline is legally limited (0.5 g/l) and unleaded gasoline vehicles are now produced. These organic lead derivatives added to gasoline play an important role in air pollution (especially in big cities) [1, 20].

Lead is taken into the organism generally through digestion (in the form of food, liquid drinks, vapor or particles) and through the respiratory tract. It is a highly toxic metal that enters biological systems by mixing with respired air and nutrients from the air, water and soil. Up to 90% of the lead in the atmosphere in solid and gaseous form is absorbed by the lungs. Small lead dust or vapor, especially in the air, cause lead compounds (oxides or salts) to enter the organism by inhalation. Very small particles can reach the alveoli, while large particles are discharged from the

upper respiratory tract or can be reabsorbed by entering the digestive tract through sputum and saliva. When organic lead compounds contaminate the skin, they can be absorbed significantly, especially from lesioned areas. When the carcinogenic effect is examined, human studies are insufficient regarding lead exposure and increased cancer risk. In animal studies, kidney tumors were reported in rats and mice orally exposed to lead. Lead is assessed as B2 (possible human carcinogen - based on evidence of adequate carcinogenicity in animals) according to the Guidelines for Carcinogenic Risk Assessment (US EPA, 1986) [1, 21].
