Understanding Lead: Chemistry, Epidemiology, Occurrence, Toxicology

Lead

Chemistry

Lead also known as plumbum (Pb) is a chemical element with an atomic number of 82 and atomic mass of 207 that is found in group four (IVa) of the periodic table. It is found in the lead group of metals. It is denser, soft, and malleable with a relatively low melting point. Although lead is not very common in the earth's crust, what is there is readily available and easy to refine. Its chief use today is in the lead-acid storage batteries such as those used in automobiles.

Most stable lead compounds contain lead in the oxidation state of 2+ or 4+. Various isotopes of lead come at the end of the natural decay series of elements like uranium, thorium, and actinium which are Pb-206, Pb-207, and Pb-208 respectively.

Lead is among the most recycled non-ferrous metals and its secondary production has therefore grown steadily in spite of declining lead prices. Its physical and chemical properties are applied in the manufacturing, construction and chemical industries.

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It is easily shaped, malleable and ductile. It mainly finds its uses in batteries, petrol additives, rolled & extruded products, alloys, pigments & compounds, cable sheathing, shot and ammunitions.

Lead can be classified into three forms namely organic lead compound where it is combined primarily with hydrocarbon and used to manufacture leaded gasoline, tetraethyl lead, and tetra-methyl lead. Another form is inorganic which comes in combination with other elements for example lead oxide and lead chloride. The last form is the elemental lead which is a common source of exposure where soldering or smelting is common.

Anthropogenic or human activity industrial as in welding and smelting activities or manufacture of consumer products (lead-acid batteries, thermometer, lead paints)32.

Approximately 85% of the total global consumption of lead is for the production of lead-acid batteries mainly used in motorized vehicles, storage of energy generated but photovoltaic cells and wind turbines and for back-up power supplies37. The increasing demand for motor vehicles as countries undergo economic development and growth in the use of renewable energy sources with the need for storage batteries is directly proportional to the increasing demand for lead-acid batteries38. The batteries contain a large amount of lead either as solid metal or lead oxide powder.

Its poisoning occurs upon exposure to either of the above three forms, raising blood concentration to more than 10mcg/dl. Eliminating lead paint will contribute to the achievement of sustainable development goals, which aims at substantially reducing the number of deaths and illnesses from hazardous chemical & air, water & soil pollution and contamination by the year 203039. It will significantly reduce the release of lead compounds to the environment hence minimizing their adverse impact on human health and the environment.

Epidemiology

Institute for health metrics and evaluation has estimated that, based on 2016 data, lead exposure accounted for 540,000 deaths and 13.9 million years lost to disability and death due to long term health effects, with the highest-burden in the developing regions40.

In Kenya, lead poisoning cases have been associated with a lead-acid battery smelter in one of the local slums in Mombasa. Thousands of slum dwellers were exposed to lead contamination through gas emissions. Effluents and other physical exposures from the smelter company. Children in the village developed strange scaly patches on their skin, the women miscarried or gave birth prematurely and infants were born with deformities. The residents complained of fainting spells, weakness, convulsion, bone pain and numb feet, which are all signs of lead poisoning.

Livestock was also affected, as those that grazed on grass growing in the effluent of the smelter would die. The soil and water got tested and it was found that the dust from the village roofs had a lead level of 14,000ppm while the soil in the village where children played had up to 11,000ppm41,42. In the United States (USA), the environmental protection agency sets a limit of 400ppm of lead in soil where children play and 1,200ppm in all other soil43.

Another study was carried out to determine the effect of occupation on blood lead levels (BLL) in Nairobi and Nyamira counties of Kenya. Four hundred subjects both males and females aged between 18-70 years were randomly recruited from four study sites in Nairobi and Nyamira county where the study used a questionnaire to asses lead exposure factors of the recruits while atomic absorption spectroscopy (AAS) and differential pulse anodic stripping voltammetry were used for determining the BLL. The street hawkers in Nairobi city center had the highest mean BLL of 36.85±16.98mcg/dl while the teachers from Nyamira town had the lowest mean BLL of 8.1±5.3mcg/dl44.

Occurrence

Lead is an abundant element found in the earth's crust. There is an estimated 13 to 20ppm. This metal ranks in the upper third element in terms of abundance. It is rarely found as a pure element in the earth rather as an ore of lead sulfide (PbS), lead sulfate (PbSO4), and lead carbonate (PbCO3).

The largest producers of lead ore are Australia, China, the USA, Peru, Canada, Mexico, and Sweden. In the USA, Missouri State produces more than 93% of all the lead. The main type of lead that is found is galena (lead sulfide) with 87% lead. Other common mineral ores of lead are cerussite (PbCO3), and anglesite (PbSO4) which occurs near the surface. When it is a hot fluid it is trapped in cracks and creates veins. If in limestone, it fills cavities to form patchy deposits.

Toxicology

Exposure to lead can occur through the following ways: inhalation where this can be a significant exposure pathway, particularly or workers in lead industries, home renovators, smokers and children exposed to second-hand smoke; ingestion is also the most common route of exposure for children; dermal absorption directly through the skin that is common among industrial workers and cosmetic users; transplacental route to the unborn can happen if the mother is exposed.

The main pathway of lead exposure from recycling used a lead-acid battery (ULAB) arises from environmental emissions, which occur at various stages in the improper recycling process. In many low-income countries, ULAB recycling and smelting operations are conducted in the open air, in densely populated and often with few or no pollution control measures. Inappropriate recycling operations release a considerable amount of lead particles and fumes emitted into the air, deposited onto soil, water bodies and other surfaces, with both environmental and human health negative impact.

After ingestion, inhalation or dermal exposure it tends to accumulate in the three compartments: blood, soft tissues, and the bone. It enters the bloodstream, in which the vast majority of circulating lead is bound to erythrocyte protein while the remaining is associated with plasma and transported to the target organ. Inorganic lead is not metabolized while the organic form is oxidized by cytochrome P450 system via different pathways, the fast hepatic dealkylation then slowly to trialkyl metabolite. It is primarily excreted in urine and bile, but the elimination rate varies depending on the tissue involved.

Poisoning with this metal can lead to multi-systemic toxic effects, affecting majorly the hematopoietic, nervous and renal system as well as the cardiovascular system (CVS), gastrointestinal and reproductive systems. The major mechanism of toxicity is due to the increased generation of reactive oxygen species (ROS) and interference with the generation of antioxidants. The ROS produced includes hydroperoxide, hydrogen peroxide and singlet oxygen. ROS are stabilized by glutathione in the body. It is estimated that 90% of glutathione in the cell exists in