**4. Health impact of heavy metals**

#### **4.1 Lead**

#### *4.1.1 Physico-chemical properties*

Lead (Pb) is one of the oldest heavy metals used in the past for hair dies, insecticides, and pottery glazes [15]. It is a naturally occurring bluish-gray metal, highly malleable and ductile and shiny when just cut. Lead is a dense metal, with a specific gravity of 11.35 and an atomic weight of 207.2 g mol−1. It is present in many inorganic forms (acetates, nitrates, carbonates, sulphates and chlorides) especially in the earth's crust and ores [5]. Lead is also found in the environment as a result of radioactive decompositions because it is a natural product of the decay of uranium. Native lead is rare, it is currently extracted from ore associated with zinc (blende), silver and copper. The main mineral source of lead is galena (PbS) [5].

#### *4.1.2 Uses*

Despite its high toxicity, lead is still used in various anthropogenic activities such as storage batteries, fossil fuel combustion, PVC production, electricity generation, alloys, glass manufacture and nuclear industry. Due to environmental pressure, lead emissions from motor vehicle gasoline have decreased significantly over the last three decades. As a result, a great improvement in air quality was observed worldwide [1, 5]. In addition, the use of lead in paints and colorants was banned in most countries after children's intoxications from lead-based pigments.

#### *4.1.3 Health hazards*

The main routes of lead exposure include ingestion of contaminated food and drinking water as well as incidental ingestion of dust and soil. Another major exposure pathway is the lead-based paint in older homes. Once inside the body, lead is distributed in blood and soft tissues and accumulated in skeletal bones [1].

One of the major mechanisms of lead toxicity is its ability to interact with proteins and to inhibit enzyme activity by competing with essential metallic cations for binding sites [2, 16, 17]. Acute toxicity can cause fatigue, irritability, sleeplessness, headache, loss of appetite, dullness, hypertension and vertigo, while chronic toxicity can result in neurological disorders, cognitive impairments, premature birth, brain injury, kidney dysfunctions, reproductive pathologies, liver damage, paralysis and even death [16, 18, 19]. Lead poisoning affects particularly children leading to hyperactivity, behavioral problems, lowered intelligence quotient (IQ ) and cognitive deficits [20]. In Chicago, after adjustment for all socio-demographic factors, it was reported that exposure to lead caused 13% of reading failure and 14.8% of math failure in children [21]. Adverse effects of lead toxicity are also found in contaminated ecosystems, including biodiversity losses, decreased growth in plants and animals and neurological damages in vertebrates [22].

### **4.2 Cadmium**

#### *4.2.1 Physico-chemical properties*

Cadmium (Cd) is a soft, bluish-white metal with an atomic weight of 112.4 g mol−1 and a density of 8.65. It melts at 594 K. Cadmium was first discovered by Friedrich Stromeyer in Germany in 1817. It is a rare element that is not found as a pure state in nature and is commonly associated with zinc (one ton of zinc provides about 2.5–3 kg of cadmium. Cadmium is also present in lead and copper ores, as well as in natural phosphates (40 ppm for Tunisian phosphates, 26 ppm for Moroccan phosphates). In the latter case, various decadmination processes can be implemented [23]. Cadmium persists in the environment and has a biological halflife of 10 to 25 years [1].

#### *4.2.2 Uses*

Cadmium is used in various industrial processes including electronics, electricity, metallurgy, plastics, PVC manufacture, pigments and paints and pesticides [1, 24]. Significant quantities of cadmium are also found in discharges from industries processing crude phosphate [23]. Due to environmental restrictions, the use of cadmium in the manufacturing of nickel-cadmium batteries decreased and has been replaced by nickel-metal hydride and lithium-ion batteries. Recently, cadmium telluride was used in semiconducting solar panels and infrared optical windows [25].

## *4.2.3 Health hazards*

Dietary intake is the most important pathway for cadmium exposure in humans. Tobacco smoke is another source of exposure containing appreciable amounts of the metal. As the absorption of cadmium from the lungs is much higher than from the gastrointestinal tract, smoking contributes significantly to the total body burden [1]. Cadmium is absorbed in the gut and the lungs, and transported to the liver by blood. Accumulation of cadmium in the body is done mainly in the liver and the kidneys [18]. Cadmium metallothionein is the form in which cadmium enters the kidneys. This complex has a protective role against cadmium toxicity through binding up to seven metal atoms per molecule. When the cadmium intake is too high, the metal is transported further to the kidneys and binds to other proteins [5, 17].

Adverse effects of cadmium toxicity include high blood pressure, reproductive disorders, fetal growth reduction, pregnancy loss, iron deficiency, gastrointestinal disorders, bone fracture, nephrotoxicity, renal dysfunction, neurological troubles, lung damage and lung cancer [1, 18, 24, 26]. In humans, blood, urine, hair and nails have been used as biomarkers of cadmium exposure and health risk evaluation [27]. Recently, Du et al. (2020) evaluated the environmental and human health hazards from cadmium exposure near an active lead-zinc mine and a copper smelter in China. They found that the cadmium concentration in hair and urine biomarkers in the mining and smelting areas were much higher than in the general population. Additionally, rice and vegetable ingestion were the two major pathways of cadmium exposure [28].
