**Abstract**

Heavy metals are among the most harmful contaminants in the ecosystems due to their persistency, bioaccumulation and high toxicity. In this chapter, we presented the sources, distribution and pathways of heavy metals in soil, water and air. The physico-chemical properties, uses, toxicity and health hazards of the purely toxic heavy metals lead, cadmium and mercury were also described. Other essential heavy metals were briefly presented and the main health effects due to their deficiency or excess were displayed in this chapter. Finally, the various methods used for the removal of heavy metals from soil and aquatic environments were discussed with a focus on nanomaterials.

**Keywords:** Heavy metals, classification, properties, uses, health hazards, removal

## **1. Introduction**

Environmental pollution has exposed humans to various contaminants such as pesticides, heavy metals or polycyclic aromatic hydrocarbons [1]. Unlike most organic pollutants, heavy metals are not removed from ecosystems by natural processes. They tend to accumulate in biotic and abiotic environments reaching toxic levels [2–4]. The introduction of heavy metals into the environment can result from natural events as volcanic eruptions, soil erosion and forest fires or anthropogenic activities including mining operations, industrial and domestic effluents and fertilizers application [3].

A number of heavy metals are considered essential for human health. They are important constituents of several key enzymes and play a crucial role in various biochemical reactions [5]. An insufficient supply of these elements leads to various deficiency syndromes. However, these micronutrients can become toxic from a threshold content in the body. Other elements are not beneficial for health and can be highly toxic even at very low levels. This is the case of lead, cadmium and mercury that are among the list of 10 chemicals of major public concern due to their high water solubility, toxicity, and carcinogenesis. Acute and chronic exposure to these elements affects human health and could cause incurable diseases leading to death [2, 3, 6, 7].

### **2. Classification of metals**

Metals are elements with a good electric conductivity and whose electric resistivity is directly proportional to the absolute temperature. Several other physical

#### *Environmental Health*

properties such as high thermal conductivity, reflectivity, malleability and ductility are common to metals [4]. The term "heavy metals" has been widely used in the environmental literature and diverse definitions, based on density, atomic mass or atomic number and on chemical properties, have been proposed for this term [8, 9]. Usually, a heavy metal is an element with an atomic weight between 63.5 and 200.6 and a density greater than 5.0 [10]. Unlike organic contaminants, heavy metals are highly persistent and tend to accumulate in tissues of leaving organisms.

Metals can be classified according to their biological effect. They are considered as "essential" when deficiency symptoms are noted with depletion or removal and "nonessential" when they have no known beneficial role to play in biological function [4]. Essential metals include zinc, copper, iron, iodine, manganese, molybdenum, selenium and vanadium. Lead, cadmium and mercury are among nonessential elements that are highly toxic even at trace amounts. It is important to note that essential elements can become potentially toxic for living organisms if they are incorporated in amounts exceeding a certain threshold [9].
