**1. Introduction**

Heavy metals (HM) represent a group of metallic elements and metalloids characterized by a relatively density higher than 5 g/cm3 , an atomic number greater than 20 and with properties like conductance of heat, current and luster surface [1–3].

Pollution or contamination of the environment with heavy metals is a major concern, due to their capacity to bioaccumulate and persistence in the environment, non-biodegradable nature, contaminate the food chains and their toxicity on the environment and living organisms (humans, animals and plants) [1–3]. Heavy metal toxicity is a concern of ecological, nutritional, evolutionary and environmental reasons [1].

Heavy metals are among the most investigated pollutants and received a higher attention by researchers, because of their toxicity [2, 4]. These elements are naturally present in the environment, but on which modern industrialization and urbanization, anthropogenic activities and use of fertilizers, led to increased levels of these metals in the environment and implicitly to a high exposure of living things to them [2, 5]. Among the heavy metals and the most toxic metalloids are chromium, mercury, arsenic, cadmium, lead, nickel, copper, zinc, but the most common heavy metals in the environment are chromium, manganese, nickel, lead, cadmium, copper and zinc [2].

Regarding their functions in biological systems, heavy metals can be essential and nonessential. The nonessential heavy metals do not possess biological functions in living organisms, being non-essential to metabolic system, both for plants and animals. Their category includes lead, cadmium, mercury, aluminum and arsenic [2, 6, 7], being able to exert toxic effects even at low concentrations [8]. The essential heavy metals are elements, which are indispensable for plant and animals, which play a vital role in biological processes and entire metabolism and may be required in living organism in different concentrations [2, 8]. These heavy metals are considered as trace elements because of their presence in trace concentrations (less than 10 ppm) in different environmental matrices [9]. The essentiality and toxicity of the trace metals depending on the dose of exposure [10]. This category includes 19 elements, among which the most important are manganese, iron, copper, zinc, nickel and chromium [2].

Trace elements or trace minerals are minerals necessary for the body, but in amounts between 1 and 100 mg/day for adults and represents less than 0.01–0.02% of the total body weight [10–12]. When they exceed these threshold concentrations, they become dangerous to the health of living organisms [1].

According to WHO classification, trace elements can be divided into three groups, such as essential elements (zinc, iodine, molybdenum, copper, selenium, chromium), probably essential elements (manganese, silicon, boron, vanadium, nickel) and potentially toxic elements (lead, cadmium, fluorine, mercury, aluminum, arsenic, barium, lithium, tin [13, 14].

Another classification of the trace elements was made by Frieden in 1981, based on their levels in biological tissues, being divided into 3 groups, namely essential trace elements (boron, cobalt, copper, iodine, manganese, molybdenum, zinc), probably essential trace elements (chromium, fluorine, nickel, selenium, vanadium) and physically promotive trace elements (bromine, lithium, silicone, tin) [13, 15].

The present chapter presents the characteristics of heavy metals, the main sources of heavy metal contamination of the environment, as well as human exposure sources. The impact of their toxicity on various environmental segments, such as water, air, soil, as well as on living organisms, animals, but especially humans, has also been described.
