**1. Introduction**

Metals are naturally occurring elements of the earth's crust, and through wind and water-induced erosion, they are dispersed as powders or dissolved into rivers [1]. However, compared to human activity, these natural processes release fewer metals into the environment. These substances spread widely in the environment, which causes them to move up the food chain. Heavy metals are metallic elements with a high density compared to water that are found in trace amounts in a variety of matrices [2]. Examples include Fe, Co, Cu, Mn, Mo, Se, Zn, Cr, and As. Since heavy metals can cause toxicity at low levels, their weight and toxicity are connected [3]. In order to maintain certain biochemical and physiological processes in humans, animals, plants, and other organisms, certain metals are necessary [3]. These trace elements, sometimes known as microelements, include cobalt (Co), copper (Cu), chromium (Cr), iron (Fe), manganese (Mn), molybdenum (Mo), selenium (Se), and zinc (Zn). Their nutritional requirements are typically minimal. Although they have varying bioavailability, they are present in a variety of matrices in trace amounts (ppb or ppm). The term "heavy metals" refers to metallic elements with an atomic number greater than 20 and possessing metallic characteristics [4]. There are trace elements, or elements necessary for the proper growth, development, and operation of living creatures, among them (such as copper, zinc, chromium, and iron) (including cadmium, lead, and mercury) [5]. However, they all share the trait of being poisonous and extremely deadly for humans, animals, and plants beyond a specific


### **Table 1.**

*Classification of heavy metals with examples.*

level [3]. Toxic heavy metals are generally determined by the level of pollution, but they can also be toxic depending on the species and age of the organism, the route taken to enter the body, the chemical's structure, how it interacts with other metals, or the body's physiological state [6]. Metals enter the body through the respiratory or gastrointestinal systems, where they are concentrated and stored before being carried by the blood to the tissues and organs [7]. According to European Union (EU) Regulation 1881/2006, essential trace elements are typically added to animal feed as nutritional supplements to enhance health and maximize output. However, prolonged exposure to these substances at higher concentrations has been associated with cellular or systemic problems and may be a source of pollution [8]. Other metals, such as As, Cd, Pb, and Hg, are regarded as pollutants and undesirable chemicals in animal feed (Regulation 2002/32/EC) since they lack biologically recognized roles [5]. Additionally, the preceding public health hazards—As, Cd, Cr, Pb, and Hg—show a high toxicity since they can cause organ damage even at low exposure levels [8]. According to their biological significance, heavy metals can be divided into four main groups: necessary, non-essential, less toxic, and highly toxic heavy metals (**Table 1**) [9]. Heavy metals can be both beneficial and harmful to the organism.
