**Abstract**

Heavy metals have been known to have great deteriorative impacts on the physiology of the body, altering the normal functioning of the body. These impacts cut across the various systems of the body including cardiopulmonary, endocrine, neurological, gastrointestinal, hematological, etc. However, not every exposure will leave such effects in the aftermath. The level of exposure to one heavy metal that is considered harmful may not be with another metal. This chapter examines the various levels of exposure that may be considered unhealthy to the human body, and the mechanisms by which the metals exert their impacts, with the aim of educating readers on how to keep exposure below such threshold level. This chapter also explains that not all heavy metals are considered unhealthy as there are essential heavy metals that may have some beneficial effects to the physiology of the human system.

**Keywords:** heavy metals, physiological changes, anxiety disorder, toxicity, essential and non-essential heavy metals

### **1. Introduction**

Heavy metals are defined as metallic elements that have a relatively high density compared to water. These metals may be toxic or poisonous even at low concentrations. They are described as those elements having atomic number greater than 20 and atomic density above 5 g cm−3 and must exhibit the properties of metals [1, 2].

Examples include cadmium (Cd), mercury (Hg), zinc (Zn), copper (Cu), chromium (Cr), lead (Pb), arsenic (As), and nickel (Ni).

Natural phenomena like weathering and volcanic eruptions have also been implicated in heavy metal pollution [3, 4]. Some of them are exploited for various industrial and economic purposes. They are grouped into essential and non-essential heavy metals.

While essential heavy metals such as iron (Fe), magnesium (Mg), copper (Cu), and the like, are essential nutrients required for various biochemical and physiological functions such as growth, metabolism, and development of different organs, non-essential heavy metals such as cadmium (Cd), antimony (Sb), lead (Pb), vanadium (V) have no

established biological functions, yet they still find their way into the body system, and have been reported to affect cellular organelles and components in biological systems [5].

There are numerous essential heavy metals required by plants as they form cofactors that are structurally and functionally vital for enzymes and other proteins. Essential elements are often required in trace amounts in the level of 10–15 ppm and are known as micronutrients.

Due to their high levels of toxicity, arsenic, cadmium, chromium, lead, and mercury are among the priority metals that are important for public health. Even at the modest exposure levels, these metallic elements are known to cause numerous organ damage and are regarded as systemic toxicants.

Despite the fact that some of these metals only affect human physiology at high amounts, others, including cadmium, mercury, lead, chromium, silver, and arsenic, have significant effects on the body even in minute quantities, leading to acute and chronic toxicities in humans [6].

Exposure to these heavy metals has been associated with certain physiological changes ranging from mental, hematological, and hormonal.

This chapter discusses major physiological changes that exposure to these metals can cause to the human body, as well as the risk factors that can lead to changes in human physiology. The impact of cadmium on the central nervous system (CNS) is used as a case study. It also discusses how these changes could be ameliorated.
