**1. Introduction**

Heavy metals, the name has so many definitions based on various parameters. Based on density the metals which are having a density values greater than 5 g/cm3 are considered as heavy metals [1]. According to this study, the heavy metals which would consider as most threat to human beings are lead, cadmium, mercury, and arsenic. Duffs [2] reviewed the usage of the term heavy metals from the history and finally, he concluded that using the term "heavy metals" is meaningless. He established that there is no relation between the density of the metal and to the usage of the term. In the case of heavy metals, metalloid arsenic also included, from this the term heaviness means may be toxicity.

Some of the heavy metals are having so much of biological importance in trace amounts [3] particularly the elements that are present in the 4th period in the modern periodic table. The biological importance of these metals is enzyme functioning (vanadium and manganese), hormone functioning, production (selenium), cellular growth (nickel), and metabolic growth (arsenic). But these metals are required for the human in trace amounts only if their amount in the body increases they cause adverse effects on human health. Overall the heavy metal should be considered as having high density and also biological importance in trace amounts.

There is a lot of importance for the determination of heavy metals in the various environmental segments, such as air, water, and soil due to their carcinogenic and toxic nature. The IARC (International Agency for Research on Cancer) declared arsenic, hexavalent chromium, cadmium, and nickel and their compounds as

group 1 carcinogens (proven carcinogens). Arsenic and their compounds cause urinary bladder, liver, and lung cancers. Hexavalent chromium causes lung cancer and nickel and its compounds cause nasal cavity and lung cancers. All these elements cause cancers to human beings by the route of exposure is through inhalation and ingestion [4]. Regarding the availability of various heavy metals in the earth's crust is about 5%, among which iron occupies nearly 95% [5].

Due to their toxicity and carcinogenic nature, most of the researchers all over the world are reported about the determination and health implications of heavy metals in the environment. Some of them are discussed hereunder.

Jyothi and Mohamed Farook [6] reported the sources, exposure, and toxicity of mercury. Suvarapu et al. [7] reviewed the heavy metal concentrations in ambient air. This study is limited to the estimation of toxicity of heavy metals in the Indian atmosphere and another study [8] they reviewed the heavy metal determination in ambient air all over the world. Kim et al. [9] reviewed heavy metal toxicity and chelating therapeutic strategies. Giller et al. [10] reviewed the toxicity of heavy metals in microorganisms in agricultural soils. This study found that the microorganisms in soil are much sensitive to heavy metal toxicity than animals and plants. Yabe et al. [11] summarized heavy metal pollution and its impact on the environment and the human population in Africa. Das et al. [12] reviewed the toxicity of cadmium in plants. Proshad et al. [13] reviewed the toxicity of heavy metals in soils of Bangladesh. In this study, they concentrated on the impact of industrialization on the concentration of heavy metals in soil. Su et al. [14] reported the heavy metal contamination in soil worldwide. In this study, they mentioned the current situation of contamination and remediation methods.

The present chapter describes the heavy metal sources, exposure, and the impact of their toxicity on various environmental segments. Based on the toxicity and nonbiodegradable nature of lead, cadmium, mercury, and arsenic, the present study mainly focused on these metals.
