**2.3 Sources, exposure and environmental impacts of cadmium**

The major exposure of cadmium by human beings is through contaminated food and water. Cigarette smoke is the way of exposure through the inhalation route. The toxicity of cadmium is due to accumulation in plants and animals for nearly 25–30 years. Microbial fermentation is one of the effective methods to remove cadmium from food [23]. Another major source of cadmium in the environment is phosphate fertilizers and the waste incineration process. Blood cadmium levels are having a huge difference between smokers and non-smokers of cigarettes [1].

The presence of lead and cadmium in the human body can reach the brain and can cause Alzheimer's disease [24]. After the exposure to the cadmium, in human, it can accumulate at the kidney, due to this reason urinary cadmium levels has been considered as biomarkers for cadmium levels in humans [25].

In case of occupational exposure to cadmium includes the workers at battery production, pigment industries, and electroplating. Because of long time accumulation in the human body even in small quantities is toxic and carcinogenic [26]. Another important source of cadmium in the soil is sewage sludge which can make the cadmium almost the same amount as fertilizers consumption (https://www. osti.gov/biblio/5478006, accessed on 1st October/2020). The types of carcinogenic effects of cadmium toxicity were explained in **Figure 2**. Cadmium shows its toxic effects on the gastric system and leads to gastric cancer, breast cancer, lung cancer, and it also affects the excretory system and leads to renal cancer.

#### **2.4 Sources, exposure, and environmental impacts of mercury**

Mercury is the metal widely studied all over the world due to its toxic nature and easily entering into the food chain. An extensive review report was published by Jyothi and Mohamed Farook [6] regarding the sources, exposure, and toxicity

*Heavy Metal Sources and Their Effects on Human Health DOI: http://dx.doi.org/10.5772/intechopen.95370*

**Figure 2.** *Carcinogenic effect of cadmium.*

of mercury. The toxicity of mercury depends on its chemical composition. Methyl mercury is more toxic than inorganic mercury. Due to its toxic nature and historical incidents like Minamata so many authors were published various facts regarding the sources, transport, and fate of mercury in the environment. Both volcanoes and forest fires are natural sources of mercury in the atmosphere. The burning of fossil fuels in power plants is one of the major anthropogenic sources of mercury (https://www.epa. gov/mercury/basic-information-about-mercury; accessed on 15th October/2020). Because of easy transportation, it is considered a global pollutant [27].

Even exposure to small quantities, shows toxic effects on various physiological systems, such as nervous and digestive systems and organs like lungs and kidneys. Due to this reason WHO declares mercury as one of the top most priority toxic metals (https://www.who.int/news-room/fact-sheets/detail/mercury-and-health; accessed on 20th October/2020). When it enters into water it largely affects the aquatic animal's life and through them, it can enter into the food chain to reach human beings.

Yokoyama [28] reported the methylmercury poisoning control measures and the current situation of its effects on fetuses and infants particularly. In this study, they addressed the global cycle of methyl mercury also. Strode et al. [29] studied the emission of mercury into the North American atmosphere due to gold and silver mining in the 19th century. The types of carcinogenic effects of mercury toxicity was explained in **Figure 3**. Mercury toxicity effects on the rectal system and leads to colorectal cancer. It shows vast effects on the central nervous system leads to brain cancer and lung cancer.

#### **2.5 Sources, exposure, and environmental impacts of arsenic**

Arsenic is a metalloid but due to its toxic and carcinogenic nature, it is discussed under the heading of heavy metal toxicity. Abdul et al. [30] reviewed the health effects of arsenic exposure to human beings. According to this study, the majority of *Heavy Metals - Their Environmental Impacts and Mitigation*

**Figure 3.** *Carcinogenic effect of mercury.*

the population expose to this toxic metal through atmospheric air, groundwater, and certain kind of foods. The health effects are not limited to damage to cardiovascular, endocrine, renal, and reproductive systems. In various parts of the world such as India, Pakistan, and Bangladesh it was observed that major exposure to the arsenic is through groundwater. Shahid et al. [31] reported about the sources and health

## *Heavy Metal Sources and Their Effects on Human Health DOI: http://dx.doi.org/10.5772/intechopen.95370*

effects of arsenic through the exposure of groundwater in Pakistan. This study predicts nearly 47 million people of Pakistan at risk due to arsenic contamination in groundwater. They found that over 50% of the well they studied are having higher arsenic levels than WHO recommended levels in drinking water. A recent study [32] describes the occurrence and mobilization of arsenic in the groundwater of Bangladesh. In this study, they found that intensive usage of land for agriculture and usage of agrochemicals are the major reasons for arsenic contamination in groundwater of Bangladesh. Ahmed et al. [33] reported the situation of arsenic contamination in groundwater in a village in Bangladesh. For this purpose, they discussed 20 years situation of its exposure. Based on their results they found that the cancer risk to the population who are exposing to arsenic has 40% more than the non-exposures. The Types of carcinogenic effects of arsenic toxicity was explained in **Figure 4**. Sources and health effects of all the above discussed heavy metals are summarized in **Table 2**. Arsenic has its toxic carcinogenic effect on prostate glands and cause prostate cancer, leukemia and cause lesions in hepatic regions leads to cause of cancer of the liver.


#### **Table 2.**

*Sources and health effects of heavy metals.*

