**2. Forms of mercury in the environment**

Mercury (Hg) is recognized by its characteristic shiny silver appearance. In the environment, it occurs in different inorganic (mercurous ion (Hg+1) and mercuric ion (Hg+2), and organic forms (methyl mercury (CH3Hg+ ), ethyl mercury (C2H5Hg+ ), and phenyl mercury (C6H5Hg+ ). The elemental mercury is the purest form [8, 9] and, at the same time, exhibits the least toxicity [10]. Under anoxic and suboxic conditions, inorganic Hg can be transformed into CH3Hg+ through sulfate-reducing and ironreducing bacteria [6].

### **2.1 Sources of mercury**

Mercury is introduced into the environment *via* natural and anthropogenic pathways. The first route results from volcanoes, weathering of rocks, forest fires, and soils [11]. The second one represents one-third of its content in nature which is related to anthropogenic activities coming from industrial processes [12] such as the burning of fossil fuels which represent up to 52.7% of Hg emissions [13], gold mining, cement production [9] and combustion of fossil fuel and agricultural additives that increases Hg levels in soil, for example, municipal [6], sludge, fertilizers, lime, and manures [10].

Long-term mining and smelting activities could bring considerable amount of Hg to the surroundings [14]. The majority of mercury's compounds are very volatile and thus travel hundreds of miles with the wind before being deposited on the earth's surface, hence contaminating the surrounding areas [15]. Generally, mercury in air can be carried by rain and eroded soils and run off to the surface waters and no one becomes safe [13]. In a study by Rodríguez Martín [16], most emitted Hg results from power plants that burn coal to create electricity; they account for about 42% of all manmade mercury emissions.

*Perspective Chapter: The Toxic Silver (Hg) DOI: http://dx.doi.org/10.5772/intechopen.111464*

The main countries, contributing to the majority of emitted mercury (kg year−1) according to the United Nations Environment Program report in 2018, are: China (572,195), India (205862), Indonesia (156766), Brazil (71470), and Russia (60,949 tonnes), representing an average percentage of 25.73, 9.25, 7.05, 3.21, and 2.74% of total emitted mercury, respectively [17].
