**1.3 Dangers of mercury exposure**

Three different types of mercury are harmful to the human body:


The human body can be exposed to mercury through [15]:


#### **Figure 2.**

*Inter-phase transfer and transport of mercury in soil, water, and air. Acronyms: Hg(0), elemental mercury; Hg(II), divalent mercury; MeHg, methyl mercury.*

The nervous system is very sensitive to all forms of mercury. Exposure to high levels of any type of mercury can permanently damage the brain, kidneys, and fetus. The effects on brain function may cause irritability, shyness, tremors, changes in vision, or problems with hearing and memory [16]. High exposure to mercury vapor can cause chest pain, shortness of breath, and fluid buildup in the lungs (pulmonary edema) that can be fatal [17].

Elemental mercury can turn into more toxic inorganic compounds as oxidized mercury (Hg2+) combines with other elements, or it can combine with carbon to form an even worse pollutant known as methyl mercury (CH3Hg). These compounds may fall to land or water through precipitation, or they may fall as dry particles and find their way into a lake or ocean [4].

Methyl mercury and metallic mercury fumes are particularly harmful because more mercury reaches the brain. Long-term exposure may cause blurring of the eye. Contact with mercury chloride can cause skin burns and permanent eye damage. Mercury also accumulates in the body [18]. Most metallic mercury will accumulate in the kidneys, but some metallic mercury can also accumulate in the brain. Most of the metallic mercury that is absorbed by the body is eventually left in the urine and feces, while small amounts leave the body in the same exhalation [19].

Humans are exposed to mercury in two ways:

1.Eating fish contaminated with organic methyl mercury

Symptoms of organic mercury poisoning from long-term exposure include a feeling of numbness or pain in certain parts of the body, tremors (uncontrollable shaking), unsteady walking, double vision, or blurry vision. Blindness, memory loss, and seizures [20].

#### *Mercury Pollution: Dangers and Treatment DOI: http://dx.doi.org/10.5772/intechopen.108390*

Mercury can enter the open seas and oceans as a result of downstream movement and re-deposition of polluting sediments from urban estuaries. The reduction and oxidation of mercury mostly occur near the surface of ocean waters. These are either driven by sunlight or microbial activity. Under ultraviolet rays, elemental mercury oxidizes and dissolves directly in ocean waters or binds to other molecules [21]. The reverse reaction reduces some Hg2+ to elemental Hg0 and returns to the atmosphere. Atmospheric fine aerosols such as ocean water droplets can act as small reaction chambers in this process providing the required special reaction conditions. Oxidation and reduction of mercury in the ocean are not very simple reversible reactions. The proposed photochemical pathway for mercury surrounding ocean aerosols was shown in **Figure 3** which indicates that it occurs through a reactive medium [22].

Photo-oxidation is suspected to be OH-driven. Roots and reduction are driven by wind and perturbations of the surface layer.

In the dark, mercury redox reactions continue due to microbial activity. Biological transformations vary and have a lower rate compared to the above sunlight-driven processes. Inorganic mercury Hg2 + and methyl mercury can adsorb in molecules. A positive correlation was observed between the amount of organic matter versus the concentration of these types of mercury, indicating that most of them are associated with organic matter [23]. This phenomenon can determine the bioavailability and toxicity of mercury in the ocean. Some methyl mercury is released into the ocean through river run-off. However, most of the methyl mercury found in the ocean is produced in ceto (within the ocean itself). Inorganic Hg methylation can occur via biotic and abiotic pathways. However, biosynthetic pathways are the most prevalent. The reactions shown in the simplified diagram below are parts of the complex enzyme-driven metabolic pathways that occur within microbial cells [24].

In abiotic reactions, hemic substances act as methylating agents, and thus this process occurs at shallow sea levels where decomposing organic matter is available to combine with inorganic Hg2+. Mercury methylation studies in polar regions have also shown a positive correlation between methylation and chlorophyll content in water. The potential for biological pathways to produce methyl mercury is

**Figure 3.** *Photochemistry of mercury on oceanic aerosols.*

**Figure 4.** *Microbial chemical conversions of mercury.*

shown in **Figure 4** [25]. The methyl mercury produced accumulates in microbes. Due to the high permeability and absence of methyl mercury degradation in other species that depend on those microbes, this highly biotoxic compound is amplified through marine food chains to top predators. Humans consume many types of marine fish that are the number one predators in food chains, putting their health at great risk. Therefore, finding possible solutions to further reduce mercury emissions and clean up existing mercury emissions and clean up existing mercury pollution [26].

Methyl mercury is toxic and can cause very harmful effects when consumed, which can happen when humans eat highly contaminated fish. Through the process of biomagnification, as shown in **Figure 5**, the concentration of methyl mercury within fish increases as one goes up the food chain. This makes eating apex predators dangerous, and especially dangerous for pregnant women and young children to do so.

Mercury levels in fish are measured in either part per million (mg kg−1) or dry weight micrograms (mcg). **Table 2** explains some common types of tuna and the concentrations of mercury in them [27].

The effects of methyl mercury can lead to risk neurological problems, especially in young children and infants, by affecting the brain and nervous system. Possible problems include cerebral palsy, delayed walking or speech, learning difficulties, tremors, irritability, poor coordination, and memory loss. Pregnant mothers in particular should not eat large fish because their babies are susceptible to these chemicals that attack developing organs [28].

The US Environmental Protection Agency (EPA) states that 0.045 micrograms of mercury per pound (0.1 micrograms per kilogram) of body weight per day are the maximum safe dose of mercury. This amount is known as the reference dose [29].

The daily reference dose of mercury is based on body weight. Multiplying that number by seven gives you the weekly mercury limit. **Table 3** shows some examples of reference doses based on different body weights [30].

#### **Figure 5.**

*Methyl mercury concentrations within the fishes.*


#### **Table 2.**

*Some common types of tuna and the concentrations of mercury in them.*


#### **Table 3.**

*Some examples of reference doses based on different body weights.*

Since some tuna species are very high in mercury, a single 3-ounce (85-gram) serving may have a mercury concentration that equals or exceeds a person's weekly reference dose.

2.Inhalation of elemental mercury (Hg) or inorganic salts (Hg2+)

Elemental mercury is toxic when ingested. When the chemical enters the body by inhalation, it travels through the bloodstream and attacks the brain and kidneys. Symptoms of inorganic mercury poisoning involve a burning feeling in the throat and/or stomach, vomiting or nausea, diarrhea, the color of the urine changes and blood in stool or vomit [20].

The atmosphere is the primary pathway for mercury transport emissions, while land and ocean processes play an important role Its role in the redistribution of mercury in terrestrial water, freshwater, and Marine ecosystems and CH3Hg production that drives The main human exposure route, fish consumption, Especially marine fish. Temporal and spatial scales of Atmospheric transport of mercury to aquatic organisms and Terrestrial ecosystems depend primarily on chemicals and physical forms. **Figure 6** illustrates Mercury Cycle in the Environment [31].

After emission, elemental mercury (Hg0 ) can be transported over long distances before oxidation and removal by dry precipitation of particles or gas in the phase or cleaning by scavenging precipitation. The atmospheric residence time of Hg0 is from several months to years and therefore mercury can be transported and deposited in remote locations such as the Arctic and Antarctic [32].

**Figure 6.** *Mercury cycle in the environment.*

*Mercury Pollution: Dangers and Treatment DOI: http://dx.doi.org/10.5772/intechopen.108390*

#### **Figure 7.**

*Current estimates of the fluxes and pools of mercury at the Earth's surface.*

The ionic particle bound Hg2+ has a shorter Atmospheric residence time of Hg0 (atmospheric residence hours to days), as a result of which it is generally deposited locally or regionally. Inputs to ecosystems occur to a large extent Hg2+, while most CH3Hg is produced within ecosystems. It is important to distinguish between primary and secondary mercury emission sources. Primary sources, both natural and Man-made mercury from the long-lived lithosphere atmospheric reservoirs. This mercury is deposited in the earth and oceans. Precipitated mercury can be reduced to Hg0 and then re-emitted [33].

Re-emissions are secondary sources of the exchange of mercury between surface reservoirs using the atmosphere car. Primary sources increase the global pool of mercury in Surface reservoirs, while secondary sources redistribute Between and within ecosystems [12].

Mercury is deposited into the atmosphere primarily as oxidized mercury (II).

By precipitation and falling or picked up by plant stomata and deposited with excreta. In soil, Hg(II) can be reduced by different pathways:

(i) Photochemical, (ii) microbial, or (iii) abiotic non-chemical reduction by natural organic matter (NOM), followed by re-emission back into the atmosphere. All forms of mercury are subject to leaching from soils with runoff or groundwater into aquatic ecosystems. **Figure 7** shows emission and re-emission sources of Hg0 [34].
