Sania Kanwal, Environmental Engineer, MUET Jamshoro

Mercury is a toxic and hazardous metal. Although mercury has no known beneficial effect in human physiology, it is estimated that an average man weighing 70 kg has an amount equivalent to 13 mg of mercury in his system. It is a highly reactive molecule that produces toxic effects by binding highly to sulfhydryl and to a lesser degree to hydroxyl, carboxyl, and phosphoryl groups. It is widely distributed as an environmental and industrial pollutant. In humans high mercury levels are found in the skin, nails, hair, and kidneys.
Mercury occurs naturally. However, natural phenomena such as the erosion of mineral deposits and volcanoes and human activities such as metal smelting, coal production, chemical synthesis and use, and waste disposal may lead to substantial contamination of the environment. Mercury is present in nature as metallic mercury, mono- and bivalent inorganic compounds, and organic alkyl, aryl, and alloxyalkyl compounds. Three main forms of mercury are found in the environment: elemental mercury or quicksilver (Hg0), inorganic mercury (Hg^ + and Hg^2+), and organic methyl-, ethyl- and phenyl mercury. They are widely distributed in the environment as a result of human activities. Methyl mercury (MeHg), which is a common organic form of mercury, tends to bioaccumulate in successive order in the aquatic food chain: from plankton and algae to herbivores and finally to the top fish predators such as sharks and fish-eating marine mammals.
The global cycling of mercury begins with the evaporation of mercury from land and sea surfaces. Volcanoes can be an important natural source. The burning of fossil fuel, especially coal and municipal waste incineration, is a major anthropogenic source to the atmosphere. Mercury vapor, a chemically stable monatomic gas, is estimated to have a residence time of 1 year in the general atmosphere. Through processes yet to be fully clarified, the vapor is oxidized in the upper atmosphere to water-soluble ionic mercury, which is returned to the earth’s surface in rainwater. This global cycling of mercury results in the distribution of mercury to the most remote regions of the planet.
Mercury is toxic and non-degradable. So this is harmful for both human and aquatic life. It is necessary to remove this heavy metal economically. Mercury is a highly deleterious environmental pollutant with recognized mutagenic and teratogenic effects. Methyl mercury is absorbed throughout the intestine; up to 90% of MeHg crosses most biological membranes. It binds in vivo to proteins such as albumin and sulfur-containing proteins. In adults recycling occurs in the enterohepatic system. It is secreted into bile and partly reabsorbed into portal circulation and thereby returned to the liver. A fraction of biliary mercury is converted by microflora to inorganic mercury, which is reabsorbed only to a small extent.
Inhalation of volatilized vapors of metallic mercury leads to chemical pneumonitis. Exposure to high levels produces acute necrotizing bronchitis and pneumonitis with symptoms of cough, dyspnea, and chest tightness. Mercury inhibits protein synthesis in general by inactivating enzymes such as aspartate and alanine amino acid transferases, although MeHg appears to stimulate protein synthesis in the brain. Many enzymes in carbohydrate metabolism, including glucose-6-phosphatase, amylase, maltase, and lactase, are inhibited by exposure to Hg^2.
Mercury is one of the toxic elements that are widely distributed in nature. Available scientific data show that the sources of mercury polluting the environment are numerous levels, as little as 0.1–0.4 micron of mercury, induce mutations in cultured cells. Therefore, it is important to know the physical and chemical properties, sources and pathways of exposure, systemic and carcinogenic health effects, current regulatory and health guidelines.
About the Author: Sania Kanwal has recently done graduation in Environmental Engineering and Management from MUET Jamshoro.