Environmental pollution by mercury and related health concerns: renotice of a silent threat.

During the 1950s, while Japanese researchers were seeking for the cause of a strange and fatal neurologic disease spread in Minamata city, it seemed ludicrous that an element located in the period 6 and group 12 of the periodic table would be to blame. Mercury in its organic form, i.e. methyl mercury, was released from the wastewater of a chemical company; bio-accumulated in fi sh and shellfi sh, and was subsequently eaten by local inhabitants. Thus, it transformed into an agent for thousands of cases of poisonings that later became known as Minamata disease. Two decades later, another disaster happened in Iraq, where around 1000,000 t of mercury-treated seed grain were mistakenly used for making bread and caused more than 6000 poisonings and 400 deaths (1). Several other stories like this have also been noted elsewhere, but concern regarding this toxic metal has shifted towards its global emission and distribution, which causes it to slowly enter into people’s homes, food and water, consequently affecting human health on a much larger scale. As a naturally occurring element, mercury can be found throughout the environment (soil, water, air) and can exist in three forms: elemental or metallic, inorganic, and organic. This metal is neither created nor degraded but its release into the environment by mankind’s manipulations has made it a global pollutant. According to a report by the US Environmental Protection Agency, natural emission from the ocean and volcanic eruptions account for one third of the world’s mercury emission, while the remaining two thirds are related to human activity. For instance, mercury is abundantly found in coal, whose burning causes its release into the air. That is why coal-fi red power plants are the largest source of mercury emission. Other mercury sources include artisanal and industrial gold mining; wastewater from metal, cement and alkaline factories; waste incineration; and products containing mercury such as linear or compact fluorescent lamps, batteries, electrical devices, paint, pesticides, fertilizers, thermometers, amalgam, pharmaceuticals, and cosmetics (2). Once released (elemental or inorganic), mercury circulates between the air, soil, and water. Under the effect of certain chemical reactions, elemental mercury can transform into its inorganic form which can later be deposited in particulates and precipitations, ultimately leading to its accumulation in rivers, lakes and streams. Once deposited in water or soil, certain microorganisms can transform it into methyl mercury, a highly toxic and bioaccumulative form, themselves later becoming food for fish and other aquatic organisms. This way, methyl mercury accumulates upwardly in the food chain in fi sh, fi sh-eating birds and animals, eventually reaching the general population (3). Humans are exposed mainly through consuming contaminated fi sh and shellfi sh or by inhaling vapours of an elemental form arising from industrial processes, or even from breaking products containing mercury. Low-level exposure through rice consumption by populations from areas with mercury-contaminated water reservoirs has also been suggested (4). Since its primary mechanism of action involves binding to sulfhydryl groups, mercury can impair the function of any cellular or subcellular structure in the body. The most susceptible targets for mercury are selenoenzymes, particularly thioredoxin; an antioxidant defence system responsible for scavenging reactive