What is Mercury?
Mercury is a chemical (element) that occurs naturally in the environment in several forms. In the metallic or elemental form, mercury is a shiny, silver-white, odorless liquid with a metallic taste. Mercury can also combine with other elements, such as chlorine, carbon, or oxygen, to form mercury compounds. These compounds are called "organic mercury" if they contain carbon, and "inorganic mercury" if they do not. In pure form, these mercury compounds are usually white powders or crystals. All forms of mercury are considered poisonous. One organic form of mercury, methylmercury, is of particular concern because it can build up in certain fish. For this reason, rather low levels of mercury in the oceans and lakes can contaminate these fish.
Uses of Mercury
There are many different uses for and sources of mercury. Metallic mercury is mined and is also a waste product of gold mining. Chemical factories that make chlorine use mercury and may release metallic mercury into the air. Thermometers, barometers, batteries, and tooth fillings all contain metallic mercury. Inorganic mercury compounds are commonly used in electrical equipment (for example, batteries, lamps) and skin care and medicinal products. Some inorganic mercury compounds are used in fungicides. Methylmercury is generally produced in the environment, rather than made by human activity. Fungicides and paints may contain other organic mercury compounds.
Mercury in the Environment
Mercury released into the environment stays there for a long time. Once in the environment, mercury can slowly be changed from organic to inorganic forms and vice versa by microorganisms and natural chemical processes. Methylmercury is the organic form of mercury created by these natural processes.
Mercury is a naturally occurring metal found throughout the environment as a result of normal breakdown of the earth's crust by wind and water. Air, water, and soil can contain mercury from both natural sources and human activity.
The mercury in air, water, and soil is thought to be mostly inorganic mercury. This inorganic mercury can enter the air from deposits of ore that contain mercury, from the burning of fuels or garbage, and from the emissions of factories that use mercury. Inorganic mercury may also enter water or soil from rocks that contain mercury, releases of water containing mercury from factories or water treatment facilities, and the disposal of wastes. Organic compounds of mercury may be released in the soil through the use of mercury-containing fungicides.
It can evaporate easily into the air and be carried a long distance before returning to water or soil in rain or snow. As mentioned before, some microorganisms in the water or soil can change inorganic forms of mercury to organic forms. Organic forms of mercury can enter the water and remain there for a long time, particularly if there are particles in the water to which they can attach. If mercury enters the water in any form, it is likely to settle to the bottom where it can remain a long time. Mercury also remains in soil for a long time. Mercury usually stays on the surface of the sediments or soil and does not move through the soil to underground water.
Small fish and other organisms living in the water can take up the organic forms of mercury. When larger fish eat these small fish or other organisms that contain organic mercury, their bodies will store most of it. In this way, large fish living in contaminated waters can collect a relatively large amount of organic mercury. Plants may also have a greater concentration of mercury in them if they are grown in soil that contains higher than normal amounts of mercury.
Exposure to Mercury
Because mercury occurs naturally in the environment, everyone is exposed to very low levels of mercury in air, water, and food. Sources of higher exposure to metallic mercury include breathing air containing mercury in the workplace or any place where mercury might have been spilled. Also, since amalgam dental fillings are about half metallic mercury, if you have them you can be exposed to mercury levels that are higher than the levels normally found in the environment. People with dental fillings containing mercury generally have more mercury in their breath than those who do not have these fillings. However, there is not enough evidence to prove that the mercury in amalgam fillings is causing health effects in humans.
Sources of exposure to inorganic mercury include swallowing or inhaling dust that contains mercury particles in the workplace and using skin care and medicinal products with small amounts of mercury in them. You can also be exposed to inorganic mercury by drinking water that is contaminated with mercury. For most people, eating contaminated fish is the major source of organic mercury exposure. Some fish contain such high levels of mercury that eating them has been prohibited. Other foods typically contain very little mercury. A greater risk of mercury exposure may occur in fetuses exposed to mercury in their mother's blood and in nursing children who may be exposed to mercury in their mother's milk.
Exposure to mercury can occur in many jobs. Most exposures on the job occur as a result of breathing air that contains mercury. Exposure occurs in the medical, dental, and other health services, and in the chemical, metal processing, electrical equipment, automotive, building, and other industries. Families of workers may be exposed to mercury in the home if the workers have mercury dust on their clothing. Dentists and their assistants may also be exposed to mercury from skin contact with dental fillings and breathing metallic mercury vapor released from these fillings.
Mercury can easily enter your body when you breathe in air containing metallic mercury. Most of the mercury that gets into your lungs as metallic mercury goes rapidly to other parts of the body. Metallic mercury that you might swallow does not enter your bloodstream very easily, and most of it leaves the body in the feces. Some metallic mercury may stay in your body, mostly in the kidney and brain. Metallic mercury can also reach the fetuses of pregnant women easily. Metallic mercury that you breathe in will leave your body in the urine, feces, and breath.
Inorganic salts of mercury (mercurous or mercuric chloride, for example) that are inhaled do not enter your body as easily. However, these inorganic forms of mercury, if swallowed, enter the body more easily than metallic mercury. Inorganic mercury can also enter the bloodstream directly through the skin. However, only a small amount would pass through your skin compared with breathing or swallowing inorganic mercury. After entering the body, inorganic compounds of mercury can also reach many tissues. Some may stay in your body, mostly in the kidneys. However, inorganic mercury cannot reach the brain as easily as metallic mercury. Inorganic mercury leaves your body in the urine or feces after several weeks or months.
Organic compounds of mercury can probably enter your body easily through the lungs. Organic mercury in contaminated fish or other foods that you might eat enters your bloodstream easily and goes rapidly to other parts of your body. It can also enter the bloodstream directly through the skin, but only a small amount would pass through your skin. Organic mercury in the body is similar to metallic mercury because it can reach most tissues including the brain and fetus. Organic mercury can change to inorganic mercury in the brain and remain there for a long time. Organic mercury that you swallow or breathe leaves your body in the feces, mostly as inorganic mercury, within weeks.
Health Effects of Mercury
The NIOSH Pocket Guide to Chemical Hazards  lists the following symptoms related to mercury exposure:
Eye and skin irritation
Cough, chest pain, breathing difficulty
Tremor, insomnia, irritability
Indecision, headache, weakness
Anorexia, weight loss
When checked on 12 December 2015, US EPA's IRIS Database stated that mercury is not classifiable as to human carcinogenicity.  When checked on 7 April 2015, the International Agency for Research on Cancer had determined that mercury and inorganic mercury compounds are not classifiable as to carcinogenicity in humans. 
 NIOSH Pocket Guide to Chemical Hazards, published by the National Institute for Occupational Safety and Health