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.
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.
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 compounds may be found in the air, soil, and water near hazardous waste sites.
Fate & Transport
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. The total amount of mercury in the environment caused by natural processes throughout the world is far greater than the total amount caused by human activities. However, the amount of mercury that exists in any one place through natural processes is usually very low. In contrast, the amount of mercury that may be at a particular waste site because of human activity can be very high. 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.
Metallic mercury is a liquid at room temperature. 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.
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 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 near hazardous waste sites is likely to occur by breathing contaminated air, having contact with contaminated soil, or drinking contaminated water.
The background or natural level of mercury found in outdoor air is generally between 10 and 20 nanograms of mercury per cubic meter of air (ng/m3). Mercury levels found in surface water are generally less than 5 ng per liter of water. Levels normally found in soil range from 20 to 625 ng of mercury per gram of soil. The Food and Drug Administration (FDA) has estimated that, on average, most people are exposed to about 50 ng of mercury per kilogram of body weight per day in the food they eat. This translates to about 3.5 micrograms of mercury per day for an average weight adult. A large proportion of this mercury is likely to come from fish. Furthermore, people who eat a lot of fish are likely to have higher exposure to mercury.
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.
Exposure to mercury can be determined by measuring amounts in blood and urine. Levels found in blood and urine may show whether health effects are expected.
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.
Long-term exposure to either inorganic or organic mercury can permanently damage the brain, kidneys, and developing fetus. The most sensitive target of low level exposure to metallic and organic mercury following short or long term exposures appears to be the nervous system. The most sensitive target of low level exposure to inorganic mercury appears to be the kidneys. Short term exposure to high levels of mercury can have similar effects. Full recovery is more likely after short term exposures than long term exposures, once the body clears itself of the contamination.
Short term exposure to high levels of metallic mercury in the air can cause skin rashes and effects on the lungs and eyes. Long term exposure to metallic mercury has been studied in workers at chlorine facilities. Some of them developed symptoms such as memory loss and shakiness. Levels of metallic mercury in air were greater than the levels normally encountered by the general population. Current levels of mercury in workplace air are lower than in the past. Because of this reduction, fewer workers have symptoms from mercury exposure. Studies in humans found there were no effects on the ability to reproduce after breathing metallic mercury for a long time.
Short and long term exposure to high levels of inorganic mercury can cause kidney effects in humans. However, recovery is likely, once the body clears itself of the contamination. There is no information on low level exposures in humans. Short and long term exposure to low levels of inorganic mercury in animals can also cause kidney and brain effects. Long term exposure to higher than normal levels of inorganic mercury from eating or drinking contaminated foods or water can lead to brain and kidney damage in some people. Long term exposure to inorganic mercury has caused effects to the fetus in animals. The general population is generally not exposed to levels high enough to produce these effects.
People who eat fish containing organic mercury or grains treated with organic mercury for a long time can have permanent damage to the brain, kidneys, and the growing fetus. The amounts of organic mercury that cause these effects are higher than the amounts to which the general population is exposed daily. Exposure to organic mercury may cause brain damage in developing fetus. Exposure to organic mercury is also dangerous for young children because their nervous systems are more sensitive to these compounds. Kidney effects occur in animals exposed to low levels of organic mercury. Low level exposure to organic mercury may also reduce the ability of animals to have babies. However, no studies are available to determine if this effect can occur in humans.
There is no information to show that mercury causes cancer in humans or animals. National Toxicology Program (NTP), EPA, and the International Agency for Research on Cancer (IARC) have not classified mercury as to its human carcinogenicity.
Information excerpted from:
Toxicological Profile for Mercury October 1992 Draft UpdateAgency for Toxic Substances and Disease Registry
U.S. Dept. of Health and Human Services