What is Mercury?
Mercury is a naturally occurring silver-colored metallic element noted for its occurrence as a liquid at room temperature and toxic to living organisms. This silvery, opaque liquid metal is very dense, yet has a high surface tension that causes it to form tiny perfect spheres in the pores of the rocks it is found in. It is designated with the chemical symbol 'Hg' in the periodic table of elements and posses the following physical and chemical properties:
- Atomic Mass Average: 200.59
- Boiling Point: 357ºC / 675ºF / 630K
- Electrical Conductivity: 0.0104 X 106 / cm Ohm
- Thermal Conductivity: 0.0834 W / cmK
- Density: 13.546 g / cc @ 300ºK
- Melting Point: -38.72ºC / -37.7ºF / 234.28ºK
- Molar Volume: 14.81 cm3 / mole
- Vapor Pressure: 0.0002Pa @ -38.72ºC
- Heat of Vaporization: 59.229 kJ / mol
- Flammability Class: Non-Combustible Liquid
- Electrochemical Equivalent: 3.742 g / amp-hr
- Electron Work Function: 4.49eV
- Heat of Fusion: 2.295 kJ / mol
Acetylene, ammonia, chloride dioxide, azides, calcium (amalgam formation), sodium carbide, lithium, rubidium, copper
Valance Electron Potential (-eV): 28.2
Mercury expands and contracts evenly with temperature changes. Elemental mercury breaks up easily into many small droplets and evaporates to form mercury vapor, a colorless and odorless gas. There are three forms of mercury in the environment: elemental, inorganic and organic mercury. Each type has its own level of toxicities.
Elemental mercury occurs naturally in three valence states: elemental (Hg 0), monovalent-mercurous (Hg 1+), and divalent mercuric (Hg 2+). Elemental mercury is the most stable form and is only slightly water-soluble. Both mercuric and mercurous mercury are thermally unstable and readily decompose to elemental mercury. Elemental mercury can vaporize at room temperature presenting a hazard if spills occur. This vapor can be inhaled into the lungs and passed into the blood stream. Elemental mercury can also be absorbed through the skin and then into the blood stream. If swallowed, however, this form of mercury is not absorbed by the stomach and usually passes through the digestive system without harm.
Inorganic mercury compounds contain ionic mercury usually as a salt compound (e.g. mercury chloride, HgCl 2). Many inorganic mercury compounds have been banned from use by local governments especially in consumer products and agriculture due to their toxicity. But inorganic mercury compounds continue to be used globally as disinfectants and pesticides. If inhaled, the mercury compounds are absorbed through the lungs. The compounds may also pass through the skin. The compounds can also be absorbed through the stomach if swallowed. Many inorganic mercury compounds are irritating or corrosive to the skin, eyes, and mucus membranes.
Organic mercury compounds are the most toxic of all especially species like methyl or di-methyl mercury. They can be chemically synthesized or biologically converted from mercury compounds by bacteria (e.g. methyl mercury). Chemically synthesized organic compounds have been used as fungicides. Some organic mercury compounds are water-soluble and can bio-accumulate in the aquatic food chain. They can enter the human body readily through absorption by lungs, skin and stomach.
Where is mercury found?
Mercury is a naturally occurring element, which is found mainly in the form of cinnabar ore (HgS). Significant levels are found in fossil fuels such as coal and trace amounts are found in minerals and rocks.
Although it is a naturally occurring substance, more than two-thirds of the mercury in the atmosphere now comes from human-made products and through the combustion of fossil fuels used to produce power. Mercury is released into the atmosphere through a variety of means such as evaporation / vaporization from water and land media but primarily the mercury is carried in stack gases exhausted from coal-fired utility boilers and waste (medical & municipal) incinerators.
Mercury enters the soil through natural breakdown of rocks containing mercury, disposal of mercury in landfills, and atmospheric depositions. It enters the watershed through runoff, atmospheric depositions and when mercury waste products are poured down drains. Once in the water cycle, free-mercury can convert into methyl mercury in the presence of microorganisms. Methyl mercury, which is fat-soluble, bio-accumulates in the tissue of fish (or other aquatic animals), and bio-magnifies (increases in concentration) up the food chain.
Where is Mercury used?
Because of its unique properties, elemental mercury had many uses in the past. It is still used in electrical equipment such as alkaline batteries, fluorescent light bulbs, contact switches, and thermostats. Mercury and its compounds are also used in extracting gold and silver from ore. In medical and scientific instruments, mercury can be found in thermometers, manometers, barometers, and mercury porosimeters. Mercury is also used in making jewelry and for coating the back planes of mirrors and it is still widely used in dental amalgams and as a catalyst in the production of polymer products and in paint.
What are the impacts of mercury exposure on humans?
Humans are exposed to mercury through their diet (primarily through fish), absorption, or even through inhalation of toxic elemental mercury fumes.
Abnormalities in newborn children caused by methyl mercury poisoning were first noted in the mid 1960's when an outbreak of cerebral palsy and micro-cephaly in newborns was reported in the fishing village of Minamata Bay, Japan. It was discovered that the mothers of the newborns had ingested high levels of mercury by eating fish from the local bay. The bay was found to be heavily contaminated by organic mercury waste discharged into the bay from a chemical company. Fetal intoxication with organic mercury since that outbreak has been referred to as 'Minamata Disease'. Methyl mercury poisoning was also observed in Iraq where seed grains contaminated with methyl mercury were mistakenly used to make bread. Infants exposed in-uteri were found to have psychomotor-retardation and cerebral palsy. Due to these historical incidents, there is a growing concern about human exposure to mercury.
Now, however, most people are not directly exposed to methyl mercury due to restricted use of the compound. Exposure is more likely to occur through inhalation of toxic elemental mercury vapor, which is present in the atmosphere and environment from coal-fired utilities, waste incinerators, cement plants, oil-fired boilers, and landfills. The health effects of elemental mercury exposure depend on several factors, including the amount inhaled and length and frequency of exposure. Young children and fetuses are most sensitive to mercury poisoning during their early growth period.
Exposure Limits set by both the U.S. OSHA (Occupational Safety and Health Administration) and NIOSH (National Institute of Occupational Safety and Health) are as follows.
Permissible Exposure Limit (PEL) 0.1 mg/m 3
Recommended Exposure Limit (REL) 0.05 mg/m 3
(TWA (total work average) over 8-hr workday, 40-hr workweek)
What are the symptoms of elemental mercury exposure?
Acute Health Effect
Exposure to high levels of mercury vapor in air will cause acute poisoning. Symptoms usually begin with coughing, chest tightness and general breathing difficulty. This can deteriorate to pneumonia, which can be fatal in those with weak or compromised immune systems.
If inorganic mercury compounds are swallowed, symptoms such as nausea, vomiting, diarrhea and kidney damage can occur.
Some organic mercury compounds (e.g. methyl mercury) are known to cause birth defects in children born to exposed mothers. Mothers are exposed to methyl mercury mainly from consumption of fish in their diet (typically large marine fish like swordfish, shark, and tuna). Experts from the United States Food and Drug Administration (US FDA) advise pregnant women to limit their consumption of such fish to no more than once a month to avoid any methyl mercury exposure to their fetuses. Consumers are advised to eat a wide variety of smaller fish species to limit possible exposure to mercury and still retain the nutritional benefits of fish in their diet.
Chronic Health Effect
Exposure to any form of mercury on a repeated basis, or even from a single high-level exposure, can result in chronic mercury poisoning. Symptoms include:
Gum Problem - gums become soft and spongy, teeth loosen and sores develop.
Mood and Mental Changes - people with chronic mercury poisoning often have large mood swings, are easily irritated, frightened, depressed, or excited for no apparent reason. Hallucinations, memory loss and inability to concentrate can occur.
Nervous System - earliest and most frequent symptom starts with minor uncontrollable tremor (shaking) of the hands. Such tremors can also occur in the tongue or eyelids. Eventually, this can progress to include trouble with balance and walking.
Other Heath Effects
There are a number of other symptoms that may be caused by exposure to mercury and compounds containing mercury:
Repeated exposures can cause a skin rash and itching.
Direct exposure to mercury vapor can cause discoloration of the lens in the eye.
Some inorganic mercury compounds can cause burns or severe irritation when in contact with the skin or eye.
How can exposure to elemental mercury be measured?
Measurement of mercury in urine is the recommended biological monitor for workers exposed to metallic and inorganic mercury. If a person is not occupationally exposed to mercury, urine levels rarely exceed 5 ug/L or 5 ug/g creatinine (standardized for a urinary creatinine of 1 gram / liter). Experts have proposed a biological threshold limit of 50 ug/g as an indicator of chronic exposure to mercury vapor.
The concentration of mercury in blood will show exposure to organic mercury as well as metallic and inorganic mercury. Exposure to organic mercury is primarily through consumption of contaminated fish. In unexposed individuals, the amount of mercury in the blood is usually less than 10 ug/L (10 parts per billion (ppb)). Early effects of mercury toxicity have been found in blood with levels exceeding 30 ppb. From the Japanese studies, toxicologists have learned that the lowest level observed in adults associated with toxic effects (paresthesia) was 200 ppb in blood, accumulated over several months to years of eating contaminated food.
The best indexes to measure the amount of exposure to methyl mercury are from hair and blood samples. The average concentration of mercury in hair for un-exposed people is about 2 parts per million (ppm).
What can you do to help prevent mercury pollution?
Two thirds of the mercury present in the atmosphere comes from human pollution. Once the mercury vapor is released in the atmosphere, it is difficult to remove. So the best practice is to prevent mercury from entering the environment whenever feasible.
Mercury is gradually being phased out of many retail products such as thermometers. However, as consumers, we should educate ourselves and not buy items containing mercury when a substitute is available.
Improper disposal of mercury-containing products adds to the level of mercury pollution. Separate household products containing mercury (like fluorescent lamps and bulbs, thermometers, batteries, etc.) and dispose of them as hazardous waste material using procedures in place for your community. Keep such products away from other domestic waste items that may be trucked to a public waste dump or municipal incinerator. Recycle and reuse as many products as possible to decrease the amount of waste that needs to be incinerated or hauled to a public dump facility.
Two-thirds of mercury pollution is generated from human activities with 80% of that value coming from the combustion of fossil fuels to produce energy. By practicing energy conservation, these electric generating stations would burn less fossil fuel and thus reduce the level of mercury emission exhausted into the environment.
Mercury Facts: Illinois Department of Public Health
Environmental & Occupational Health, University of Minnesota
Illinois Teratogen Information Services: Mercury
Maryland Department of the Environment: General Mercury Information
Environmental Health & Safety, Oklahoma State University
United States Occupational Safety & Health Administration
National Institute of Occupational Safety & Health
United States Geological Survey: Mercury Contamination of Aquatic Ecosystems
United States Geological Survey: Mercury in the Environment