To detect excessive exposure to mercury
When you have symptoms of mercury poisoning, to evaluate a known exposure to mercury, or to monitor occupational exposure to mercury
A blood sample taken from a vein in your arm and/or a urine collection, rarely; hair, breast, milk, nails, faeces and breathe.
Mercury is an element that exists in three forms:
- Metallic or elemental mercury. This form can be found in dental fillings, some thermometers and batteries.
- Inorganic mercury where it has combined with other elements such as oxygen or sulphur to form a salt.
- Organic mercury where it has combined with carbon to form a variety of organic compounds, the most common of which is methyl mercury. This form can be found in larger, older and predator fish such as shark, swordfish and marlin due to absorption over time and bioaccumulation in the food chain.
This test measures the amount of mercury in blood, urine, or (rarely) hair to detect acute or chronic excessive exposure.
Mercury is a naturally occurring element, but much of what is found in the environment today is from past industrial activity. It is released by the breakdown of minerals in rocks and soils and as a by-product of fossil fuel combustion and waste incineration. It is inhaled with the air that we breathe, absorbed through the skin, and ingested with food. The tiny amounts to which the vast majority of people are exposed to do not generally cause health concerns but people who are exposed to dangerous concentrations of mercury (such as might be found at a hazardous waste site) or are exposed chronically to mercury (such as those who work with “heavy metals” in their occupation) may have mercury-related symptoms and complications.
Dental amalgam used in dental fillings, contains mercury, but where already placed it is not considered a health risk for the general population (EU Scientific Committee on the Emerging and Newly Identified Health Risks).
Exposure to excessive amounts of mercury can be toxic. The amount of mercury absorbed by an individual and its effects on their health depends on the type of mercury, its concentration, and the exposure time. According to the Agency for Toxic Substances and Disease Registry (ATSDR), very little metallic mercury (less than 0.01%) is absorbed by the body, even if it is swallowed. However, if the same mercury is inhaled as a vapour, about 80% is absorbed into the bloodstream. About 95% of methyl mercury (the type found in fish and other seafood) is absorbed by the stomach and intestines. The most common source of human exposure to methyl mercury is as a result of eating contaminated seafood. Fish that come from contaminated waters and larger predator fish – fish that have eaten smaller fish – may have significantly increased concentrations of methyl mercury. This is why it is recommended that you know the source of the fish you consume and that you limit the quantity of large predator fish you eat.
During pregnancy it is recommended that shark, swordfish and marlin are avoided and that only 2 portions per week of oily fish such as salmon, trout and mackerel are consumed due to mercury levels found in them.
Once mercury is absorbed, it finds its way into a variety of body organs, including the kidney and brain. The body will slowly get rid of mercury through the urine and stool, but if excessive amounts are present, it can permanently damage the kidneys, nervous system, and brain. Pregnant women with elevated blood concentrations of mercury can pass it on to their foetus, affecting development especially the foetus’s brain, kidneys, and nerves. Mercury can be passed from mother to baby through breast milk during nursing.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm, or a random or 24 hour urine sample may be collected. Rarely, another sample such as hair, breast milk, nails, faeces or breathe may be tested.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is it used?
Mercury testing is carried out to detect the presence of an excessive amount of mercury within the body. It may be requested by your doctor to determine whether you have been acutely or chronically exposed to increased concentrations of mercury. It may also be requested to monitor those who are exposed to mercury in the workplace.
More than one type of sample may be collected and tested.
- Blood is primarily tested to detect the presence of methyl mercury. Other forms of mercury can also be detected in the blood, but according to the Agency for Toxic Substances and Disease Registry (ATSDR), the amount present in blood will decrease by a half within 1-3 days as the mercury moves into other organs such as the brain and kidneys. Blood testing must be done within days of suspected exposure.
- Urine is used to test for metallic mercury and inorganic forms of mercury, but it cannot be used to determine exposure to methyl mercury.
- Hair testing may be useful to detect methyl mercury exposures that occurred several months previously, but hair testing is relatively complex and is not used frequently.
- Although not routinely requested tests, mercury has been shown to be present in nails, breast milk, faeces, and breath.
When is it requested?
Mercury testing may be performed when a patient has symptoms suggesting excessive exposure to mercury. Acute symptoms may include:
- burning in the mouth and lungs
- cough, difficulty breathing, chest tightness
- decreased urine output
- nausea, vomiting, diarrhoea
- increased heart rate
Those who are chronically exposed to mercury may have non-specific symptoms that involve the lungs, kidneys, and nervous system. Some of the chronic symptoms may include:
- problems with hearing, taste and smell
- blurred vision or sometimes tunnel vision
- tingling or tremors in the arms or legs
- difficulty walking
Testing may also be performed when a patient is known to have been exposed to mercury, in order to help evaluate the extent of the exposure. Mercury levels may be evaluated regularly as a monitoring tool for those individuals who work in industries that utilise mercury and may be requested, along with lead and/or other heavy metals, for individuals who work with a variety of potentially hazardous materials.
What does the test result mean?
Normal concentrations of mercury in blood and urine indicate that it is unlikely that the patient has been exposed to excessive levels of mercury, at least not in the window of time that the test is measuring.
Elevated concentrations of mercury in blood or urine indicate that excessive exposure to mercury has occurred, but it does not indicate the form or quantity of mercury to which a person was exposed. Increased blood concentrations suggest a relatively recent exposure to mercury, while a 24-hour urine sample gives more of an average past history of exposure to metallic or inorganic mercury.
Increased concentrations of mercury in hair may indicate exposure to increased concentrations of methyl mercury but hair samples are rarely used because of issues involving testing standardisation, sample contamination and the fact that hair is subject to many pre-analytical variables (hair exposure to dyes, bleach, shampoo, etc.).
Mercury is considered to be a non-essential trace element in humans, therefore low levels of mercury are usually not of any concern.
Is there anything else I should know?
Measures have been taken in recent years to reduce and control the public’s exposure to mercury. Stricter regulations and recommendations have lowered the amounts allowable in the air, water, soil, food and in the workplace.
The high concentrations of mercury found in certain fish may harm the developing nervous systems in unborn babies and young children. The NHS recommends that pregnant women, women who may become pregnant, young children and nursing mothers avoid eating shark, swordfish, and merlin and that girls, women planning pregnancy, and pregnant and breastfeeding women should eat no more than 2 portions of oily fish per week.
What is thiomersal?
Thiomersal (known in the US as thimerosal) is an organic mercury compound that is used in small amounts as a preservative in some vaccines. Although it is generally regarded to be safe, the use of thiomersal is now being phased out and it is no longer used in routine childhood vaccinations.
Do silver teeth filling contain mercury?
Yes. Dental amalgams (teeth fillings) are made up of approximately 50% metallic mercury. Some people feel that the tiny amount of mercury vapour released when a person chews may affect their health, but internationally most major health organisations consider the amalgams to be safe at this time. The intention is to phase out the use of dental amalgam, on environmental grounds, by 2030, and there are already restrictions on its use in children under 15 and pregnant or breastfeeding mothers.
Elsewhere On The Web
Mercury: health effects, incident management and toxicology - GOV.UK (www.gov.uk)
Agency for Toxic Substances and Disease Registry (ATSDR): Toxicological Profile for Mercury
U.S. Environmental Protection Agency: Mercury
Environmental Protection Agency, What You Need to Know about Mercury in Fish and Shellfish