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Mercury tests: fact sheet

Mercury exists in elemental, inorganic and organic forms (see below), all of which may be toxic. The toxic manifestation depends on the form of exposure.

For elemental and inorganic forms, the preferred test depends on whether the patient is suspected of having acute or chronic mercury exposure.

Whole blood should be measured in very recently exposed patients with suspected acute toxicity - whole blood can reflect very large acute exposure but is less reliable as redistribution to tissues takes place

24 hour urine should be measured in patients with suspected chronic (medium to long term) exposure

For organic mercury the preferred test for determining toxicity is whole blood . Because organic mercury is eliminated via the faecal route, urine mercury testing is less useful.

Blood and urine mercury tests measure the total mercury level.

The testing of urine mercury following a dose of a chelating agent (e.g. DMSA) is not a validated method for assessing mercury toxicity. Urine mercury always increases after administration of a chelating agent, and this has been used to mislead people into believing they have mercury toxicity.

Mercury and its salts can enter the body by inhalation, ingestion or absorption. Organic mercury is usually the major fraction of blood mercury.  Fish intake is the strongest determinant of blood mercury levels, in people not occupationally exposed to mercury.   Mercury levels are highest in large predatory fish at the top of the food chain (e.g.  tuna, swordfish, shark). 


Most people have blood mercury levels less than 50 nmol/L, and levels up to 100 nmol/L are often seen in fish eaters with no adverse effects.


In women of childbearing age

It is desirable that women of child bearing age have low blood mercury level. Although a 'safe' level during pregnancy is not well established some health authorities suggest <40 nmol/L. The toxic effects of high antenatal mercury exposure to newborns has been well described e.g. from Japan. However, at low level especially between 40 and 100 nmol/L, there is no clear relationship with toxicity.

Fish is the main source of mercury in most people. It is advisable to avoid or have limited intake of fish species known to have high mercury content when planning for, or during, pregnancy. On the other hand, that should be balanced against the health benefits that fish species with low mercury content can bring as part of diet. Information on recommended servings for specific fish species can be found at https://www.mpi.govt.nz/food-safety/food-safety-for-consumers/food-and-pregnancy/list-of-safe-food-in-pregnancy/ .


Dental amalgam fillings make only a minor contribution to body mercury burden, and do not cause a significant increase in blood mercury level. Claims that amalgam fillings cause mercury toxicity are not supported by good evidence.


Elemental mercury

Elemental mercury, also known as quicksilver, is a vapour at room temperature. The CNS is the major site of deposition for mercury derived from inhalation. Sources of exposure include manufacturing, gold mining, dentistry, metal refineries.


Inorganic mercury (salts)

Inorganic mercury is present as mercuric salts. Exposure is via ingestion or absorption across skin or mucosa. The kidney is an important site of deposition for inorganic mercury compounds. Exposure is from chloralkali industries and those that use electroplating, or use of laboratory chemicals or medications


Organic mercury

Organomercuric compounds such as methylmercury are absorbed from the GI tract and are distributed to the CNS and can cross the placenta. Dimethylmercury is particularly toxic. Exposure to organic mercury is mostly via consumption of contaminated fish. The thiomersol used in vaccines was considered a source of toxicity. There is little evidence to suggest it caused paediatric neurodevelopmental disorders, however its use has been reduced. Other sources include wood preservation and some paints.

The half-life of organic mercury   in blood is   45-70 days.


Toxic effects of mercury: 

Irreversible neurological damage (central and peripheral nervous system) is the most important, but other organs are also affected e.g. kidney tubule damage. Toxic effects have been clearly demonstrated at blood mercury levels >1000 nmol/L,  and levels >500 nmol/L should be regarded as indicating high risk.

The clinical effects of levels between 100 and 500 nmol/L are uncertain.  Obvious toxicity in adults has not been reported at these levels.  However the foetal brain is more sensitive to mercury, and subtle effects on cognitive and fine motor performance in children have shown correlations with cord blood mercury levels in this range in some studies. 

Non-specific symptoms such as memory loss, cognitive decline, depression, or chronic fatigue syndrome are not a sufficient indication for measuring blood or urine mercury.   There is no evidence that mercury has any causal relationship to autism spectrum disorder or to Alzheimer's disease.


1.  Mercury: Your Health and the Environment: A Resource Tool.  Health Canada Mercury Issues Task Group, 2004  (http://www.hc-sc.gc.ca)

2.  Clarkson TW, Magos L. The toxicology of mercury and its chemical compounds. Crit Rev Toxicol. 2006;36:609-662

3.  McKelvey W, Gwynn RC, Jeffery N et al. A biomonitoring study of lead, cadmium, and mercury in the blood of New York city adults. Environ Health Perspect. 2007;115:1435-1441

4.  Bates MN. Mercury amalgam dental fillings: an epidemiologic assessment. Int J Hyg Environ Health. 2006;209:309-316

5. Legrand M, Feeley M, Tikhonov C et al. Methylmercury blood guidance values for Canada. Can J Public Health. 2010;101:28-31

6.  Eyeson J, House I, Yang YH, Warnakulasuriya KA. Relationship between mercury levels in blood and urine and complaints of chronic mercury toxicity from amalgam restorations. Br Dent J 2010; 208(4) :E7

7.  A comprehensive literature review by the American Dental Association is available at https://www.ada.org/en/~/media/ADA/Member%20Center/Files/amalgam_literature_review_1009

8.  More notes on the Anti-amalgam movement by Robert S. Baratz: http://www.dentalwatch.org/hg/hearings/fda(2010).html

9.  Mercury Toxicity - http://www.uptodate.com/contents/mercury-toxicicty

Last updated at 12:35:33 10/08/2021