Mercury and health

 Key points about mercury

  • Mercury is an element found naturally in water, air and soil so everybody is exposed to some level of mercury.
  • Exposure to even small amounts of mercury can cause serious health problems and it's considered by the World Health Organisation to be a major public health concern.
  • It's a threat to the development of the child in utero and early in life and may have toxic effects on your nervous, digestive and immune systems, and on lungs, kidneys, skin and eyes.
  • You are mainly exposed to methylmercury (an organic compound) when you eat fish and shellfish containing it.  
Man fishing off a rock by the sea
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Mercury is a natural element found in the earth's crust and exists in various forms:

  • Elemental (or metallic) and inorganic – you may be exposed through your occupation, eg, by inhaling mercury vapours from industrial processes.
  • Organic (eg, methylmercury) – you may be exposed through your diet.

Mercury can be released into the environment from a range of natural activities (volcanic activity and weathering of rocks) and human ones such as mining, coal-fired power stations, waste incinerators, home heating and cooking with coal-based products and various industrial processes. 

Bacteria in the environment can transform mercury into organic methylmercury which can then accumulate in the food chain as plankton or shellfish, eaten by fish, which are eaten by larger fish and then by humans. This means that larger, predatory fish are likely to have higher levels of mercury as they have accumulated it from eating other fish. Other fish with higher levels of mercury are those that live for a long time and those living in lakes or rivers supplied by geothermal water.

Image credit: Canva

Mercury is not affected by heat so cooking doesn't protect us from mercury in our foods.

Mercury in all its forms is toxic to humans and animals resulting in damage to our nervous, digestive and immune systems, lungs, kidneys, skin and eyes. The severity can be mild through to fatal.  

Symptoms of mercury exposure or toxicity can include:

  • tremors
  • insomnia
  • memory loss
  • skin and eye burns from direct contact to corrosive inorganic salts of mercury
  • gastrointestinal damage from ingestion
  • neuromuscular effects
  • headaches
  • cognition and thinking difficulties
  • behavioural disorders
  • motor dysfunction
  • proteinuria through to kidney failure.

Anyone can be exposed to mercury poisoning, but 3 groups are at higher risk:

  • Pregnant women and their unborn babies as mercury is toxic to their developing brains (results in learning difficulties, poor coordination, attention, vision and cognition problems).
  • Workers employed in industries where mercury can be released into the environment or the vapour inhaled.
  • Populations relying on fish and shellfish as a staple food, eg, fishing communities in Brazil, China, Canada, Columbia and Greenland.

The World Health Organisation and other large groups have identified key changes to reduce and prevent mercury exposure. These include:

  • promoting the use of clean energy sources that don't burn coal 
  • stopping the use of mercury in gold mining
  • stopping the mining of mercury
  • phasing out non-essential mercury-containing products such as batteries, thermometers, dental fillings, lamps, skin lightening products and other cosmetics
  • careful recycling of mercury
  • safer work practices where mercury is used.

Advice on eating fish

The Ministry for Primary Industries in Aotearoa New Zealand suggests there are certain fish with higher levels of mercury you should be aware of either if you're pregnant, or if you eat a lot of fish and are exposed to more mercury. These are listed as:

  • cardinalfish
  • dogfish (excluding rig)
  • Lake Rotomahana trout
  • lake trout from geothermal regions
  • school shark (greyboy, tope)
  • marlin (striped)
  • southern bluefin tuna
  • swordfish.

If a mercury thermometer breaks (or other items containing around ¼ teaspoon or less of mercury), you may be able to clean it up yourself by following the Health New Zealand | Te Whatu Ora information sheet.

If you spill more than this amount, you should follow this advice and seek help from professional hazardous waste disposal companies to clean up the spill. Contractors can be contacted through the Yellow Pages under “Waste Disposal”.

View Health New Zealand | Te Whatu Ora's advice about mercury spills(external link).

Pullout guide to food safety in pregnancy(external link) Ministry for Primary Industries, NZ, updated Sept 2020


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