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Heavy metals

What are they?

Heavy metals are found naturally in the Earth's crust. Living organisms, including humans, require trace amounts of some heavy metals, such as copper, iron, and others. These are known as trace elements. Excessive levels of heavy metals can be harmful to living organisms, even when they are necessary for good health.

Where are they found?

Human activities have altered the natural (biochemical and geochemical) cycles and levels of some heavy metals. Heavy metals are stable in the environment and cannot be degraded or destroyed. Therefore, they tend to build up in the atmosphere, soils, sediments and water. Excessive levels of metals in the seas and oceans can affect marine life and pose risk to people consuming seafood.

Currently NZFSA’s Imported Food Programme targets the following foods and food containers for heavy metals as they have been assessed as being of concern:

What illnesses do they cause?

• Antimony:

Everyone is exposed to antimony in the environment. Many paints and pigments use antimony. Ingestion causes nausea, vomiting, and diarrhoea. Heart and lung damage can be caused by the cumulative effects of antimony.

• Arsenic:

In the environment, arsenic is combined with oxygen, chlorine and sulphur to form inorganic arsenic compounds. Arsenic in plants and animals combines with carbon and hydrogen to form organic arsenic compounds. Inorganic arsenic is the toxic form of arsenic for humans. It is a known carcinogen and exposure has been linked with vomiting, diarrhoea, anaemia, liver damage and cardiac abnormalities.

• Cadmium:

Cadmium had many uses including batteries, pigments and metal coatings. Ingestion of cadmium causes irritation to the stomach, resulting in vomiting and diarrhoea.

• Copper:

Copper is a metal that occurs naturally in the environment, and also in plans and animals. As a trace metal, low levels of copper are essential for maintaining good health. High levels can cause harmful effects such as irritation of the nose, mouth and eyes, nausea, vomiting, diarrhoea and stomach cramps.

• Lead:

Lead is used in the production of batteries, solder, ammunition and devices to shield x-rays. Due to chemical similarities, lead can substitute for calcium and be included in bone. Lead that is stored in bone is not harmful, but the lead can be released from bone if the calcium intake increases. Lead can affect almost every organ and system in the body. The central nervous system is most sensitive, especially in children. Lead also damages the kidneys and reproductive systems.

• Mercury:

Mercury occurs naturally in the environment and it can also be released into the air through industrial pollution. Bacteria present in water can change mercury into methylmercury. Methylmercury binds tightly to the proteins in fish tissue. As smaller fish are eaten by bigger fish the methylmercury accumulates. Predatory fish species (such as shark) tend to have higher levels than non-predatory fish or species lower down the food chain. High amounts of mercury can damage the central nervous system which can cause memory loss, slurred speech, hearing loss, lack of coordination, loss of sensation in fingers and toes, reproductive problems, coma, and possibly death.

• Selenium:

Selenium is a trace element. Due to chemical similarity, selenium can substitute for sulphur. Too much selenium can cause skin and nail abnormalities.

How do people absorb heavy metals?

Many organisms are able to regulate the metal concentrations in their tissues. Research has shown that aquatic plants (seaweed) and some molluscs, crustaceans (shrimps/prawns) and fish are not able to successfully regulate metal uptake, and as a result, tend to suffer from metal accumulation.

Heavy metals may enter the food chain in several ways. Small amounts are absorbed by organisms directly from the water through their gills and other tissues. However, most of the pollutants found in aquatic organisms arrive there through the food chain. First, bacteria, and other small organisms absorb these materials. In turn, these are eaten by larger animals, eventually being eaten by people.

The processes described above are completely natural; however, industrialisation for our convenience has provided new ways of absorbing heavy metals. The manufacture of cans involves either welding or soldering. Where lead solder is used, the lead can be leached from the solder to the food stored in the can. The same applies to copper food containers with soldered seams.

Ceramic and enamelware are made from or coated/painted with pigments which contain heavy metals. If the finish on the food container is not perfect, heavy metals can potentially contaminate the food stored/placed in them.