Mercury is a toxic heavy metal that is present in the environment both naturally (e.g. volcanic eruptions) and through industrial processes (e.g. mining, burning coal or fuel oil). Mercury also enters the food chain, and thus food, through deposition in soil and water. There are different forms of mercury that have different health effects: elemental (metallic) mercury (Hg0), inorganic mercury (iHg), and organic mercury compounds such as methylmercury (MeHg).
Inorganic mercury can be found in all food groups: in fish and seafood as well as in plant products and in animal products from land animals. In water, inorganic mercury is converted by bacteria into the even more harmful organic methylmercury. There, it is taken up by aquatic organisms, which in turn feed some fish. Long-lived predatory fish that feed on fish already contaminated with methylmercury contain particularly high concentrations of methylmercury. Therefore, this mercury compound is found exclusively in fish and seafood (crustaceans, mollusks, squid).
Elemental mercury is primarily absorbed by inhalation through the respiratory tract, whereas dietary intake of elemental mercury through the gastrointestinal tract is vanishingly small and thus not significant.
Inorganic mercury accumulates primarily in the kidney. It can also affect the liver, nervous system, reproductive system and immune system.
Organic mercury compounds, such as methylmercury, are considered a particularly dangerous form of mercury in food. Methylmercury can cross the blood-brain barrier and placenta, resulting in neurological damage. The development of the nervous system in the unborn child is particularly sensitive to these mercury compounds.
In the 1950s, mass methylmercury poisonings occurred in Minamata, Japan: industrial wastewater containing mercury was discharged into the sea. In the sea, methylmercury accumulated in fish and seafood, which were the staple foods of coastal residents. Due to the high intake levels, numerous acute poisonings occurred, resulting in irreversible neurological deficits, deaths, and brain damage in newborns.
Since 2000, the mining of mercury ores has been stopped in the EU (UBA, 2014). In 2013, the United Nations adopted measures to reduce global mercury emissions in the Minamata Convention. The convention includes, for example, a ban on the activation of new mercury mines and a phase-out plan for existing mining sites (UNEP, 2013).
Situation in Austria
Results from Austria and the EU show that contamination levels of mercury in food of terrestrial origin, i.e. in plant products and in animal products from terrestrial animals, are very low. In the majority of these foods, the concentrations of mercury were so low that they could not be measured. The situation is different with regard to mercury in fish and seafood: Here, measurable concentrations are present in most cases. In fish, 80 to 100 percent of the total mercury is present as methylmercury, in seafood (shrimp, mussels, squid) 50 to 80 percent. The remaining mercury is inorganic mercury.
Maximum levels for mercury in food
Mercury is regulated as total mercury in the Contaminants Regulation (Regulation (EC) No. 1881/2006 as amended).
With regard to mercury, certain fish species, seafood and food supplements are thus regulated in the form of maximum levels. For the majority of fish, the maximum level is 500 µg/kg fresh weight. For certain fatty fish, especially predatory fish, which are at the bottom of the food chain and can accumulate contaminants at an increased rate, the maximum level is 1,000 µg/kg fresh weight. Foodstuffs may only be placed on the market if their contaminant content does not exceed the maximum levels listed.
Furthermore, maximum residue levels for mercury in other foodstuffs are regulated according to the Pesticide Regulation (EC) No. 396/2005. On January 18, 2018, new M RLs were published in the Official Journal of the European Union in Regulation (EU) No. 2018/73, which have been valid since February 7, 2018.
Investigations of mercury in fish
We investigate mercury in fish and seafood as well as in foods of terrestrial origin such as cereals, fruits, vegetables, dairy products, meat, baby food, sports food and dietary supplements. From 2007 to 2015, a total of 1,751 samples of fish and seafood were analyzed for their mercury levels. In the case of freshwater fish, the main species analyzed were trout, carp, char and pikeperch, both from domestic waters and imported products. Furthermore, exotic freshwater fish such as pangasius and tilapia and popular marine fish such as tuna, cod (codfish), herring, mackerel, sardine, anchovy, plaice, gilthead bream (dorade), halibut, Alaska pollock (Alaska pollack), sea bass, saithe (coalfish), sprat, snapper and butterfish were analyzed. Additional focus was on crustaceans (shrimp), aquatic mollusks (squid and mussels), and fish products (fish sticks and surimi). In addition, individual samples of more than 30 other fish species were tested. The limits were exceeded in seven marine fish.
The level of mercury contamination is strongly dependent on the fish species: Predatory fish at the end of the food chain contain particularly high mercury concentrations. Of the fish species popular in Austria, trout, carp, char, sardine, sprat, herring, salmon as well as the "Alaska pollack", which is often processed into fish sticks, have low mercury levels. Seafood such as shrimp, squid and mussels also have low levels of contamination, according to the data. Exceedances of the maximum levels for methylmercury, on the other hand, occur in tuna, swordfish, snapper, marlin, and butterfish, which is found in certain sushi dishes, for example.
Overview of mean values of methylmercury in fish from Austrian food surveys from 2007-2015:
- Trout, char, carp, salmon, Alaska pollock, sprats, sardines, herring, pangasius and tilapia are low contaminated with an average of 10 - 40 µg/kg. The mercury contamination of seafood (shrimp, squid and mussels) is also low at 18 - 24 µg/kg.
- For pikeperch, cod, mackerel, anchovies, plaice, gilthead bream, halibut and sea bass the mean values are in the range of 45 - 100 µg/kg.
- High mercury mean values occur in tuna (184 µg/kg) and snapper (256 µg/kg) as well as in butterfish (677 µg/kg).
- In domestic fish, trout (20 µg/kg), char (33 µg/kg) and carp (26 µg/kg) have low levels. In pikeperch, levels average 93 µg/kg.
Focus actions regarding mercury in fish since 2015
In the course of a focus action in 2017, 60 food samples from all over Austria were examined with regard to (methyl) mercury in food. Thereby, the total mercury content of two samples (one tuna, one swordfish) was in the range of the maximum value of 1.0 mg/kg. In the marine fish tested (50 percent of the total samples), mercury was present almost exclusively in the form of methylmercury. All four tuna samples tested were contaminated (0.339 mg/kg to 0.800 mg/kg fish), plus one swordfish sample (0.901 mg/kg), one butter mackerel (0.698 mg/kg), and other predatory fish (monkfish, red snapper, barracuda, Greenland halibut: 0.110 mg/kg to 0.223 mg/kg). Inorganic mercury levels were negligible in these fish except for one sample of marlin (0.543 mg/kg). Freshwater fish (such as trout, walleye, pangasius) and seafood (mollusks, crustaceans, squid) did not have significant levels of mercury.
In another focus action in 2018, mercury levels were analyzed in 102 samples of marine fish. In three of the samples, the maximum mercury level was exceeded, two of which were assessed as "unsafe - unfit for human consumption" based on the exposure assessment. In two samples, the mercury content was in the range of the maximum level.
Fish is rich in iodine, contains high-quality protein, omega-3 fatty acids and significant amounts of vitamin D. Therefore, the Austrian Ministry of Health recommends eating 1 serving of domestic fish (such as char, trout, carp) and 1 serving of fatty sea fish (such as salmon, mackerel, herring, tuna) per week (1 serving = 1 palm-sized, finger-thick piece).
Recommendations for sensitive groups of people:
- When following fish consumption recommendations, care should be taken to ensure that children do not eat predatory fish such as tuna, swordfish, halibut, pike, butterfish, snapper, shark, marlin, swordfish, king mackerel, tilefish, and butter/snake mackerel every week. Babies, small children, pregnant women, nursing mothers, and women of childbearing potential should avoid these predatory fish altogether
- In this way, the beneficial nutritional effects of regular fish consumption can be achieved without ingesting alarming levels of organic mercury compounds, such as methylmercury. When avoiding marine fish altogether, it is recommended to eat an additional 1 tablespoon of canola oil/day
For inorganic mercury and methylmercury, TWI values have been derived by the European Food Safety Authority (EFSA). TWI means tolerable weekly intake. This is the amount of a substance that can be ingested per week over a lifetime without causing health effects.
The TWI value for inorganic mercury is 4 µg/kg body weight (bw) and the TWI value for methylmercury is 1.3 µg/kg body weight. A 70 kg person may ingest 91 micrograms of methylmercury in a week without any health effects. A 30 kg child may ingest 39 micrograms of methylmercury in a week without experiencing health effects.
To assess exposure to methylmercury, the tolerable weekly intake was compared to data on fish consumption of the Austrian population (Österreichischer Ernährungsbericht 2012).
Exhaustion of the daily tolerable intake of mercury.
Using the Austrian Nutrition Report data on fish consumption, adults consume on average 9% of the TWI for methylmercury (circa 111 grams of fish & seafood per week). Children consume on average 18% of the TWI value (circa 124 grams of fish & seafood per week). Adults with high fish consumption (approx. 750 grams of fish & seafood per week) use up 66% of the TWI value. Children with a high fish consumption (approx. 570 grams of fish & seafood per week) use up 116% of the TWI value. This means that the TWI is exceeded. However, the type of fish consumed plays a major role. If only low-polluted fish such as trout, char, carp, salmon or fish sticks are consumed, even very high consumption does not lead to an exceedance. In the case of highly contaminated species such as tuna, even a 150 g can of tuna per week can lead to an exceedance in children.
Mercury and fish consumption
Comparing average levels of mercury from fish, the following examples of weekly tolerable intake for children and adults can be calculated:
|Fish species/product||average mercury content||child with 30 kg, TWI max. 39 µg||Adult with 70 kg, TWI max. 91 µg|
|Trout||20 µg/kg||1.95 kg||4.6 kg|
|Salmon||23 µg/kg||1.7 kg||4 kg|
|Carp||26 µg/kg||1,5 kg||3,5 kg|
|Herring||36 µg/kg||1 kg||2,5 kg|
|Mackerel||51 µg/kg||765 g||1,7 kg|
|Pikeperch||93 µg/kg||419 g||978 g|
|Tuna||290 µg/kg*||135 g||314 g|
|Butterfish||677 µg/kg||58 g||134 g|
Data based on the mean values of methylmercury from the Austrian food investigations of the years 2007-2015.
* Mean value for tuna from the opinion of the European Food Safety Authority (EFSA, 2012). For tuna, a large difference was shown between the Austrian (184 µg/kg) and the European mean value (290 µg/kg). As a precaution, the higher European value was used for this calculation.
EFSA (European Food Safety Authority), 2012: Scientific Opinion on the risk for public health related to the presence of mercury and methylmercury in food. EFSA Panel on Contaminants in the Food Chain (CONTAM). EFSA Journal 2012;10(12):2985. 1-241.
Last updated: 21.01.2022