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.

The European Food Safety Authority (EFSA) has derived TWI values for inorganic mercury and methylmercury. 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 for inorganic mercury is 4 µg/kg body weight (bw) per week and the TWI for methylmercury is 1.3 µg/kg bw per week. A 70 kg person may ingest 91 µg of methylmercury weekly without adverse health effects. A 30 kg child may ingest 39 µg of methylmercury weekly without causing health effects.

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 land 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, while in seafood (shrimp, mussels, squid) 50 to 80 percent of the total mercury is methylmercury and 20 to 50 percent is inorganic mercury.

Maximum levels for mercury in food

Mercury is regulated as total mercury in fish, seafood, food supplements and salt in Regulation (EU) No. 2023/915, as amended. While a maximum level of 300 µg/kg is set for some fish as well as seafood, the maximum level for the majority of fish is 500 µg/kg. For certain fatty fish, especially predatory fish, which are at the end of the food chain and can accumulate pollutants to a greater extent, the maximum level of 1,000 µg/kg applies. Foodstuffs may only be placed on the market if their contaminant content does not exceed the maximum levels specified. For drinking water as well as for natural mineral waters, a parameter value of 1 μg/l applies in Austria according to the Drinking Water Ordinance and the Mineral Water and Spring Water Ordinance, respectively.

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 MRLs were published in the Official Journal of the European Union in Regulation (EU) No. 2018/73, which have been valid since February 7, 2018. In Austria, the use of mercury compounds in plant protection products is prohibited (ban on plant protection products containing certain active substances, BGBl. II No. 308/2002).

In addition, mercury is regulated in Directive 2002/32/EC on undesirable substances in animal feed.

Analysis 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 and food supplements. From 2016 to 2022, a total of 1,099 samples of fish and seafood were analyzed for their mercury levels. Among freshwater fish, the main species analyzed were trout, carp, char, catfish 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, Alaskan pollock (pollack/ saithe/chum), sea bass, and butter mackerel were analyzed. In addition, crustaceans (shrimp), aquatic mollusks (squid and mussels), and fish products (canned fish, fish sticks, spreads) were analyzed.

Levels of methylmercury in fish from the 2016-2022 Austrian food surveys:

Mercury is present in fish and seafood mainly in the form of methylmercury (80 - 100% of total mercury). The level of methylmercury contamination is highly dependent on the type of fish: Fish fillets of fatty predatory fish at the end of the food chain, such as swordfish, buttermackerel and tuna, contain particularly high concentrations of mercury. Canned tuna contains significantly lower mercury concentrations compared to tuna fillets. The size of the processed fish may play a role here, because the accumulation of mercury in fish increases with age.

Of the fish species popular in Austria, trout, carp, char, herring, salmon, and "Alaska pollock," which is often processed into fish sticks, were low in contamination.

According to data from 2016-2022, seafood such as shrimp, squid and mussels were low contaminated.

Exceedances of the maximum levels for methylmercury were found in three tuna samples, two swordfish samples, and one sample each of bluefish, butter mackerel, and butterfish, which is found in certain sushi dishes.

Fish recommendations

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 consuming 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).

Table: Number of servings of selected fish species that could be consumed weekly without exhausting the TWI (serving size: adults: 150g, children: 70 g)

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

Recommendations on fish consumption of the National Nutrition Commission

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 for inorganic mercury is 4 µg/kg body weight (bw) per week and the TWI for methylmercury is 1.3 µg/kg bw per week. A 70 kg person may ingest 91 µg of methylmercury weekly without adverse health effects. A 30 kg child may ingest 39 µg of methylmercury weekly without experiencing health effects.

To assess exposure to methylmercury, the tolerable weekly intake was compared to the calculated intake based on data on fish consumption of the Austrian population (Österreichischer Ernährungsbericht 2017).

Utilization of the daily tolerable intake of mercury in Austria.

At average consumption of fish fillets, adults exhaust 9% and children exhaust 24% of the TWI for methylmercury. With a high consumption of fish fillets, adults exhaust the TWI value by 74% and children by 162%. Thus, the TWI value is exceeded in children. 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, for example, the consumption of 2 portions of tuna (fillet) per week can lead to an exceedance. When canned tuna is consumed, on the other hand, the utilization of the TWI value is significantly lower.