Dietary Intake of Mercury based on Austrian Data of the Years 2007-2015

Mercury is emitted into the environment via natural sources (e.g. evaporation, volcanism), as well as via industrial processes (e.g. mining, waste incineration, coal combustion). Soil- and waterborne mercury can enter the food chain. Mercury has three different forms: as elemental Mercury (Hg0), in inorganic mercury compounds (iHg) and in organic mercury compounds. Elemental mercury is not relevant as a food contaminant. Inorganic mercury may occur as a contaminant in all food categories. Among the organic mercury compounds, methylmercury (MeHg) is the most important food contaminant. Methylmercury only occurs in fish and seafood and has particularly adverse health effects. For this reason, the risk of mercury uptake is frequently discussed in the context of fish consumption.

Inorganic mercury and methylmercury have different toxicological properties and require separate risk assessments. Inorganic mercury is primarily enriched in the kidneys and can, therefore, cause kidney damage. Methylmercury is able to cross the blood-brain barrier, as well as the placenta, and can affect the central nervous system. The prenatal development of the central nervous system is particularly sensitive to methylmercury. The Tolerable Weekly Intake (TWI) is 4 μg per kg body weight per week for inorganic mercury and 1.3 μg per kg body weight per week for methylmercury. The TWI is the amount that can be taken up weekly for a lifetime without an appreciable health risk.

This report presents the occurrence of mercury in foods on the Austrian market. The assessment was based on food-control data from the Austrian Agency for Health and Food safety (AGES) collected in 2007 - 2015. These results were combined with food consumption data from the Austrian Nutrition Report 2012 to calculate the dietary uptake of inorganic mercury and methylmercury in adults and in children of 6-9 years (exposure assessment). The uptake levels calculated were compared to the respective TWI values to assess health risks for Austrian consumers.

Occurrence of Mercury in Food on the Austrian Market

Mercury in food was measured as “total mercury”. Concentrations of inorganic mercury and methylmercury were calculated from total mercury concentrations using standard conversion factors. All the mercury present in foods of terrestrial origin is inorganic mercury. Foods of terrestrial origin comprise plant products and products from terrestrial animals -- i.e. all food categories except fish and seafood. In fish, 80-100 % of the total mercury is methylmercury and, in seafood, it is 50-80 %, while the remaining fraction is inorganic.

Mercury contents were generally low in foods of terrestrial origin. In 93 % of the 3,695 terrestrial food samples analyzed mercury concentrations were below the quantification limit mean values were calculated using both the lower bound (LB) and the upper bound (UB) method, given this large proportion of left-censored data. The LB method underestimates the actual mean value, whereas the UB method overestimates this value. The resulting mean value range for total mercury in foods of terrestrial origin was 1 - 7 μg/kg (LB-UB). From fish and seafood, 1,751 samples were analyzed and the mean value for total mercury was 64 - 66 μg/kg (LB-UB), with pronounced variations between fish species. Top predators in the aquatic food chain with a long lifespan accumulate the highest amounts of mercury.

Inorganic mercury exposure assessment

Exposure to inorganic mercury was calculated assuming that 100 % of the total mercury in terrestrial foods, 20 % of the mercury in fish and 50 % of the mercury in seafood was inorganic. The resulting values for total dietary uptake of inorganic mercury did not exceed the TWI values. In the most conservative scenario (based on the UB), average uptake was 50 % of the TWI for adults and 71 % of the TWI for children. For high consumers of fish and seafood (95th percentile of the consumption data) inorganic mercury uptake was 54 % of the TWI (UB) among adults. Inorganic mercury uptake was 78 % of the TWI (UB) for high consumers of fish and seafood among children.

Methylmercury exposure assessment

Exposure to methylmercury was calculated assuming that 100 % of the total mercury in fish and 80 % of the total mercury in seafood was methylmercury. Austrian adults consumed on average about 111 g of fish and seafood per week, resulting in an average methylmercury uptake of 9 % of the TWI. For children in the age range of 6-9 years, the average fish and seafood consumption was about 124 g per week and the average methylmercury uptake was 18 % of the TWI. High consumers of fish and seafood among adults (95th percentile of the consumption data) consumed about 750 g fish and seafood per week, about 66 % of the TWI. High consumers among children consumed about 570 g fish and seafood per week and had an uptake of 116 % of the TWI for methylmercury.

Methylmercury uptake is highly dependent on the fish species consumed. For example, one standard serving (150 g) of trout per week will expose adults to 3 % of the TWI and children to 8 % of the TWI, based on the Austrian mercury data. In contrast, one standard serving of tuna will expose adults to 30 % of the TWI and children to 71 %.

The Austrian health authorities recommend one serving of regional species (e.g. trout, carp, Salvelinus) and one serving of fat marine fish (salmon, mackerel, herring, tuna) per week. Adults adhering to these recommendations take up 7 - 35 % of the TWI for methylmercury, children 17 - 84 %, depending on the species preferentially consumed.

The average concentrations of mercury in fish and seafood observed in Austria were tendentiously lower than the European averages recently reported by EFSA (European Food Safety Authority). The Austrian data are likely more representative for the regional freshwater species. The European data may more accurately reflect the situation in internationally traded marine fish. Based on the European data, adults take up 48 % and children 112 % of the TWI for methylmercury with one standard serving of tuna per week. Therefore, the above-cited nutritional recommendations should be amended with the advice not to serve children predatory fish such as tuna, swordfish, halibut or pike regularly. Toddlers, pregnant and lactating women, as well as women intending to get pregnant, are advised to avoid such fish species completely. Consumers aware of these limitations can benefit from the positive nutritional effects of regular fish consumption without ingesting critical amounts of methylmercury.