People eating a regular amount (intake) of fish, and buying the most common species available in the shop, have a mercury exposure below the tolerable weekly intake (TWI) for mercury. People eating more than one portion per week of fish species that often contain high concentrations of mercury, or of fish caught in contaminated areas, may have an exposure exceeding the TWI.
People’s exposure to mercury occurs mainly through their diet, where fish and other seafood is the main source of the exposure.
On request from the NFSA, VKM has investigated which mercury concentration in fish that may lead to exceedance of the TWI for mercury, in light of the amounts and type of fish eaten.
In the Terms of Reference, the NFSA states that recreational fishing and the consumption of self-caught fish is common in parts of the population. Hence, VKM has focused on fish caught in coastal areas and in freshwater.
NFSA asked VKM to focus on groups of the population vulnerable to mercury exposure.
Mercury concentrations in fish and TWI
VKM has made reasoned assumptions to specify ‘low’ and ‘high’ mercury concentrations in fish. The concentrations were used in scenarios estimating the exposure at different compositions of fish with ‘low’ and ‘high’ mercury concentration in the diet. ‘Low’ mercury concentration in fish is set at 0.051 mg/kg and ‘high’ is set at 0.33 mg/kg in the estimations. The ‘low’ mercury concentration is a combination of the mean mercury concentrations in Atlantic cod (60%) and farmed Atlantic salmon (40%).
The European Food Safety Authority (EFSA) has established a TWI for methylmercury (the form of mercury present in fish) at 1.3 microgram per kg body weight.
Eating fish with a ‘low’ mercury concentration will not lead to an exposure exceeding the TWI, not even at a high intake of fish, i.e. one kilogram per week.
If the proportion of fish with a ‘high’ mercury concentration increases (1/3 ‘low’ concentration and 2/3 ‘high concentration), the exposure is equal to the TWI if two portions of fish is consumed per week (300 g of fish).
If only fish with a ‘high’ mercury concentration is consumed, the exposure will exceed the TWI when eating one portion of fish per week (150 g).
Pregnant women and their foetus are vulnerable groups
Pregnant women and their foetus are vulnerable to mercury exposure as mercury is harmful to the developing central nervous system.
The mean weekly fish consumption (217 g) in pregnant women participating in the Norwegian birth cohort (MoBa) may lead to mercury exposure exceeding the TWI, if the women only eat fish with a ‘high’ mercury concentration.
Fish species with a high concentration of mercury
VKM has estimated that if three portions of fish is consumed per week, the fish can contain a concentration of mercury at 0.28 mg/kg before the TWI is reached.
Some species of fish can be expected to have mean mercury concentrations higher than 0.28 mg/kg. These species are the marine fish blue ling and tusk, and the freshwater fish burbot, Northern pike and European pike. In addition to the species above, the estimated 95-percentile concentration is higher than 0.28 mg/kg in the marine fish Atlantic cod, common ling, rose fish, European hake and Atlantic halibut, and the freshwater fish brown trout and Arctic charr. For many of the fish species, the mercury concentration increased with the length of the fish.
Mercury exists in organic (methylmercury) and inorganic forms. The forms are usually measured together as total mercury. Almost all (100%) of the total mercury in fish is methylmercury.
VKM received from the NFSA 26 361 measurements of mercury in 36 different fish species from 305 locations. VKM excluded data points from fish not caught in coastal areas, data points from before 2008, and data points for fish species with less than 20 measurements. The final data set used in the report consisted of 8906 measurements of mercury in 21 fish species, which are common to consume in Norway.
The matrix used to estimate the mercury exposure from fish was based on six levels of consumption (from 150 g to 1000 g fish per week) and four different compositions of fish in the diet (varying from a diet consisting of only fish with a ‘low’ mercury concentration to a diet consisting of only fish with a ‘high’ concentration of mercury).
VKM also estimated mercury concentrations in fish leading to an exposure equal to the TWI, given different compositions of fish in the diet and the number of portions of fish eaten.
The project group consists of:
Heidi Amlund, member of the Panel on Contaminants (chair)
Helle Knutsen, member of the Panel on Contaminants
Anders Ruus, member of the Panel on Plant Protection
Jostein Starrfelt, statistician, VKM secretariat
Kirsten Eline Rakkestad, project manager, VKM secretariat
The project group is formed on the basis of relevant expertise and experience with risk assessments.
VKMs Panel on Contaminants is responsible for the report.