Released naturally and by anthropogenic activity, mercury is found in the environment in three forms: elemental mercury, inorganic mercury and organic mercury. Highly toxic when ingested by humans, safe consumption levels have been established around the globe to protect consumers, and to guide and regulate the food industry. Exposure to methylmercury occurs primarily through the diet, with fish and fish products as the dominant source.
Inorganic mercury occurs naturally in the environment and as result of human industries, and is toxic to humans. Mercury is released into the atmosphere through fossil fuel combustion, volcanic activity, the weathering of rocks, mining and the manufacture of cement and pesticides.
What Is Methylmercury And How Is It Formed?
Methylmercury is created when inorganic mercury evaporates into the atmosphere from rock and soil before falling back to earth as rain and being washed into rivers, lakes and oceans. In inorganic form mercury sinks to riverbeds and to lake and ocean floors. In the aquatic environment, inorganic mercury is converted to methylmercury by microorganisms living in sediment, by a process call methylation. The mercury is absorbed by phytoplankton species, which are eaten by plankton consumers. These are then eaten by ever-larger fish, and thereby enter the food chain.
Mercury Accumulation In Seafood
Methylmercury is commonly found in fish, both freshwater and saltwater. The accumulation of most metals (except mercury) in fish and seafood is usually inversely related to an organism’s age and size. However, mercury and methylmercury concentrations in seafood increase with the organism’s age. Unlike other heavy metals, mercury accumulates in the tissue of fish and shellfish, particularly muscle tissue. Biomagnification, the accumulation of mercury as one organism consumes more and more organisms that contain mercury, means that the higher up the food chain fish and seafood are, the higher the concentration of mercury in their tissue. For example, sardines contain about 0.01 ppm of mercury, while sharks contain from 1 ppm to 4 ppm (EPA, 2006).
This principle applies equally to saltwater and freshwater fish.
Healthy Balanced Diet
In moderation, fish and seafood should form part of a healthy balanced diet, being healthy, nutritious and an excellent source of protein. Understanding how mercury and methylmercury enters the environment, how it accumulates in fish and seafood and its impact on human health, means educated decisions can be made about the products delivered to and eaten by consumers.
Harmful to human health
Mercury, in both its organic and inorganic forms, is highly toxic to humans. The human body however does not absorb inorganic mercury forms, as it does organic mercury. Methylmercury in fish and seafood is easily absorbed on its journey along the digestive tract. It can cause health issues including:
- Impairment of the peripheral vision.
- Disturbances in sensations (“pins and needles”, usually in the hands, feet, and around the mouth).
- Lack of coordination of movements.
- Impairment of speech, hearing, walking.
- Muscle weakness.
Sources of past exposure to methyl mercury include fungicide-treated grains and meat from animals fed such grain.
The presence of mercury and methylmercury in fish and seafood can be detected using high performance liquid chromatography (HPLC) coupled with inductively coupled plasma-mass spectrometry (ICP-MS). With this LCICP/MS method, inorganic and organic mercury are separated on a phenomenex synergi hydro RP C18 column after extraction of both forms with a L-cysteine solution. After separation they are quantified by means of ICP/ MS, which is the preferential detector for metals. The limit of quantifications (LOQ) of 0.05 mg/kg for mercury and methylmercury with this methodology are obtained.
Methylmercury in extraction solution decomposes over time. To ensure accurate quantification of methylmercury, extracts must be analysed within 8 hours of preparation.
Many government agencies, including the United States Environmental Protection Agency (EPA), the United States Food and Drug Administration (FDA), Health Canada, and the European Union Health and Consumer Protection Directorate-General, as well as the World Health Organization (WHO) and the United Nations Food and Agriculture Organisation (FAO), have issued guidance that is designed to limit mercury exposure from fish consumption and increasingly discriminates between the organic and inorganic forms (see box below).
Guidance: Mercury Exposure Form Fish Consumption
(fresh/saltwater, finfish, crustaceans, molluscs)
|Methylmercury||1.0 ppm.2 |
NOTE: An older FDA requirement had a 0.5 ppm maximum for specific fish, such as tuna. This requirement is still listed in some US State regulations.
Retail fish (with exceptions)
Exceptions: fresh/frozen tuna, shark, swordfish, escolar, marlin, and orange roughy
(inorganic and organic)
|Oceania (Australia & New Zealand)||
Crustacea, fish and molluscs:
Gemfish, billfish (including marlin), southern bluefin tuna, barramundi, ling, orange roughy, rays and all species of shark, and Fish for which insufficient samples are available to analyse in accordance with clause 6
Mean 0.5 mg/kg
Mean 1.0 mg/kg
|Taiwan||Seafood||Methylmercury||0.5-2 ppm |
NOTE: MRL varies by seafood variety.
|Europe||Fishery products and muscle meat of fish, except specific exclusions*
|Mercury||0.5 mg/kg 1.0 mg/kg|
For further information on SGS please visit our website at: www.foodsafety.sgs.com.
Ron Wacker, PhD
Global Food Testing Business
t: +49 40 301 012 65