Tuna is one of the most loved foods on tables around the world, but it is often associated with the presence of mercury, a heavy metal potentially dangerous for human health. But why does mercury accumulate in this fish? Where does it come from? And what are the implications for our diet?
Where does mercury come from?
Mercury is a natural element found in the earth’s crust, however, human activities have significantly increased its concentration in ecosystems, especially in the seas. Major sources of mercury include industrial emissions, such as coal-fired power plants, mining processes, and metal smelting; mining activities: such as gold extraction, which uses mercury compounds. The incineration of waste, especially electronic waste.
According to a report from United Nations Environment Program (UNEP)approximately 2,000 tons of mercury are released into the environment every year due to human activities. Once emitted into the atmosphere, mercury can settle in oceans and freshwater, thus entering the marine food chain.
The most mercury polluted seas and the case of the Mediterranean
Mercury is concentrated in the seas all over the world, but not uniformly, some areas are particularly affected, such as the Mediterranean, which stands out for contamination levels higher than the global average. This peculiarity is linked to geographical, historical and industrial factors.
The Mediterranean is a semi-enclosed sea, with limited water exchange through the Strait of Gibraltar. This characteristic makes it vulnerable to the accumulation of pollutants. Mercury, released mainly by industrial activities along its densely populated coasts, settles in marine sediments and spreads through the food chain. Historically industrialized countries such as Italy, Spain and Turkey have contributed to this contamination with mining, coal-fired power plants and unregulated waste.
Furthermore, the Mediterranean has a history of intensive exploitation of its resources. Mining activities, such as the extraction of cinnabar, a mineral from which mercury is obtained, have been common along its coasts, particularly in the Iberian Peninsula. The residues of these activities continue to flow into the sea, worsening the situation. Added to this are discharges from polluted rivers, such as the Po and the Nile, which bring large quantities of mercury accumulated in river basins into the Mediterranean.
The Mediterranean climate further contributes to transforming mercury into methylmercury, the most toxic and bioaccumulative form. High temperatures and a high concentration of microorganisms in the water favor this chemical process, amplifying the impact on marine fauna and, consequently, on human health.
Why does mercury accumulate in fish?
Mercury released into water turns into an organic form called methylmercuryhighly toxic and easily absorbed by living organisms. This compound enters the food chain through:
- Plankton and microorganisms: which absorb methylmercury from water.
- Small fish: which feed on plankton and accumulate mercury in their tissues.
- Largest marine predators: like tuna, which eat smaller fish.
This process, known as bioaccumulationcauses mercury levels to increase as you move up the food chain. Tuna, being a large and long-lived predator, accumulates significant quantities of mercury in its tissues.
How much mercury is in tuna?
The mercury content in tuna varies depending on the species and size of the fish. Larger tuna species, such as bluefin tuna and albacore tuna, tend to contain higher levels of mercury than smaller species.
A report fromEFSA (European Food Safety Authority) highlights that average mercury concentrations in tuna can reach 0.5–1.5 milligrams per kilo, exceeding safe limits for regular consumption, especially for pregnant women and children.
One of Bloom’s latest investigations into mercury in tuna cans analyzed 148 samples in European countries such as Italy, France, Germany and Spain, and found that 100% of the samples were contaminated with mercury. Furthermore, 57% of the cans exceeded the 0.3 mg/kg limit applied to other fish species, with a record peak of 3.9 mg/kg found in a Petit Navire brand package purchased in Paris.
What are the health risks?
Mercury, particularly in its methylmercury form, poses a serious risk to human health. Its toxicity mainly affects the nervous system, with particularly serious effects during the neurological development phase in fetuses and children, making it an element of great concern for pregnant women. Furthermore, mercury tends to accumulate in the kidneys and liver, progressively damaging these vital organs. Recent studies have also highlighted a possible connection between exposure to this heavy metal and the onset of cardiovascular diseases.
The World Health Organization (WHO) highlights the importance of reducing the consumption of fish high in mercury. Opting for low-content fish species, such as sardines, mackerel or salmon, is a safer and more responsible choice to minimize exposure and protect health, especially in the most vulnerable categories.
For those who love tuna, the advice is to vary their consumption by choosing smaller species or products from companies that carry out checks on heavy metals. The mercury problem in tuna is not just a personal health issue, but an indicator of the environmental consequences of human activities. Reducing mercury emissions globally is critical to protecting not only the health of consumers, but also that of marine ecosystems.
