In a rare showing of interdepartmental cooperation, the Environmental Protection Agency (EPA) and the Food and Drug Administration (FDA) have issued guidelines for seafood consumption in order to minimize the risks of mercury exposure. A plethora of studies have shown that nearly all fish and shellfish species are contaminated with mercury to some degree, which can be harmful to fetal development and lead to neurological problems in adults.
The most at risk individuals are women who are pregnant or breast-feeding, children under the age of 10, and women of childbearing age (15-44 years old) who could become pregnant.
According to EPA guidelines, fish with mercury levels <0.5 ppm are safe for all individuals to consume. Those with levels between 0.5 and 1.5 ppm should be limited to once a week for adults and once a month for at risk individuals. All groups should avoid fish with mercury contamination above 1.5 ppm.
Sharks, king mackerel, tilefish, and swordfish are consistently the most contaminated species and should be avoided by all individuals. Safer options include pollock, haddock, cod, catfish, and salmon, all of which have levels well below the 0.5 ppm threshold.
When mercury is consumed, it is absorbed by the gastrointestinal tract and quickly enters the bloodstream. From there, it is transported throughout the body and can take months to fully remove. Symptoms of mercury poisoning include impaired memory and cognition, parasthesia (pins and needles feeling in the extremities), tunnel vision, deafness, immune system suppression, and in extremely rare cases, death. All of these symptoms are a result of neuron degeneration.
Mercury in the blood of pregnant women can easily pass to their developing fetus and impact the child’s brain development. After birth, mercury can be passed on through contaminated breast milk.
Mercury poisoning is generally considered a chronic threat with tissue buildup over years. However, there is also the possibility of neurological symptom onset after consuming extremely contaminated fish. Mercury can remain in the human blood stream for months, infecting tissues and leading to neurological problems. While the concentration needed to impact adults is relatively high, mercury can cross the placental barrier and there is a strong correlation of fish consumption during pregnancy and mental retardation in newborns. Last year, over 300,000 infants were exposed to unsafe levels of mercury in utero.
Popular fish species such as tuna and swordfish account for over half of the mercury consumption of the US population. Tuna are highly migratory, large, and long lived fish. All of these characteristics result in the accumulation of mercury in their muscle tissue and livers. In general, bigeye tuna have mercury concentrations nearly twice that of most other species, partly due to their vast migratory patterns and long lifespan. Therefore, they have a longer period of time to accumulate mercury in their tissues.
Mercury concentrations of fish mirror the water chemistry of the area. The North Atlantic has the highest levels of any ocean and many studies have shown that fish caught in this region also tend to have correspondingly high contamination rates. As 90% of mercury exposure occurs through a tuna’s diet, larger individuals are more likely to be highly contaminated. Fresh tuna steaks generally come from these large individuals and are therefore more highly contaminated than light canned tuna.
The popularization of sushi has put an even greater number of Americans at risk. Eating as little as six pieces of sushi a week may expose diners to more mercury than the EPA recommends.
The EPA reminds Americans that consuming fish is integral for maintaining a healthy diet, as they are an especially good source of omega-3 fatty acids. These omega-3 fatty acids are critical to a developing fetus’ future motor and cognitive skills, as well as preventing heart disease and stroke in adults. For pregnant women who want to receive the full benefits of omega-3 fatty acids without potentially exposing themselves or their child to mercury, fish oil capsules have been found to be a safe alternative.
In addition to contaminating fish, small concentrations of mercury are found in the air. Between 10-20 nanograms of mercury per cubic meter is found in urban air, 100 times lower than what is deemed safe by the Center for Disease Control (CDC). Air contaminated with unsafe levels of mercury is generally found around hazardous waste sites or fossil fuel power plants.
Certain occupations also put individuals at greater risk of exposure. Workers at chemical processing plants, manufacturers of automotive parts, metal processing plants, and dentists that perform amalgam fillings (which are 50% metallic mercury) are more likely to breathe in toxic mercury fumes.
While some mercury is the environment is natural, nearly 2/3 is released into the environment through many facets of industrial life. This includes paper and wood processing, iron, steel, and chemical manufacture, the burning of fossil fuels, and gold mining operations. Mercury in the atmosphere can be transported globally before it is finally rained out. The contaminated water permeates the soil, tainting not only terrestrial ecosystems but also groundwater and ocean sediments. In water, it settles out in the highly toxic form of methylmercury which is absorbed by bacteria before making its way up the food chain, eventually to humans.
The problem of mercury-contaminated food and air is unlikely to go away any time soon. Over 200,000 tons of mercury have been released into the environment since 1890, with an additional 158 tons produced annually in the US. It is estimated that even if all mercury production were eliminated, it would take more than 50 years to return to pre-industrial levels. Currently, levels are increasing 1.2-1.5% annually.