Toxic mercury in fish

As early as 1500 BC, the metal substance called mercury has been found in Egyptian tombs. Used in the 18th century, mercury was an ingredient in the anti-syphilitic agents, and the following century, the phrase “mad as a hatter” was used to describe those sickened by mercury poisoning in the hat-making industry.

In the 1940s and 1950s, products with mercury was found to be the cause of a disease called acrodynia, also known as pink disease. Exposures to mercury was noted in Minamata Bay in Japan in the 1960s; methyl-mercury treated grain in Iraq and contaminated fish in Canada in the 1969-70. A cosmetic cream from Mexico, called “Crema de Belleza-Manning,” sold in the United States in 1996 was found to have mercury.Mercury was an essential ingredient in many medicines for centuries, but not today. The use of fossil fuels and agricultural products containing mercury poses a major concern about environmental pollution. Methyl-mercury has a biological half-life of approximately 65 days.

Where else is mercury found?
This substance is found in thermometers, barometers, batteries, blood pressure manometers, dental amalgams and antiseptic agents, in anti-fungus medications for agricultural industry. Before 1990, mercury was also used in paints, to prevent mildew. Today, larger fish, like Tuna (fresh or frozen, like Albacore), Swordfish, Shark and Tilefish fishes, might contain higher levels of methyl-mercury. For fresh water fish, bass, pike, muskellunge, and walleye  are considered to have high levels of mercury too.

How prevalent is mercury toxicity?
The American Association of Poison Control Centers’ Toxic Exposure Surveillance System, in 2003, had documented 3362 exposures to mercury, 569 of them in children younger than 6 years of age, and 1705 in individuals older than 19.

What sources are the major concerns?
The three well-publicized areas of mercury exposure concerns include fish consumption, especially in pregnant women and children, dental fillings or amalgams, and vaccines. According to the Public Health Service “dental amalgams do not pose a serious health risk,” and that vaccines are safe.

How about fish consumption?
Eating mercury contaminated fish is the primary source of mercury toxicity. Since the fetal brain is more sensitive to mercury exposure, the Environmental Protection Agency (EPA) “reduced the allowable intake of methylmercury from 0.5 mcg to 0.1 mcg of mercury per kilogram per day, which is lower than the amount allowable according to other regulatory agencies. The Food and Drug Administration (FDA) has “recommended that pregnant women, breastfeeding mothers, and young children avoid eating fish with a high mercury content (>1 ppm), such as shark, swordfish, tilefish, and king mackerel… This also includes fresh and frozen tuna (mercury content between 0.5 ppm and 1.5 ppm) but not canned tuna, which consists of smaller, shorter-lived species with lower mercury levels.”
The above stated recommendation translates into consumption of one can (198 g or 7 oz) of tuna for an adult per week, and not more. For children and women of child-bearing age, only one can per month.

What about thimerosal and autism?
Thimerosal, which contains mercury, is used as a preservative in vaccines. There used to be fear that it caused autism in children. After extensive studies, and independent investigations, including those from other countries, the Public Health Service agencies reported “no harmful effects have been reported from thimerosal at doses used in vaccines,” and the published official conclusion was that “the evidence favors rejection of a causal relationship between thimerosal-containing vaccines and autism.”

How do fish get contaminated with mercury?
The power plants that burn fossil fuel, particularly coal, is the primary source of the mercury. It accumulates in streams, rivers, oceans, and with the aid of bacteria, it is chemically transformed into methyl-mercury, which is highly toxic. As the fish feed on aquatic organisms, it absorbs methyl-mercury from the water. Larger fish eat smaller fish, live longer, and so they contain higher level of methy-mercury.

Does cooking reduce the mercury?
No, cooking does not get rid of the methyl-mercury. The FDA regulates commercially sold fish and seafood. The EPA monitors the concentration of methyl-mercury in the environment and regulates industrial releases of mercury to surface water and air. The recommendation from the National Academy of Science states, “Because of the beneficial effects of fish consumption, the long-term goal needs to be a reduction in the concentration of mercury in fish, rather than the replacement of fish in the diet by other less healthier food… In the interim, the best method of maintaining fish consumption and minimizing mercury exposure is the consumption of fish known to have lower methyl-mercury concentrations,” meaning the smaller fish.

How about canned tuna?
One of the most popular fish is tuna. Canned tuna are usually the smaller fish, rather than the larger tuna seen in the market. The recommendation is to choose “chunk light” or “chunk” canned tuna since they contain less mercury, compared to canned tuna labeled as “solid white” or “chunk white,” like albacore tuna, which children and pregnant or lactating mothers should stay away from.

Which fish have low levels of mercury?
The commercially available fish/seafood considered to have lower level of mercury besides canned “chunk light” or “chunk” tuna: catfish, cod, flounder-sole, haddock, herring, mahi-mahi, ocean perch, rainbow trout, tilapia, farmed trout, farmed salmon, wild salmon, sardines lobster, crab, scallops, shrimp, and oysters.

-Philippine News