Mercury in Seafood
Exposure to mercury in its many forms can cause a string of harmful health effects, including ginvigitis, blindness, paralysis, birth defects, and neurological and muscular disorders. The California Office of Environmental Health Hazard Assessment has issued a health advisory for fish caught in the San Francisco Bay, recommending that adults eat no more than two fish per month due to the mercury contamination. Because of their increased susceptibility to toxins and the concentration of mercury in breast milk, children and pregnant women are advised to eat no more than one fish per month.
Mercury in the Bay: Where does it come from?
Most of the mercury pollution in the San Francisco Bay comes from old gold mines operated in the Sierra Nevada foothills in the 19th and 20th centuries. Because of it's unique chemical properties, mercury was used to separate the valuable mineral from the raw ore. Although the technique is no longer used in the U.S. and most of the mines are closed, the mercury that entered the soil and river sediment near these operations still leaches into our waterways, many of which flow into the San Francisco Bay.
Other sources of mercury pollution come from improper management of everyday products like mercury thermometers, fluorescent lights, and mercury-containing switches in automobiles, washing machines, and other equipment. Mercury is also released to the atmosphere by burning of fossil fuels at power plants. Mercury is so toxic to the environment that the small amount contained in a mercury thermometer is enough to contaminate five million gallons of water.
Bioaccumulation: Eating higher up the food chain means higher exposure
Both natural and manmade sources release Inorganic (elemental) mercury into the aquatic environment. where certain anaerobic microorganisms convert some of it into methyl mercury. Although all forms of mercury are hazardous to human health and the environment, methyl mercury is considered the most toxic form because, unlike elemental mercury, it is readily and completely absorbed through skin and in the stomach.
The methylated mercury passes up the food chain as plankton feed on the microorganisms and small fish feed on the plankton. When larger fish feed on the smaller fish, they are also ingesting the methyl mercury built up in each small fish. Finally, when people and other animals eat the larger fish, they are exposed to all of the methyl mercury that has accumulated in each level of the food chain.
This process, known as bioaccumulation, means the animals highest up on the food chain end up with the highest levels of exposure to the methyl mercury. Methyl mercury can be found in both saltwater and freshwater fish.
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