Fish and shellfish contain a form of organic mercury called methylmercury, which is toxic to mammals. When mercury--usually airborne--gets into the water, bacteria process it into methylmercury, according to the U.S. Geological Survey, or USGS. The bacteria either are consumed or they excrete methylmercury into the water, where it sticks to plankton. Methylmercury makes its way up the food chain. Larger, older fish are the most contaminated because of methylmercury from the other fish they consumed.
Natural Causes
Mercury is a naturally occurring element found mainly in cinnabar ore and is the only common metal that is liquid at room temperature. It enters the atmosphere and the environment through volcanic activity and normal breakdown of rocks and soil by water and weather. Rain or wind transport mercury into oceans, rivers or lakes where it is converted into methylmercury and consumed, is buried in sediment at the bottom, or evaporates back into the air, where the process begins again.
Human Activities
Humans mine the mercuric sulfide of cinnabar ore and vaporize the mercury in it, capturing and cooling the vapors to form the liquid mercury used in thermometers and fluorescent lights, including compact fluorescent lights. Mercury pollution occurs when mercury-containing items are not disposed of properly or undergo processes that allow vapors to escape into the atmosphere. For example, melting down mercury-based light switches from older cars causes significant atmospheric mercury pollution, which ends up in the oceans to be converted to methylmercury, according to the Natural Resources Defense Council. Efforts to recover mercury switches from cars are ongoing, and the National Vehicle Mercury Switch Recovery Program hopes for an eventual 80 percent to 90 percent recovery rate.
Industrialization
The amount of mercury in the environment has increased since the beginning of the industrial age. The U.S. Environmental Protection agency notes that today, the main source of mercury pollution is air emissions from power plants and industrial waste disposal. Generating electricity by burning coal releases mercury into the air, as do gold and mercury mining, and alkali and metal processing. USGS sediment cores in North America show 3 percent to 5 percent higher mercury concentrations than pre-industrial cores. Increased concentrations of mercury in the atmosphere are responsible for contaminating previously uncontaminated areas and species. For example, the USGS cites over a century of increasing mercury concentrations in the feathers of fish-eating birds from the northeastern Atlantic Ocean.
References
- U.S. Geological Survey: Mercury in the Environment
- Natural Resources Defense Council: Mercury Contamination in Fish
- Ecology Center: EcoLink: Auto Mercury-Switch Recovery Reaches Million Marker
- U.S. Environmental Protection Agency: Human Health Criteria: Methylmercury Fish Tissue Criterion
- U.S. Environmental Protection Agency: Fact Sheet: Mercury in Compact Fluorescent Lamps (CFLs)
Member Comments
gotmercury July 1
An excellent tool to gauge how much potential mercury is in the fish you are eating is the free on-line calculator found at www.gotmercury.org