Mercury is a naturally occurring element that is present throughout the environment. It becomes a toxic air pollutant when released into the air, water and soil by human activity.
Reactive, inorganic mercury is emitted to the atmosphere primarily from coal-burning power plants and incinerators that combust mercury-containing wastes.
Air currents and rainfall convey this mercury from the atmosphere to the earth's surface. Some of the deposited mercury ends up in wetlands, lakes, and streams where bacteria convert a portion of it into methylmercury, a toxic form that builds up (bioaccumulates) in the tissues of animals at each link in the food chain.
Mercury may accumulate in sport fish to levels that would be toxic if eaten by humans over a prolonged period of time or by wildlife that prey upon those fish. This effect occurs worldwide but is particularly acute in the marshes of the Florida Everglades. There, largemouth bass, which feed primarily on other fish in the food chain, have been found with five times the level of mercury considered safe for human consumption, and wading birds are ingesting amounts of mercury close to levels that could reduce their populations.
Human beings are believed to have some tolerance for mercury. Based on this, the Florida Department of Health has established the following guidelines. Fish that have more than 1.5 parts per million of mercury in the edible flesh are considered unsafe for any consumption. Those containing less than 0.5 parts per million are considered safe for unlimited consumption. Consumption should be limited for fish with concentrations from 0.5 to 1.5 parts per million of mercury in edible flesh. Women of childbearing age and children should limit consumption of these fish to a single serving per month. Other adults should limit consumption of these fish to a single serving per week. These values are based on a body weight of 156 pounds and an 8-ounce (half-pound) serving of fish. If a person weighs less, it would be safer to consume less. A conservative approach for eating largemouth bass from untested waters would be to follow the advice given for limited consumption - one serving per month for women of childbearing age and children, and one serving per week for other adults.
Last updated: September 12, 2013