Mercury poses dangerous health risks, especially to children and the unborn fetuses of pregnant women. The essence of what I have copied below (I bolded the important parts) are that the way we can use it as an educational tool is that the greatest threat it poses is to children. That of course will have a large impact in the same way that society views children as victims of tobacco. There is also a website, http://www.gotmercury.org/, which let's you calculate the amount of mercury you're getting. In one can of tuna, a 147 pound person gets 115% of his weekly allotment. (Yes I entered my own weight). Anyway, the site also discusses the excessive levels of mercury in swordfish and canned tuna. Didn't someone just mention something about tuna and greenpeace? Read the stuff below and I'll do more research.
Methyl mercury effects
For fetuses, infants, and children, the primary health effect of methylmercury is impaired neurological development. Methylmercury exposure in the womb, which can result from a mother's consumption of fish and shellfish that contain methylmercury, can adversely affect a baby's growing brain and nervous system. Impacts on cognitive thinking, memory, attention, language, and fine motor and visual spatial skills have been seen in children exposed to methylmercury in the womb. Recent human biological monitoring by the Centers for Disease Control and Prevention in 1999 and 2000 (PDF) (3 pp., 42 KB, About PDF) shows that most people have blood mercury levels below a level associated with possible health effects. More recent data from the CDC support this general finding.
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In addition to the subtle impairments noted above, symptoms of methylmercury poisoning may include; impairment of the peripheral vision; disturbances in sensations ("pins and needles" feelings, usually in the hands, feet, and around the mouth); lack of coordination of movements; impairment of speech, hearing, walking; and muscle weakness. People concerned about their exposure to methylmercury should consult their physician.
From http://www.epa.gov/mercury/effects.htm.\\ 
Other Risks:
The fish may have bacteria, virus, parasites and other disease causing organisms. In most cases there are no regulations to verify the fish catch for such stuff. Sometimes the fish go directly from the fishermen to the consumers without any sort of regulating body. sometimes the fishermen themselves are the consumers which complicates the situations further. It is specially a greater problem if the fish is caught in recreational fishing like sports angling.
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