The health harms of mercury are well known: mercury is a potent neurotoxin that accumulates in animals and the environment. Fetuses and young children are particularly vulnerable as mercury poisons the developing brain, causing delayed development, memory and attention problems, decreased IQ and mental retardation.
Higher doses similarly impair adults and can also increase the risk of heart attacks. It is estimated that up to 100,000 Illinois women have sufficiently high blood-mercury levels to put an unborn child at risk of developmental problems. People get most of their mercury from eating fish—and now tuna sushi, like canned tuna and many local sport fish, has been shown to be contaminated with too much mercury to be eaten safely on a regular basis.
To test your mercury levels for various types of fish go to: http://gotmercury.org/