Facts: Unlike some toxic substances whose hazards are known only from tests done on animals, the hazards of methylmercury have well been documented in studies of human populations exposed to mercury in their diets, from eating fish (and sometimes also whale meat). We know for a fact that mercury causes health damage in people who are exposed to too much of it. What is known with less certainty is how much mercury is “too much.”
The scientific evidence that establishes the reality of risk from methylmercury in fish, has been reviewed repeatedly by authoritative bodies such as the National Research Council/Institute of Medicine of the National Academy of Sciences, and the World Health Organization of the United Nations. There is no doubt at all that mercury risk is real. The questions that require closer analysis are, how large is the risk, and who is at risk? Both are discussed elsewhere on this web site.