Nature moves mercury into the atmosphere by various means, such as volcanoes, but human sources are now three times as large as all other natural sources of mercury. This new data has come as something of a shock to scientists, who had thought humans were a rather puny actor on this particular stage. Economic projections by the Washington think tank, Resources for the Future, indicate that humans are likely to double their mercury releases during the next 50 years. Methylmercury is much more toxic than elemental mercury. Scientists now know that mercury becomes methylated when it enters a body of water, especially an acidic body of water, though the exact role of the acid is still unclear.
Big fish taken from acid waters are dangerous to humans already.
And, as other sources of protein become more expensive or come to be regarded as unhealthful, such as red meat, eating fish becomes more popular. Fish are relatively insensitive to mercury, so they can build up a level in their tissues that is toxic to humans and other fish eaters. Mercury mainly attacks the central nervous system, chiefly the brain. In adults, the first symptoms of serious exposure are loss of sensation in the extremities of the hands and around the mouth. The effect of mercury on humans began to be understood during the 1970s.