Debate is brewing over safe levels of exposure to mercury and whether new limits should be imposed on mercury emissions from coal-burning power plants. While most industrial uses of mercury are declining, concentrations of methylmercury, a particularly toxic form of mercury, are increasing in the environment and the food chain. Consumption of fish is the major dietary source of methylmercury in humans, and ingesting the mercury from fish even in small concentrations has been shown to lead to slower reflexes, reduced coordination, and poorer vision. A National Wildlife Federation study released in September blamed coal-burning power plants for more than half of the excess mercury in rain in several Midwestern cities. A major study by the National Academy of Sciences on mercury toxicity is due to be completed next June. Congress is requiring the EPA to wait for completion of the study before imposing tougher requirements on power plants.