A study in the journal Health and Place documents a correlation between mercury emissions from coal plants and autism rates in 1200 Texas school districts. According to the abstract,
There was a significant increase in the rates of special education students and autism rates associated with increases in environmentally released mercury. On average, for each 1000 lb of environmentally released mercury, there was a 43% increase in the rate of special education services and a 61% increase in the rate of autism. The association between environmentally released mercury and special education rates were fully mediated by increased autism rates.
No one has been able to figure out what is happening with autism. Some studies have suggested the incidence of autism has risen by a factor of 10 in recent decades, while other argue it can be explained by increased diagnosis. It will be a long while before we really understand what is happening and why.