Updated June 08, 2015.

A new study from the University of Texas seems to suggest a relationship between industrial mercury and autism. According to an article in Science Daily:

• A newly published study of Texas school district data and industrial mercury-release data, conducted by researchers at The University of Texas Health Science Center at San Antonio, indeed shows a statistically significant link between pounds of industrial-release mercury and increased autism rates. It also shows -- for the first time in scientific literature -- a statistically significant association between autism risk and distance from the mercury source.
  “This is not a definitive study, but just one more that furthers the association between environmental mercury and autism,” said lead author Raymond F. Palmer, PhD, associate professor of family and community medicine at the UT Health Science Center San Antonio. The article is in the journal Health & Place.
  
  Dr. Palmer; Stephen Blanchard, PhD, of Our Lady of the Lake University in San Antonio; and Robert Wood of the UT Health Science Center found that community autism prevalence is reduced by 1 to 2 percent with each 10 miles of distance from the pollution source.
  

This fairly large study looked at statistical evidence from more than 1,000 Texas school districts and compared their distance to mercury-producing plants. Frustratingly, while it does suggest a relationship between the location of the plants and increased levels of autism, it does not provide any conclusive causal connection. This means that, while parents may have legitimate concerns about the environmental impact of plant emissions, the study does not actually prove that environmental mercury causes autism.
In fact, the authors themselves cite several limitations to their study:

• Dr. Palmer and his colleagues pointed out the study did not reflect the true community prevalence rates of autism, because children younger than school age are not counted in the Texas Education Agency data system. The 1:500 autism rates in the study are lower than the 1:150 autism rates in recent reports of the U.S. Centers for Disease Control and Prevention.
  Furthermore, the authors note that distance was not calculated from individual homes to the pollution source, but from central points in school districts that varied widely in area.
  

Neither the article nor the study abstract indicate a margin of error in the study, so it's hard to know, without digging further, whether the differences in autism rates are as significant as the article suggests. Bottom line, though, it can't possibly be healthy for infants to live in close proximity to high levels of neurotoxic emissions. It will be important to keep an eye on this research to see whether it is supported by other similar studies or whether other researchers can actually zero in on a causal connection between mercury emissions and autism levels.
_References:

Palmer RF, Blanchard S, Wood R.]Proximity to point sources of environmental mercury release as a predictor of autism prevalence.Health Place. 2008 Feb 12 [Epub ahead of print].

Palmer RF, Blanchard S, Stein Z, Mandell D, Miller C. Environmental mercury release, special education rates, and autism disorder: an ecological study of Texas. Health Place. 2006 Dec;12(4):749-50; discussion 751-2.