Generation Rescue are a non-profit organisation that highlights the link between autism and mercury poisoning.  Mercury is slightly toxic in its liquid state but highly toxic if inhaled or in various compounds. Its known to cause liver and brain damage.
Surprisingly mercury is used in vaccinations given to children mostly in a chemical called Thimerosal which acts as a preservative. A study on mice found that it did have an effect on development but a Danish study found no link between autism and Thimerosal. In fact autism rates increased after Thimerosal was banned,
Mercury is also released by coal firing power stations.  In a study published in Health and Place researchers found that "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."