One of the world's leading medical journals, The Lancet Neurology, has just published a study linking autism, ADHD, and dyslexia with many of the chemicals found in our world. (You can read about the study here.) The guilty chemicals - lead, methylmercury, polychlorinated biphenyls, arsenic, toluene, manganese, fluoride, chlorpyrifos, dischlorodiphenyltrichloroethane, tetrachloroethylene, and polybrominated diphenly ethers - are often found in our food and water.
Even though some of these chemicals are banned in the US, that doesn't mean we aren't exposed to them. Some of the chemicals that were used in the past still persist in our soil. Also, we import our food from some countries that continue to use those dangerous chemicals.
Here's an eye-opening infographic on the dangers of pesticides (which are just some of the chemicals we need to avoid):