Fluoride

Multiple studies conclude that fluoride exposure in children will (most likely permanently) decrease their intelligence. Fluoride appears to be as big a factor as lead in IQ suppression.
While evaluating this research, keep in mind that the CDC recommends water fluoridation at a level of 0.7 mg/L. Before 2011, the U.S. Public Health Service would recommend levels as high as 1.2 mg/L.
But exposure also comes from other sources, such as dental treatments, toothpaste, pesticides, and infant formula. Besides lower intelligence, higher levels of fluoride can also cause tooth discoloration, bone and joint pain and deformities.
This graph compares the effects of fluoride and lead on IQ:

Fluoride also crosses the placenta, which has implications for unborn children. A study from Canada of pregnant women who lived in areas where fluoride was added to the drinking water found significantly lower IQ scores in their children when they reached 3 to 4 years of age, compared to those living in areas with non-fluoridated tap water.
- Government Report: Multiple Studies Show Fluoridation Can Lower Child IQ
- Proposed HHS Recommendation for Fluoride Concentration in Drinking Water (2011)
- Association Between Maternal Fluoride Exposure During Pregnancy and IQ Scores in Offspring in Canada
- Top 5 Ways to Reduce Fluoride Exposure from Infant Formula