Can environmental exposures cause mental retardation?

We don’t have evidence to clearly show that environmental exposures cause MR. Some studies have found that children with MR have higher levels of certain environmental chemicals than children without MR. The significance of these findings are unknown, but they may suggest a link between environmental exposures and MR. [3] Each chemical associated with MR (mercury, lead, cadmium, tobacco smoke, PCBs) will be discussed separately.

Mercury:
Mercury is an element naturally found on the surface of the earth. There are three basic types of mercury that can be found in the environment: elemental mercury, mercury salts, and organic mercury compounds. To learn more about mercury, click here.

Many human studies link high levels of methylmercury exposure with intellectual impairment, including MR. [10-15]

Several human studies suggest that lower levels of methylmercury exposure decrease intellectual function in children. [16-17] However, not all studies support this relationship. [18-19]

Lead:
Lead is a heavy metal naturally distributed in small amounts on the surface of the earth. To learn more about lead, click here.

Two studies show an association between exposure to high levels of lead in drinking water and intellectual impairment, including MR, in children. [8][9]

Many human studies have found that exposure to lower levels of lead produce intellectual impairment as well. [5-7]

Cadmium:
Cadmium is a heavy metal released into the environment from various human activities (burning fossil fuels and medical/city wastes, mining, certain fertilizers, battery disposal, etc.).

Some human studies link cadmium exposure with intellectual impairment, including MR. [20-23]

Several animal studies also give us reason for concern. [24-26]

Tobacco Smoke:
Several human studies have linked parental smoking during pregnancy with decreased intellectual function in children. [35-38] However, the intellectual impairments found are generally mild.

Not all studies support this relationship. [39-40]

PCBs:
PCBs, or polychlorinated biphenyls, are members of a chemical family that were widely used in the past in industry as lubricants, coatings, and insulation materials for electrical equipment like transformers and capacitors. To learn more about PCBs, click here.

Several human studies have linked exposure to higher levels of PCBs with decreased intellectual function. [27-34] However, these intellectual deficits were generally modest.