Thimerosal, a preservative once used in many childhood vaccines, may be related to abnormal brain development in children. However, a panel of doctors appointed by the Institute of Medicine recently reported that there is not enough evidence to either prove or disprove a link between thimerosal and developmental disorders. Nevertheless, the group did find that such a link is "biologically plausible."

Thimerosal is a preservative that was developed in the 1930s for use in maintaining the clinical quality of certain vaccines. In the late 1990s, the Food and Drug Administration (FDA) ordered vaccine makers to stop using thimerosal. As a result, all newly manufactured vaccines are thimerosal free. However, government experts point out that many vaccines made prior to the thimerosal ban remain available to doctors. Healthcare professionals recommend that parents request that their pediatricians immunize their children with thimerosal free vaccinations. However, in the unlikely event that the only available vaccine dose does contain thimerosal, experts say the benefits from the vaccination outweigh any risks associated with the preservative.

Once injected into the body, thimerosal breaks down into a type of mercury known as ethylmercury. Many scientists are concerned about mercury's impact on developing children. In fact, there are studies currently underway investigating the relationship between mercury exposure and autism, attention deficit and hyperactivity disorder, and delayed language development.