source exposure, such as contaminants in a municipal water supply; however, individual water consumption is still likely to vary because of the use of well water or bottled water or a nonuniform distribution of contaminants within a geographic area. Ecologic studies do not include information about exposure and outcome within individuals, so they are considered to be the weakest type of observational studies. They are subject to what is referred to as the ecologic fallacy in that relationships observed (or not observed) between exposure and outcomes at the ecologic level may not apply at the individual level. Thus, ecologic studies alone cannot provide direct evidence of causation, although their results can provide supporting data concerning a possible causal relationship.

Acknowledging that ecologic data alone are not sufficient to demonstrate whether or not an association is causal, the committee found that they can provide evidence bearing on possible associations and reached the following conclusions regarding the proposed association of perchlorate exposure with various health end points:

• Congenital hypothyroidism. The available epidemiologic evidence is not consistent with a causal association between perchlorate exposure and congenital hypothyroidism as defined by the authors of the studies reviewed by the committee. All studies of that association were negative.

• Changes in thyroid function in newborns. The available epidemiologic evidence is not consistent with a causal association between exposure during gestation to perchlorate in the drinking water at up to 120 ppb and changes in thyroid hormone and TSH production in normal-birthweight, full-term newborns. Most of the studies show neither significantly lower T₄ production nor significantly higher TSH secretion in infants born in geographic areas in which the water supply had measurable perchlorate concentrations. However, no data are available on the association of perchlorate exposure with thyroid dysfunction in the groups of greatest concern, low-birthweight or preterm newborns, offspring of mothers who had iodide deficiency during gestation, or offspring of hypothyroid mothers.

• Neurodevelopmental outcomes. The epidemiologic evidence is inadequate to determine whether or not there is a causal association between perchlorate exposure and adverse neurodevelopmental outcomes in children. Only one pertinent study has been conducted: an ecologic study that examined the association of perchlorate exposure with autism and attention-deficit-hyperactivity disorder (ADHD). Although the committee considers the inclusion of ADHD plausible, it questions the appropriateness of autism as an end point given that autism has not been observed in the spectrum of