Low lead levels and mental retardation.

Borderline and mildly retarded children attending the hospital developmental evaluation clinic were divided into two groups on the basis of the presence or absence of a pred with a paediatric control group using blood-lead concentration as the independent variable. Children with a history of diagnosed lead posisoning were excluded from the study. The group of mentally retarded children "aetiology unknown" had statistically significantly raised blood-lead concentrations but the mentally retarded sample with "probable aetiology" showed no significant difference in lead concentrations from those of the normal controls. It is concluded that the association between lead and mental retardation extends over a much wider range than hitherto suspected and that the nature of this association is independent of a history of "encephalopathic" lead poisoning. It is suggested that physicians should consider raised lead levels in their examination of all children suspected of mental retardation and that the numerical definition of lead toxicity should be re-evaluated.