Mental retardation and "nontoxic" lead levels.

The authors studied the blood lead concentrations of children with IQs of 55-84, divided according to the presence (N = 48) or absence (N = 35) of a probable etiology for their retardation, and of 40 control children. The mean lead level of the retarded children with unknown etiologies was 25.03 micrograms/dl, which was significantly higher than those of the other two groups; 54% of the group with unknown etiologies had levels above 25 micrograms/dl, compared with 15% of the retarded children with probable etiologies and 17% of the control children. In the group of retarded children with unknown etiologies there was a significant negative correlation between lead levels and IQ.