Table 4.
Unadjusted and Adjusted Changes in IQ for Each Increase in the Blood Lead Concentration of 1 µg per Deciliter for Children with Peak Blood Lead Concentrations below 10 µg per Deciliter.*
| Type of Blood Lead Measurement |
No. of Children |
At 3 Years of Age | At 5 Years of Age | Overall | |||
|---|---|---|---|---|---|---|---|
| β±SE (95% CI) | P Value | β±SE (95% CI) | P Value | β±SE (95% CI) | P Value | ||
| Unadjusted estimate† | |||||||
| Lifetime average | 101 | −2.30±0.67 (−3.64 to −0.96) |
<0.001 | −2.54±0.74 (−4.01 to −1.07) |
<0.001 | −2.42±0.63 (−3.67 to −1.17) |
<0.001 |
| Peak | 101 | −2.09±0.58 (−3.25 to −0.93) |
<0.001 | −2.12±0.60 (−3.32 to −0.91) |
<0.001 | −2.10±0.53 (−3.16 to −1.04) |
<0.001 |
| Concurrent | 101 | −2.19±0.49 (−3.18 to −1.21) |
<0.001 | −2.56±0.58 (−3.71 to −1.40) |
<0.001 | −2.38±0.45 (−3.26 to 1.49) |
<0.001 |
| Average in infancy (6–24 mo) |
105 | −1.29±0.67 (−2.61 to 0.04) |
0.06 | −1.58±0.67 (−2.92 to −0.24) |
0.02 | −1.43±0.61 (−2.65 to −0.21) |
0.02 |
| Adjusted estimate‡ | |||||||
| Lifetime average | 101 | −1.22±0.66 (−2.53 to 0.09) |
0.07 | −1.52±0.71 (−2.94 to −0.09) |
0.04 | −1.37±0.60 (−2.56 to −0.17) |
0.03 |
| Peak | 101 | −1.36±0.55 (−2.46 to −0.27) |
0.02 | −1.44±0.56 (−2.55 to −0.33) |
0.01 | −1.40±0.48 (−2.37 to −0.44) |
0.005 |
| Concurrent | 101 | −1.36±0.51 (−2.37 to −0.35) |
0.009 | −1.79±0.60 (−3.00 to −0.60) |
0.004 | −1.58±0.46 (−2.50 to −0.65) |
0.001 |
| Average in infancy (6–24 mo) |
105 | −0.58±0.58 (−1.75 to 0.59) |
0.32 | −0.92±0.59 (−2.09 to 0.25) |
0.12 | −0.75±0.51 (−1.78 to 0.28) |
0.15 |
The lifetime average blood lead concentration was estimated at the ages of 3 and 5 years by computing the area under the blood lead curve (AUC) from 6 through 36 months and from 6 through 60 months, respectively, and then dividing the AUC by its corresponding age span to yield an average on the microgram-per-deciliter scale. The peak blood lead concentration was the child’s highest measured blood lead concentration through the age of three or five years. The concurrent blood lead concentration was the concentration measured on the day of cognitive testing, and the average blood lead concentration in infancy was the AUC from 6 through 24 months. A total of 71 children were found to have a peak blood lead concentration below 10 µg per deciliter at both ages; an additional 15 children had a peak concentration below 10 µg per deciliter at three years of age but at five years of age had a higher concentration or were not tested, and another 15 children had a peak concentration below 10 µg per deciliter at five years but were not tested at three years. The total number of children in the analysis of the average concentration in infancy is 105 because in 4 children the peak blood lead concentration occurred after the age of 24 months. CI denotes confidence interval. b values are the estimated unstand-ardized regression coefficients.
The unadjusted model includes only classification factors for age and for individual children.
Estimates were adjusted for maternal IQ, race, level of education, use of tobacco during pregnancy, household income, and Home Observation for Measurement of Environment Inventory score, and the child’s sex, birth weight, and iron status.