Temporal pattern in the effect of postnatal blood lead level on intellectual development of young children

To determine the temporal pattern of the effect of postnatal blood lead lev el on the General Cognitive Index (GCI) of the McCarthy Scales of Children' s Abilities, we used data from 112 children of the Mexico City Prospective Lead Study with complete evaluations from 36 to 60 months of age at 6-month intervals. We measured blood lead level every 6 months from 6 to 54 months . We controlled for 5-min Apgar, birth weight, birth order, sex, socioecono mic level, maternal IQ, and maximum maternal educational level in a repeate d measures ANCOVA using child blood lead level grouped by 6-18 month (geome tric mean 10.1 mug/dl, range 3.5-37.0 mug/dl), 24-36 month (geometric mean 9.7 mug/dl, range 3.0-42.7 mug/dl), and 42-54 month (geometric mean 8.4 mug /dl, range 2.5-44.8 mug/dl) averages. There were significant interactions b etween the 6-18 month blood lead level and age with GCI as the endpoint and between 24-36 month blood lead level and age. The regression coefficient o f blood lead at 6-18 months became more negative with age until 48 months, when the rate of decline moderated (linear polynomial contrast p = 0.047). The regression coefficient of blood lead at 24-36 months with CGI became mo re negative as well from 36 to 38 months but then started decreasing toward zero from 48 to 60 months (quadratic polynomial contrast p = 0.019). Signi ficant between-subjects lead effects on GCI were found for 24-36 month bloo d lead level at 48 months (p = 0.021) and at 54 months (p = 0.073). The gre atest effect (at 48 months) was a 5.8-point GCI decrease with each natural log unit increase in blood lead. Significant between subjects lead effects on GCI were found for 42-54 month blood lead level at 54 months (p = 0.040) and at 60 months (p = 0.060). The effect of postnatal blood lead level on GCI reaches its maximum approximately 1-3 years later, and then becomes les s evident. Four to five years of age appears to be a critical period for th e manifestation of the earlier postnatal blood lead level effects. (C) 2000 Elsevier Science Inc. All rights reserved.