The effects of lead levels on the growth of word recognition in middle childhood.

The relationship between dentine lead levels and the development of word recognition abilities from 8 to 12 years of age was studied in a sample of 636 children drawn from a birth cohort of 1265 New Zealand children. Data were analysed by growth curve modelling methods to characterize the relationship between early lead levels and subsequent development of word recognition abilities. The results of this analysis showed: i) Before adjustment for confounding and other factors children who had lead levels of > or V 8 p.p.m. had mean test scores which were consistently 5 points lower than children with levels of 0-3 p.p.m. There was no evidence to suggest either catch-up or deterioration in the performance of those with levels of > or = 8 p.p.m. ii) After adjustment for both confounding covariates and potential sample selection bias these differences reduced to about 3 points between those with levels of > or = 8 p.p.m. and those with levels of 0-3 p.p.m. These differences, however, remained statistically significant (P < 0.05) or marginally significant (P < 0.10). It was estimated that the 3-point difference in word recognition skills amounted to approximately 4-6 months' delay in the word recognition abilities of those with mildly elevated lead levels. The results of this study are generally consistent with the view that early exposure to lead results in very small, statistically detectable but apparently enduring deficits in cognitive abilities.