An investigation of the effects of lead on children’s cognitive abilities.

Abstract

This study explores the relationship between children‘s cognitive abilities and lead (Pb) exposure within the theoretical framework provided by the Cattell-Horn-Carroll (CHC) taxonomy of cognitive abilities. An abundance of research has centered upon the environmental neurotoxicant Pb and the outcomes of severe Pb poisoning (like brain damage and coma) are undisputed. Whilst people in industrialised societies have 500-to-1000 times more Pb in their bodies than their prehistoric ancestors, successful abatement programs have meant that Pb levels in humans are currently their lowest in 50 years. Paradoxically, questions have emerged about the effects of even these low levels of Pb exposure on children‘s cognitive abilities. Indeed, research (Lanphear, Hornung, Khoury, Yolton, Baghurst, Bellinger et al., 2005) has suggested that lower levels of Pb exposure may have a more deleterious impact on children's cognitive abilities than exposure at higher levels. This study investigates the relationship between low-level Pb exposure (mean blood lead (PbB) concentration = 4.97 μg/dL, standard deviation (SD) = 3.52, range = 1.0 – 19.3) and child outcomes in two Australian communities (Port Pirie and Broken Hill) where Pb derived from ore bodies through mining and smelting remains a source of exposure. One hundred and six children (mean age = 7.96 years, SD = 0.59) were assessed using a battery measuring broad factors delineated in CHC theory by supplementing Wechsler IQ (Wechsler Intelligence Scale for Children-Fourth Edition; WISC-IV) scores with measures of CHC abilities (e.g., subtests from the Woodcock Johnson-III Tests of Cognitive Abilities). Information about parental cognitive functioning and a range of potential demographic, familial, psycho-social and environmental and pre- and post-natal variables was also collected. In unadjusted analyses, moderate, inverse significant associations were identified between PbB levels and performance on the WISC-IV and CHC factor scores. The shape of the curve of the association between PbB levels and WISC-IV Full Scale IQ (FSIQ) and the g factor, respectively, was non-linear. In covariate adjusted analyses (controlling for maternal IQ (Wechsler Adult Intelligence Scale-Third Edition), birth weight, Middle child Home Observation for Measurement of the Environment Inventory (MC HOME) scores, number of stressful life events, annual combined family income, smoking during pregnancy and duration of breast-feeding), consistent findings emerged that suggested that low-level Pb may detrimentally impact children‘s speed of information processing capabilities (across the three measurement approaches used in this study: WISC-IV Processing Speed Index, the Speed of Information Processing factor and Gs Invaders). When the PbB terms were added to models of WISC-IV Working Memory Index and the Woodcock Johnson-III Tests of Cognitive Abilities (WJ-III) Long-term storage and retrieval factor, these PbB variables contributed significantly to variance in children‘s memory performance above and beyond the variance already explained by variables considered to impact cognitive development. The variables that consistently explained the most variance in cognitive performance, aside from PbB level, were incidence and duration of breastfeeding and family income level. Higher PbB levels were significantly associated with lower paternal cognitive ability, parental education, combined family income and quality of the home environment, larger family size and later birth order. This research supports the assertion that there is no safe level of paediatric Pb exposure and therefore contributes to the ongoing debate about whether the intervention level for childhood PbB levels should be reconsidered. In addition, this thesis discusses the confounding effects of socio-cultural and environmental factors that influence children‘s cognitive abilities.