Adult:Child Differences in the Intraspecies Uncertainty Factor: A Case Study Using Lead

Abstract

Adults and children differ in their susceptibility to the toxic effects of lead. Lead was therefore used as a case study to evaluate intraspecies differences by comparing the adult and child minimal Lowest Observed Adverse Effect Level (LOAEL) or the No Observed Adverse Effect Level (NOAEL), allowing an evaluation of the ten-fold intraspecies uncertainty factor (UF). The lead intakes (in µg/kg/d) necessary to achieve target blood lead (PbB) levels reflecting the minimal LOAEL or NOAEL were determined using biokinetic slope factors (BKSFs), which relate lead uptake to PbB levels. The analyses assumed chronic, low-level oral exposure to lead, and the response of a typical adult and child. Child analyses used a target geometric mean (GM) PbB of 4.6 µg/dL (95% of population <10 µg/dL), resulting in lead intakes of 1.9 µg/kg/day (assuming 100% soluble lead) and 4.9 µg/kg/day (assuming 25% soluble lead and 75 % soil lead). Adult analyses assumed intake of 100 % soluble lead, and used target GM PbB levels of 4.2 µg/dL (95% of population <11.1 µg/dL) and 11.4 µg/dL (95% of population <30 µg/dL), resulting in lead intakes of 1.9 µg/kg/day and 5.1 µg/kg/day, respectively. The results indicate that despite the greater vulnerability of young children to the effects of lead as compared to adults, the minimal LOAEL or NOAEL for lead is remarkably similar between children and adults. In this case, the application of a tenfold intraspecies uncertainty factor to adjust the adult minimal LOAEL or NOAEL for a child would be unnecessary, despite the well-established vulnerability of children to lead.