Sources of Lead Exposure/Current Blood Lead Levels
The main sources of lead in children's environments are diet, lead-based paint in older housing, lead in soil and dust from contaminated leaded paint and gasoline, or past and present mining and industrial activity (Mielke 2002; Mielke and Reagan 1998). Exposure from air and waterborne sources has been greatly reduced with the introduction of unleaded gasoline and the replacement of lead water pipes and water tanks with nonlead alternatives. However, lead in soil and dust continues to be a major source of exposure. Indoor floor dust accounts for approximately 50% of a young child's total lead intake [Institute for Environment and Health (IEH) 1998]. Although dust is a major source of lead intake throughout the first 1-2 years of childhood, lead-contaminated window sills in older housing become an increasingly important source of lead as children become mobile and stand upright.
Blood lead levels peak in children at around 2 years of age, and hand-to-mouth behavior and pica (eating substances not normally eaten e.g., soil or paint chips) are significantly associated with elevated blood lead levels (Lanphear et al. 2002). Children typically ingest < 50 mg/day of soil on average (Stanek and Calabrese 1995). However, in the case of pica, this amount can be ≥ 5 g a day (Mielke and Reagan 1998), and some children have ingested 25-60 g during a single day (Calabrese et al. 1997). Indeed, from the point of view of risk assessment, Calabrese and colleagues urge that soil pica be seen "as an expected, although highly variable, activity in a normal population of young children, rather than an unusual activity in a small subset of the population." Soil abatement and paint hazard remediation programs have attempted to reduce children's exposures to lead and other heavy metals, with mixed outcomes (Elias and Gulson 2003; Lanphear et al. 2003).
Children's blood lead concentrations have fallen substantially in a number of countries in the last few decades, including the United States, Australia, Mexico, Germany, Poland, Sweden, and the United Kingdom (Delves et al. 1996; IEH 1998; Jarosinska and Rogan 2003; Meyer et al. 2003a). By 1999 the geometric mean blood lead for U.S. children 1-5 years of age had fallen from 15 µg/dL in the late 1970s to 2.0 µg/dL. A survey of 774 Swedish children over the period 1995-2001 showed blood lead levels had stabilized at 2 µg/dL at 7-11 years of age (Strömberg et al. 2003). In the United Kingdom, blood lead levels of 584 children measured during 1995 in the Avon Longitudinal Study of Pregnancy and Childhood (ALSPAC) showed a geometric mean of 3.44 µg/dL at 2.5 years of age (Golding et al. 1998). Despite these falls in blood lead levels, childhood lead poisoning continues to be a major public health problem for certain groups of children, specifically low-income, urban, African-American children in the United States (Roberts et al. 2001), children suffering from abuse and neglect (Chung et al. 2001), children living in rural mining communities (Lynch et al. 2000), and children in developing countries (Falk 2003; Fewtrell et al. 2004).
Lowering of exposure guideline levels reflects concern over the growing body of evidence that low levels of lead exposure have subtle effects on the nervous system of children. Since 1971 there have been four reductions in the CDC guideline level above which children are considered to have an elevated lead level. This level currently stands at 10 µg/dL (0.483 µmol/L). In 1997 the CDC estimated that 4.4% of children in the United States 1-5 years of age have blood lead levels ≥ 10 µg/dL (Lynch et al. 2000). In a recent report of blood lead levels in children 6 months to 5 years of age living in New Orleans, Louisiana, USA, 29% had levels ≥ 10 µg/dL (Rabito et al. 2003). In Wuxi City, China, 27% of children 1-5 years of age had blood lead levels > 10 µg/dL (Gao et al. 2001), whereas in Johannesburg, South Africa, the blood lead levels of 78% of schoolchildren ≥ 10 µg/dL (Mathee et al. 2002) and in Dhaka, Bangladesh, 87% of children 4-12 years of age had blood lead levels > 10 µg/dL (Kaiser et al. 2001). In the United Kingdom, large-scale blood lead monitoring programs ceased in the late 1980s, and there is a paucity of recent data on blood lead levels in young children. The proportion of children with blood lead levels > 10 µg/dL ranged from 0.74 to 5% according to recent reports from three different regions of England (IEH 1998; Lewendon et al. 2001; O'Donohoe et al. 1998), and there is growing concern that significant numbers of children under 5 years of age remain at risk from lead exposure in the United Kingdom (Grigg 2004).
Environ Health Perspect. 2004;112(9) © 2004 National Institute of Environmental Health Sciences
Cite this: Recent Developments in Low-Level Lead Exposure and Intellectual Impairment in Children - Medscape - Jun 01, 2004.