IQ effects of childhood lead exposure persist with age.
Mazumdar, M, DC Bellinger, M Gregas, K Abanilla, J Bacic and HL Needleman. 2011. Low-level environmental lead exposure in childhood and adult intellectual function: a follow-up study. Environmental Health http://dx.doi.org/10.1186/1476-069X-10-24.
Even small decreases in IQ can increase the number of people in society who need extra help in school or who may have difficulty finding work. Although it is well known that lead exposure during childhood can decrease cognitive function in children, the results of this study suggest there are long-term consequences of environmental lead exposure.
Lead exposure is a significant public health hazard because it is related to various developmental problems in children and health effects in adults. Too much causes poisoning and possible death, while low-level exposures are associated with more subtle effects on brain and nerve function.
Children are especially vulerable to any lead exposure because they are at a stage of rapid brain development. Numerous studies show adverse impacts of lead exposure on child cognition – test scores, memory and learning, fine motor skills – and behavior – hyperactivity, aggression. In adults, high blood pressure, joint pain, and memory and concentration problems are linked to chronic lead exposure.
Scientists know that lead exposure during childhood harms the developing brain and nervous system, but it is not clear if childhood lead exposure has long-lasting effects on adult brain function. Changes in adult intelligence quotients – IQs – is a way to measure effects of lead and other neurotoxic substances.
Lead is a highly toxic metal found in many consumer products, including electronics, some toys, jewelry and ceramics. It can be released from industrial processes and garbage incineration. In the United States, its past use as an antiknock agent in gasoline and as an additive in paint led to high exposures. After lead was banned from these products in the 1970s, blood lead levels in children dramatically declined in most populations.
Lead prevention efforts are lauded as a public health triumph. Yet, millions of houses still contain leaded paint, which continues to be a main source of exposure. Many children have blood lead levels that exceed the "level of concern" set in 1991 at 10 micrograms per deciliter of blood (µg/dL) by the Centers for Disease Control and Prevention (CDC).
Currently, there is no lowest threshold identified for adverse effects of lead in children. This means blood lead levels lower than 10 µg/dL can be associated with neurological impacts.
The researchers examined effects of childhood lead exposure on adult IQ by extending a long-term Boston study by another two decades.
They started with data collected from the initial 1980 study that included 249 infants who were followed and checked at 6, 12, 18, 24 and 57 months. At 10 years, 148 of the original participants were checked again. Blood lead levels were determined from blood samples in each of the follow-ups. Demographics, socioeconomic factors, lifestyle and medical history were collected through questionnnaires filled out by the children's parents.
Twenty years later, Mazumbar and colleagues followed up with 43 of the original cohort, who were an average of 29 years old. During the adult follow-up, participants were tested using a standardized adult cognitive function test called the Wechsler Abbreviated Scale of Intelligence (WASI). The test provided a full-scale IQ as well as two components: verbal IQ and performance IQ.
The researchers compared the associations between several indicators of childhood lead exposure – which reflect the blood lead level change during childhood – and the adult IQ scores. The statistical models adjusted for covarites – including sex, race, birth weight, and the mother's education, marital status and alcohol/tobacco use – that might influence the association between the exposure and the adult's IQ score.
Overall, higher lead exposures during childhood was associated with lower IQ as an adult.
The average blood lead levels of the 43 study participants in the adult follow-up reached maximum at two years old (10 µg/dL) and then declined to 6.7 µg/dL at four years of age and then 3 µg/dL at 10 years of age.
Adult full-scale IQ was significantly associated with several indicators of childhood lead exposure, including blood lead concentrations at ages of 6 months, 4 years and 10 years and average blood lead levels from 4 to 10 years.
Among the several indicators, the average blood lead levels from 4 to 10 years had the strongest association with the adult full-scale IQ. For each 1 µg/dL average blood lead levels, the adult full-scale IQ deficit was about two IQ points.
The association between late childhood blood lead levels and adult cognitive function was also evident in verbal and performance IQs.
Adult IQ scores are associated with childhood lead exposures that were measured at or below the CDCs level of concern. The scores declined with higher blood lead levels.
The results strongly support that childhood exposure to lead in the environment has long term consequences on intelligence. Even though the study had a relatively small number of participants, the findings suggest that the impact of lead exposure on child cognitive function may persist – or be irreversible – in adulthood.
This study is unique because it followed a cohort of infants up to 30 years, which is extremely difficult to do, and therefore, was never done before. However, only the long-term follow-up studies like this can provide significant insight into the adverse effects of lead exposure across several decades and life stages.
The results indicate lower adult IQs following higher childhood lead exposure. The lead levels measuring in the older children – 4 to 10 years old – bear the strongest association. This suggests that lead exposure throughout childhood is still important and should be prevented.
Evidence is strong that in the United States, the child blood lead levels peak at about two years of age. Even though lead levels can decline after age two, exposure during preschool and school age should be prevented to minimize long-term effect on adult IQ.
The study population had blood lead levels mostly at or below the CDC's level of concern. However, the results still report adverse effects on adult cognitive function. The study suggests children with blood lead levels below 10 µg/dL can still be at risk for long-term cognitive deficits. Therefore, prevention of any lead exposure is disireable.
In this study, the participants were fairly similar and mostly from families with college-educated parents. Most of the participants had also completed college. Future studies should focus on investigating the effects of lead exposure on adult cognitive function in the general population.
The individual IQ deficit of a couple of IQ points can only be detected by standardized cognitive tests. An individual or physician cannot usually discern this drop in IQ. However, at a societal level, any decline of population IQ scores results in lower intellectual ability, career achievement and possible increased costs for extra help in school and health care. If the source of damage is preventable, efforts should be taken to reduce the adverse effect of environmental exposure.
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Lead and IQ