Low lead exposures lower child intelligence.

Feb 25, 2008

Jusko TA, CR Henderson, BP Lanphear, DA Cory-Slechta, PJ Parsons and RL Canfield. 2007. Blood lead concentrations less than 10 micrograms per deciliter and child intelligence at 6 years of age. Environmental Health Perspectives 116:243–248.




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Exposure to lead at concentrations below the US safe standard level decreases childhood intelligence, according to a study of children living in Rochester, New York. The findings show a significant decline of 5 intelligence quotient (IQ) points in children with blood lead levels between 5 and 9.9 micrograms per deciliter (mcg/dL), which is lower than the Centers for Disease Control's safe limit of 10 mcg/dL.

A seemingly modest national decline of 5 IQ points could impact society by causing a 57 percent increase in the number of children categorized as mentally deficient (<70 points) and a 40 percent decrease in the number of children categorized as gifted (>130 points), according to the authors.

 

 

Context

Children are most commonly exposed to lead via breathing or eating lead-contaminated paint, dust and soil. Contaminated household paint is a primary source, particularly in homes built prior to the 1977 ban on lead-based paints. Older homes are also more susceptible to lead contamination of drinking water via lead pipes and plumbing solder.

Another big source is the exhaust from industrial smelters and contaminated soil near industrial or high-density traffic areas, primarily due to historical use of leaded gasoline. Lead may be present in old or imported toys; lead-glazed or lead-painted pottery; leaded crystal; and some inks, plasters, hobby and sports materials.

Children are especially vulnerable to lead exposure. They generally are in close contact with items that may be coated with lead-based paints (e.g., toys).

Infants, children under six years of age and pregnant or nursing women are at greatest risk of adverse health effects of lead exposure. Health effects include delays in children's physical and mental development. Adults chronically exposed to lead can suffer kidney damage, reproductive disorders, nerve disorders, memory problems and/or elevated blood pressure.

Levels of lead in young children have been declining steadily over the past 10 years (CDC). However, current data suggest that one child in a hundred has blood lead levels above 10 mcg/dL and one in 10 has a level of 4 mcg/dl or greater

Furthermore, “blood lead levels once considered safe are now considered hazardous, with no known [safe] threshold," according to the Centers for Disease Control.

Jusko et al.'s report is the next step in a study of lead in children that began in the mid 1990's. Originally, 276 children born in three urban hospitals in Rochester, New York, were recruited at 6 months of age and followed for two years. The children were primarily black (60 percent) and low income (71 percent).

The study's first stage determined that dust control alone did not prevent childhood lead exposure. (Lanphear 1999). 

In phase two, the children's blood lead levels continued to be measured and IQs were measured at 3 and 5 years of age. The results showed blood lead levels less than 10 mcg/dL affected IQ, with greater intelligence declines at the lower blood lead levels measured (Canfield et al. 2003).

In this phase of the study, Jusko et al. examined whether lead levels below the CDC-defined safe threshold of 10 would affect intelligence in 6-year-old children. Blood lead levels were measured at 6 years old in the 174 children who still remained in the study. Lead levels of these children were known at 6, 12, 18 and 24 months and 3, 4 and 5 years old.

Lead exposure was assessed through four different blood lead measures: 1) lifetime average level, 2) concurrent level at age of intelligence test (6 yrs), 3) average level at infancy and 4) peak level.

Jusko and colleagues used the Wechsler Preschool and Primary Scale of Intelligence test (a different assessment tool than was used in Canfield), a standard test that includes subtests to assess visual-spatial skills (the performance tests: object assembly, geometric design, block design, mazes and picture completion) and verbal skills (the verbal tests: information, comprehension, arithmetic, vocabulary and similarities). All together, the scores for these tests provide the total IQ score.
 

What did they find?

Measures of blood lead levels (lifetime average, concurrent, infancy and peak levels) were below 10 mcg/dL in 75 percent of the children completing the study. The average IQ score at 6 yrs of age was 85.

Taking into account other variables (maternal IQ, family income, gender, maternal education level, race, parental smoking, birth weight, etc.), the authors determined that all four blood lead measurements were inversely associated with intelligence. That is, the higher the blood lead levels, the lower the IQ score.

Of particular note, children who had a lifetime blood lead average between 5 and 9.9 mcg/dL scored nearly 5 IQ points lower than children with very low levels, < 5 mcg/dL. The decline in IQ was consistent among all calculation methods for blood lead levels.

The negative effects of lead were most pronounced for visual-spatial performance tests. Also of interest is that the biggest drop per unit dose of lead was found below the CDC threshold at the lower exposure levels measured, between 0-5 and 5-10 mcg/dL, rather than at the higher levels measured, between 5-10 and 10+ mcg/dL.


What does it mean?

The results of this study show that blood lead levels lower than the CDC safe threshold of 10 mcg/dL can adversely affect children’s intellectual development. The study design is very strong in that children were followed for several years, blood lead was measured multiple times during the study and several different measures of lead exposure were examined. Consistent results were found between all exposure measures.

Although the observed decrease of 5 IQ points was modest, the societal costs of an overall reduction in intelligence is large in terms of additional education expenses and loss of intellectual contribution to society.

Overall, these data suggest that the safety criteria for lead exposure in children should be re-examined. Increased effort to reduce exposure through education and clean up are necessary and important to prevent further IQ losses.


Resources

Canfield, RL, CR Henderson, Jr., DA Cory-Slechta, C Cox, TA Justko and BP Lamphear. 2003. Intellectual impairment in children with blood lead concentrations below 10 µg per deciliter. The New England Journal of Medicine 348: ;348:1517-26.

Lanphear BP, C Howard, S Eberly, P Auinger, J Kolassa, M Weitzman, SJ Schaffer and K Alexander. 1999. Primary prevention of childhood lead exposure: A randomized trial of dust control. Pediatrics 103(4 Pt 1):772-7.

 Lead Poisoning Prevention Program. US Centers for Disease Control.

Lead Survellance Program, 1997-2006. US Centers for Disease Control.

Lead toxicity. Agency for Toxic Substances Registry.

 National Lead Information Center. US Environmental Protection Agency.

 

 

 

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