Federal safety standards for blood lead may be too high to prevent prenatal damage resulting in diminished intelligence later in childhood. According to a study recently accepted for publication in Environmental Health Perspectives, maternal blood lead levels well below the current standard of 10 micrograms per deciliter (ug/dL) during pregnancy had a significant negative impact on children's intelligence up to age 10. Low-level lead exposure may be more harmful to the developing brain before birth than after birth.

Previous research has firmly established that low-level lead exposure during infancy and toddlerhood negatively affects a child's intellectual development. There is less known about the effects of prenatal lead exposure on intelligence, and only a few studies have included measures of both prenatal and postnatal lead exposure. The current study is based on tests of mothers' blood at regular intervals during pregnancy and on tests of their children's blood for 10 years after birth.

Mothers were recruited for the study during their first trimester of pregnancy, and maternal blood lead levels were measured at 12, 20, 28, and 36 weeks of pregnancy and at birth. One hundred fifty children entered the study as healthy, full-term infants born between 1987 and 1992 and remained enrolled until age 10. Umbilical cord blood was tested at birth, and children's blood was tested twice a year from 6 months to 5 years and yearly from 6 to 10 years. Researchers assessed the children's IQ yearly beginning at age 6 and also collected information on factors such as mothers' intelligence and home learning environment that could affect IQ scores.

Data analysis revealed that a mother's blood lead concentration during the third trimester of pregnancy, particularly at 28 weeks, was linked in an inverse dose-response fashion to her child's IQ scores at ages 6 to 10. Mothers' blood lead levels ranged from 1 to 32 ug/dL, with an average level of 8 ug/dL. Half of the decline in children's adjusted intelligence scores occurred within the first few micrograms of the mothers' exposure, with most of the decline observed by the current safety standard of 10 ug/dL.

The third trimester is a period of considerable brain development as nerve cells actively organize along the proper pathways and complete the finer details of the brain's structure. The researchers observe that the fetal brain seems susceptible to lower lead concentrations than those established by government standards and urge further investigation of their findings. "If we continue to accept the current action limit," they write, "we also accept that most of the 'damage' to the IQ of children associated with third trimester lead exposure in our sample is permissible."

The lead author of the study was Lourdes Schnaas of the National Institute of Perinatology in Mexico City. The other authors were Stephen J. Rothenberg, Maria-Fernanda Flores, Sandra Martinez, Carmen Hernandez, Erica Osorio, Silvia Ruiz Velasco, and Estela Perroni. Funding for the study was provided by the Mexican Secretariat of Health's National Council of Science and Technology and the U.S. Environmental Protection Agency. The article is available free of charge at ehp.niehs.nih/docs/2005/8552/abstract.html.

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