DDT is currently banned in the United States, but officials in Africa are considering expanding its use to combat mosquitoes that spread malaria.
The new findings -- along with the potential benefits of DDT use to reduce malaria -- "need to be considered by the policymakers," said study author Brenda Eskenazi, a professor of epidemiology and maternal and child health at the University of California, Berkeley.
Environmentalists have long expressed concern that DDT is toxic to humans, and its use has been limited or banned across the world. In the United States, it's been banned since 1972.
While it does appears to be hazardous to some animals, DDT's health effects on humans are still being studied, said Dr. Walter Rogan, senior investigator in the National Institute of Environmental Health Sciences' epidemiology branch. It's not clear, he said, that DDT has ever killed anyone.
Rogan's agency helped pay for the study, but he did not participate in it.
The study is the first to look at the direct effects of DDT on infant development rather than the effects of DDT's byproducts.
The researchers examined blood levels of DDT and one of the breakdown products -- known as DDE -- in 360 pregnant women from California's Central Valley who are participating in a long-term UC Berkeley project called the Center for the Health Assessment of Mothers and Children of Salinas (CHAMACOS).
The project is designed to examine the effects of pesticides and other environmental factors on the health of pregnant Latina women and their children living in what the researchers call "one of the most intensely farmed regions in the world."
Ninety percent of the women in the study were born in Mexico, where DDT was widely used in agriculture during the 1970s, then used to control mosquitoes until 1995. In 2000, a complete ban went into effect.
The researchers tested the mental and physical skills of the women's infants at 6, 12 and 24 months of age. The researchers adjusted their findings to account for a variety of outside factors, such as education level and income.
The babies of mothers with the highest DDT exposure showed signs of delayed mental development at 12 months and 24 months.
For each tenfold increase in DDT levels measured in the mother, the researchers found a corresponding two- to three-point decrease in the child's mental development scores at 12 and 24 months. In physical skills exams, there were two-point decreases in children's scores at 6 months and 12 months for each tenfold increase in DDT levels in the mothers. No decrease was found at 24 months.
The researchers said changes in individual children due to DDT exposure might not be readily noticeable. However, "if this association is uniform across the population, you would see more children with problems in the population," Eskenazi said.
The findings are published in the July issue of Pediatrics.
The researchers also found that breast-feeding seemed to help the infants of mothers who were heavily exposed to DDT. They developed more normally, even though DDT is transmitted through breast milk.
People are typically exposed to DDT by coming into contact with the pesticide spray or by eating food that has been sprayed, Rogan said.
It's not clear what DDT physically does to the brains of infants whose mothers are exposed to it, he said. It's also not known if the developmental effects in infants will be permanent.
As for countries considering the use of DDT to fight malaria, Rogan said, "They have to entertain the idea that DDT is not an entirely innocuous compound. If you think about it, it's implausible that it would be," he said, adding that, after all, DDT is a poison designed to kill living things.
More information
Learn more about DDT from Duke University.
