The team also found that the longer the children nursed, the better they scored on developmental tests, despite the fact that DDT is also transmitted through breast milk. This was the case even with mothers who had high accumulations of the pesticide in their bodies, a finding that suggests that the benefits of nursing may outweigh the potentially harmful effects of DDT transmission through mother's milk.
The study is the first to examine the effects of maternal levels of DDT, rather than its breakdown products, on child neurodevelopment - that is, the development of mental and physical skills. At a time when health authorities around the world are considering increasing use of this pesticide to combat malaria, the study is one of the first to suggest that DDT may be harmful to child development. As such, it provides important health information for decision makers, said Brenda Eskenazi, the study's lead author.
"People need to consider these data if they are going to continue using DDT or reintroduce it in countries where it's been banned," said Eskenazi, a professor of epidemiology and maternal and child health at UC Berkeley's School of Public Health. "Given the impact of malaria on child health, I'm not saying that we shouldn't use it. But if we do, we need to think of ways to protect women and children."
The study is published in the July issue of the journal Pediatrics.
Eskenazi and her team measured blood levels of DDT and one of its breakdown products, DDE, in 360 pregnant women. Then they tested the mental and physical skills of the women's babies at six, 12 and 24 months using tests known as the Bayley Scales of Infant Development, a well-known method for developmental assessment of young children.
For each tenfold increase in DDT levels measured in the mother, the team found a corresponding two- to three-point decrease in the children's mental development scores at 12 and 24 months. No decrease was found at six months. The highest in utero DDT exposures in children were associated with a seven- to 10-point decrease in Bayley mental scores, compared to the lowest exposures.
In the physical skills evaluations - known as psychomotor testing - there were two-point decreases in children's scores at six and 12 months for each tenfold increase in DDT levels in the mothers. No decrease was found at 24 months.
When it evaluated the effects of DDE on development, the team found associations that were similar to those for DDT, but not as strong.
Other findings, reported earlier by the team, were that mothers' levels of DDT and DDE did not affect the length of their pregnancies or their infants' birth weights.
In its analysis of the data, the team took into account many factors, including the mother's age and number of years she had lived in the United States, income, education, marital and work status, the child's gender, duration of breastfeeding and the quality of the home environment for young children.
Although small decreases on the Bayley tests may not be apparent in individual children, Eskenazi said, the decreased scores her team found are relevant for populations. (The Bayley test is designed so that the average score is 100 and scores below 85 indicate a possible developmental delay.)
"If you had a whole population with a downward shift like this, you'd be seeing more kids with developmental problems," Eskenazi said.
Eskenazi hopes to be able to follow these children until they reach school age to determine whether the effects of DDT exposure persist. "We need to know what's happening further down the road," she said. "What's critical is to find out whether these levels of exposure are affecting a child's ability to perform academically and to function in society."
The mothers in the DDT study are participants 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 California's Salinas Valley, 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. Its use gradually declined there until 1995, when applications were banned other than to control malaria by targeting mosquitoes. In 2000, a complete ban went into effect. As a result, levels of DDT in most women in the CHAMACOS study are considerably higher than those of the general population in the United States, where DDT has not been used since 1972.
Classified as an organochlorine, DDT persists in the environment long after use, accumulating in the food chain and in fatty tissues of animals and humans. Over time, it degrades into DDE and DDD, compounds with chemical and physical properties similar to DDT that are also highly persistent. Thirty-three years after its use was banned in the United States, DDT is still detectable in about five to 10 percent of this country's residents, and DDE is detectable in nearly everyone, Eskenazi said.
While earlier studies have shown that DDT is harmful to the neurodevelopment of animals, until now no one has studied its effects on the neurodevelopment of humans. A few studies have examined DDE's effects on people, but with conflicting results.
Since 2001, more than 100 countries have committed to the Stockholm Convention on Persistent Organic Pollutants, which calls for the elimination by the year 2025 of DDT and 11 other persistent pollutants. Because DDT is inexpensive and effective in controlling mosquitoes - at least in those regions where insects have not yet become resistant to it - the convention permits its use for malaria control until safe, affordable and effective alternatives can be found.
Other authors of the study are Amy Marks, statistician, and Asa Bradman, associate director of CHAMACOS, both with the Center for Children's Environmental Health Research at UC Berkeley's School of Public Health; Caroline Johnson, pediatric neuropsychologist in private practice in Berkeley; Nicholas Jewell, professor of statistics and biostatistics at UC Berkeley's School of Public Health; Laura Fenster, epidemiologist in the Occupational Health Branch at the California Department of Health Services; and Dana Barr, research chemist at the Centers for Disease Control and Prevention in Atlanta.
Funding from the National Institute for Occupational Safety and Health, the National Institute of Environmental Health Sciences and the EPA helped support this research.