Because DDT and DDE are so similar, scientists were surprised that DDT seemed to harm brain development while the other had little impact. That suggests that the mental abilities of U.S. children born between World War II and the early 1970s -- when DDT was routinely sprayed -- could have been affected, but not those born years later and exposed to old residue in the environment.
Nevertheless, the researchers reported that breastfeeding is beneficial to babies even when the milk contains large doses of DDT. The children's test scores increased with every month of nursing even for the most contaminated mothers.
Doctors know that breastfeeding boosts a baby's intelligence, yet they have long wondered if contaminants in the milk erase that benefit. The new study "provides additional evidence that breastfeeding may help to compensate for the subtle perinatal insult associated with DDT/DDE exposure," the authors wrote. The insecticide's damage probably occurs in the womb, not during breastfeeding.
The researchers tested the women for other pesticides, but only DDT was connected to neurological effects.
The study is part of a federally funded UC Berkeley project that assesses whether agricultural chemicals in the Salinas Valley, one of the world's most intensely farmed areas, are harming children.
Although animal tests have repeatedly shown that DDT causes neurological damage, the new study is the first in the United States to find such effects in humans.
In North Carolina and New York, large studies in the 1980s and 1990s detected no effect on babies' mental abilities, but they tested for DDE, not DDT. A smaller study in Spain did report some neurological effects.
The insecticide, which mimics estrogen, also affects reproduction. California women exposed in the womb are more likely to experience delays in getting pregnant decades later, according to a 2003 UC Berkeley study. Again, the effect was predominantly found with DDT, not its older residue.
Developed as an insecticide in 1939, DDT was popular because it killed insects but wasn't acutely poisonous to people or animals. But by the 1950s, it was accumulating in food chains, nearly wiping out eagles and other birds.
Canadian scientists recently reported that DDT still contaminates farm soils and will seep into the air for another generation.