Although banned for decades in most rich countries, the insecticide DDT may be influencing whether babies born today and in the future develop autism. A study in Finland has found that mothers that show signs of high DDT exposure in their blood may be more likely to have children with autism.
DDT was sprayed in large amounts from the 1940s onwards, to kill disease-carrying mosquitoes. But it was widely banned in Western nations in the 1970s and 1980s, after evidence mounted that it caused cancers in laboratory animals and impaired reproduction in wildlife.
But the insecticide takes decades to break down, so people are still absorbing it from contaminated water and food. Once consumed, it lodges in the body’s fat, and circulates in the blood – and is known to pass to fetuses during pregnancy.
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To see if this might be linked to autism, of Columbia University in New York and his colleagues analysed blood samples taken in Finland between 1983 and 2005 from more than a million women during the early stages of pregnancy. Like in the UK and US, DDT was used widely in Finland before it was banned.
The team screened these blood samples for DDE – a long-lived breakdown product of DDT – and found that, on average, DDE levels were higher in mothers who went on to have autistic children.
Small difference
The mothers of children without autism had, on average, 811 picograms of DDE present in each millilitre of their blood, but the average was 1032 picograms in the mothers of autistic children.
Comparing women with and without autistic children who had the highest levels of DDE, the team calculated that high exposure to DDT raises the likelihood of having a child with autism by around a third.
The team also screened the samples for other long-lived pollutants called polychlorinated biphenyls, or PCBs, which were used as insulation in electrical transformers. But they found no association between these and autism.
Brown says this tallies with previous research showing that DDE, but not PCBs, have been linked with low birth weight and prematurity – two well-established factors in autism.
However, Brown says the team’s findings shouldn’t concern people. “I would argue against worrying, because even among those with high levels, most won’t have children with autism,” he says.
Further work is needed to determine if DDT really is linked to autism, and whether DDT itself is a cause of autism, or instead if the two are linked by some other, causative factor. “Ideally, we’d like to see the same finding replicated in at least three studies to feel confident about the association,” says  of the A. J. Drexel Autism Institute in Philadelphia.
“I wouldn’t dismiss it as unlikely,” says Rosa Hoekstra of King’s College London. But she says any effect of DDT is very small compared with the genetic factors that contribute to autism.
American Journal of Psychiatry
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