The infamous pesticide DDT has been further implicated as a possible trigger for the development of autism spectrum disorder by an international group of epidemiologists and psychiatrists studying children who were exposed in utero.

Their findings, now published in the American Journal of Psychiatry, present the first evidence of a link between the banned chlorine-based chemical and the neurological condition that was drawn from cases where the mother was confirmed to have DDT in her body – past research has relied on estimated exposures calculated by each woman’s residential proximity to DDT-polluted areas.

“DDT is still persisting in the environment and is detectable in almost everyone,” lead author Dr Alan Brown told IFLScience. “I would not say women should be concerned but it is important to be informed that at least this one chemical exposure is related to increased risk.”

DDT was first synthesized by chemists in 1874, but it wasn’t widely used until its insecticidal properties were discovered in 1939. For the next several decades, the compound was freely and generously sprayed on food crops worldwide and directly into public spaces and people’s homes.

But based on mounting evidence that the pesticide was harmful to both animals and humans, DDT was banned in the US in 1972 and in the European Union in 1986. In the time since, additional studies have confirmed that DDT accumulates and lingers in ecosystems, and that when ingested or inhaled, it can lead to reproductive issues and cancer.

Hoping to substantiate past work, Dr Brown’s group drew data from a large, nationwide Finnish study that took blood serum samples from women during more than 1 million pregnancies that occurred between 1987 and 2005. Subjects of the current investigation included 778 of the resulting non-twin children who had been diagnosed with autism at any time point up to 2007, as well as 778 control children who were each matched, one-to-one, to each subject with autism in regards to date of birth, place of birth, and place of residence.

The level of DDT in the mother’s system was quantified by a laboratory test for DDE, the chemical that DDT is metabolized into.

Examining this data revealed that the likelihood of autism was 32 percent higher in children whose mothers harbored DDE levels in the 75th percentile (of the range of concentrations in all subject’s mothers), after statistical adjustment for the influence of maternal age, family history of psychiatric disorders, number of siblings, and others. Furthermore, the odds of autism with intellectual disability were a staggering 121 percent higher if the mother had DDE levels above this threshold.

The study also looked at the risks associated with a class of industrial chemicals called polychlorinated biphenyls (PCBs) that were banned in 1979 in the US and in 1985 in Europe, but no association with autism was found. Confusingly, a similarly designed 2016 study did find a link to PCBs and failed to identify one with DDT.

Of course, DDT is not the only environmental toxin suspected to drive epigenetic changes during fetal development. Dr Brown hopes his group’s results will help clarify which chemicals to study further based on the developmental pathways they alter. Both DDT and PCBs affect the actions of male sex hormones in utero – which are being increasingly implicated in autism – yet do so differently. DDT is known to inhibit the production and function of receptors for these hormones, whereas PCBs increase the number of receptors.