Exposure to the notorious pesticide dichlorodiphenyltrichloroethane (DDT) during pregnancy could raise the risk of a child developing autism.
The agent was famously criticised for its environmental impacts in the 1960s, and has since become banned throughout most of the world. Yet a number of countries still use the pesticide, and the decision to ban it worldwide isn't straight-forward.
A research team led by Columbia University used data collected from Finnish mothers for an earlier prenatal study on autism.
They identified 778 children born between 1987 and 2005 who were diagnosed with autism, matching them with maternal blood samples taken during pregnancy. Another 778 control groups were then selected for comparison.
Blood samples were tested for both the pesticide DDT and the metabolite that it forms as it breaks down in our body – p,p'-dichlorodiphenyl dichloroethylene (p,p'-DDE).
In addition, the research team measured the levels of another group of potentially toxic pollutants called polychlorinated biphenyls (PCBs).
There were no signs of a relationship between autism and PCBs. But they did find a significantly increased risk that a child would be diagnosed with autism if the mother was among the top 25 percent of serum DDE levels.
The child was also twice as likely to have an intellectual disability compared with cases of autism among the lowest 75 percent of DDE levels.
Neither DDT nor PCBs have a sterling reputation as it is. We can thank mid-20th century environmentalism for that.
DDT was a common synthetic pesticide developed in the 1940s for the purpose of wiping out mosquitoes on a large scale. It was so effective, it was used on everything from crops to the average kitchen bench, keeping virtually any unwanted insect at bay.
A marine biologist by the name of Rachel Carson was the first to ring the bell on its potential for causing environmental harm in her book Silent Spring.
Thanks to her popular writing, the idea that some potentially toxic chemicals can concentrate as they are passed up the food chain has become public knowledge. In 1972, ten years after the book's publication, the US government ceased the insecticide's use.
It has been lauded as a win for the environment and public health, but not everybody has been happy with DDT's vilification.
The pesticide is incredibly effective for managing the spread of malaria by killing its vector, the mosquito. While we might have little concern for this deadly pest in the affluent western nations, it's still one of humanity's biggest killers.
Even today, the World Health Organisation recommends DDT's indoor use in areas where malaria is deemed to be a significant enough threat. Many African countries continue to spray regularly with the pesticide, and so the controversy continues.
The chemical itself is listed only as moderately toxic, meaning you'd have to have a fairly large exposure to die from its effects.
Over the decades, evidence of subtle but serious health impacts has started to pile up, and DDT's reputation as a villain has been reinforced. Now we can consider adding autism to its list of concerning side-effects.
The study itself can't tell us how DDT might affect brain development, or even make the claim that its metabolite causes autism, especially given the disorder's complex nature.
The condition is characterised by difficulties in sensory processing, communication, and socialising, and seems to involve diverse neurological pathways and brain functions.
With concerns that ASD seems to still be on the rise in the west, pollutants and other features of our fast-paced modern world are often the first to be blamed.
It can be hard to tease apart the multitude of causes behind this complicated condition, and as always, further research is desperately needed.
Studies like this one might not point to easy solutions, but we need them to make any decisions we do make as informed as possible.
This research was published in the American Journal of Psychiatry.