But a new study from Drexel University indicates that that might not be the case. Older readers may remember the infamous pesticide DDT, which was banned in 1972, ten years after the publication of Rachel Carson’s seminal book Silent Spring, which detailed the many ways in which DDT was ravaging the environment. (The pesticide might be most infamous for very nearly pushing the bald eagle to extinction.)
DDT is a member of a group of pesticides known as organochlorines, all of which were banned at that same time. In the same general family are polychlorinated biphenyls, usually abbreviated to PCBs, which throughout the 20th century were produced as a byproduct of various manufactured goods and, frequently, dumped directly into such important waterways as the Great Lakes and the Hudson River.
The new study found a distinct correlation between babies who tested positive for higher levels of certain organochlorines and PCBs and the likelihood that those babies will later be diagnosed with autism and/or intellectual disability. In fact, those children who tested at the highest levels of particular organochlorines were about 80 percent more likely to be diagnosed with autism than those who tested at the lowest level (or presented no organochlorines at all). The organochlorine p,p’DDE, formed by the breakdown of DDT, was found to be correlated with increased risk of intellectual disability.
The study is fairly simple, though it uses data that hasn’t been put together in quite this way before. Some children born in the past few decades were given blood tests in their second trimester, and those blood tests will show the presence of these chemicals in the fetus. Comparing that data with autism spectrum diagnoses (and a control group from the general population) yields the results.
In case you’re wondering, organochlorines are widely known to have not really decomposed in the past 40 years; they seeped into groundwater, were absorbed by plants, and can sometimes be found in the fats of livestock that we eat today.
Nobody is saying these pesticides cause autism; only that a higher level of them has a correlation with higher rates of autism.
The caveats for this study are important. For one thing, this is absolutely a correlation; nobody is saying these pesticides cause autism or intellectual disability, only that a higher level of them has a correlation with higher rates. And the sample size is small, relying on less than 1,500 cases, all from Southern California, and all of whom had enrolled in a specific program that administered those blood tests. Even further, the study adjusted for some variables – gender, time of birth, maternal age and ethnic identification – but not others, like socioeconomic status, location, or family medical history.
That said, of course, the study indicates – not proves, but indicates – just how damaging pesticides and toxic byproducts might be in the environment and in generations still to come. Even if they’ve been banned for almost half a century.
Correction: An earlier version of this article contained errors about the precise connection between certain chemicals and their specific correlations. We apologize for this mistake.