A study has been making the rounds on social media claiming an association between prenatal exposure to pesticides and the risk of autism and developmental delay. This means that I am getting asked by many people what the study actually shows. Spoiler alert – not much. But let’s break it down.
Autism spectrum disorder (ASD) is a neurological disorder involving brain development resulting in decreased communications among neurons in the brain and characterized by reduced social ability. Our current scientific understanding is that ASD is largely a genetic disorder. While environmental factors cannot be ruled out, it seems that genes are the primary factor. It’s reasonable to search for environmental risk factors, but so far none have been clearly established.
Those who feel there likely is an environmental factor also tend to believe that there is an autism epidemic – that the incidence of autism is increasing in a way that is not easily explained by genetics, and therefore suggests and environmental factor. While it is uncontroversial that the number of ASD diagnoses has been increasing over the last two decades, this does not necessarily mean that the true incidence of ASD has been increasing.
The evidence actually shows that diagnostic substitution, broadening of the definition of ASD, and increased surveillance account for much of the increased recorded incidence. It’s possible that changes in diagnostic behavior entirely accounts for the apparent increase. It’s also possible that a subset is due to a true increase, but that has not been clearly established.
This still leaves us with the conclusion that an environmental factor is possible in ASD, but not necessary.
What does this current study show? The study in question is a case-control study using data from the CHARGE study and data about pesticide use in California. A case-control study is a retrospective epidemiological study. It looks at two or more populations based upon whether or not they have a condition, in this study there are three groups – ASD, developmental delay (DD) and typical. The groups are then compared based on exposure to a potential risk factor to see if it is correlated with the condition.
In this study the authors looked at proximity to pesticide use prior to conception and during each trimester of pregnancy. They concluded:
“This study of ASD strengthens the evidence linking neurodevelopmental disorders with gestational pesticide exposures, and particularly, organophosphates and provides novel results of ASD and DD associations with, respectively, pyrethroids and carbamates.”
Orac has already reviewed this study and I agree with his assessment – this conclusion is not justified by the data presented.
First, there is a fatal flaw in the study design . . .
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