For more than ten years, French researchers have been studying the effects of a common pesticide ingredient on child development, and they recently found that the chemical may be linked to behavioral problems.
Pyrethroids are the key component of piperonyl butoxide, a chemical synergist that doesn't have any pesticidal effect on its own, but enhances the bug-killing abilities of other chemicals. It's found in flea collars, mosquito repellants, indoor and garden pesticides, clothes and bedding treated to repel insects, and shampoos used to get rid of parasites like fleas and lice.
In use since the 1940s, pyrethroids are seen as a safer alternative to other common insecticides, known as organophosphates. That's one reason why, in recent decades, European Union countries and the United States have seen a decrease in use of organophosphate insecticides and an increase in pyrethroids.
The effects of pyrethroid neurotoxins on childhood development, however, are just beginning to be studied. A 2014 study found that the kids of mothers who resided near agricultural operations where the air was thick with pyrethroids during pregnancy had a greater chance of being on the autism spectrum. A 2015 study linked traces of a pyrethroid metabolite in the urine of children aged eight to 15 to ADHD.
The French research team, most of whom were affiliated with the University of Rennes, recruited 3,421 pregnant women from the region of Brittany from 2002 to 2006. After the unenviable task of collecting a urine sample from each woman, they randomly selected 571 women to follow up with. Of those, 287 agreed to allow a psychologist to do a behavior assessment of their child at age six. The mothers also filled in a detailed questionnaire on socioeconomic factors, lifestyle, and other environmental exposures. Psychologists then visited the kids at their homes to score them on some standardized tests and collect yet more pee.
Pyrethroid traces in the urine of mothers-to-be was associated with children who showed what psychologists call "externalizing behaviors," which includes cheating, stealing, rule-breaking, physical aggression, and destruction of property. Kids with the highest levels of pyrethroid metabolites in their own pee were about three times more likely to display abnormal behavior.
Researchers note that this is an "observational study." Ethical concerns would of course not allow for a study in which pyrethroid was more carefully controlled. The limits of such studies are such that scientists do not draw any firm conclusions from them, but the suggestion from this decade-spanning research is that pyrethroid exposure may harm the neural development of children.