The study, which was published on March 1 in the journal Occupational & Environmental Medicine, found that children who had higher levels of certain pyrethroids — which are a group of synthetic chemical insecticides — in their system were more likely to display abnormal behaviors at age 6 compared to those who didn’t.
For the study, researchers from the University of Rennes and University Hospital in France analyzed the urine of 287 women during pregnancy and that of their children six years later. They found that higher levels of a certain pyrethroid in the urine of the pregnant women was associated with an increased risk of “internalizing” behaviors in their children, such as being anxious and withdrawn.
The researchers also found that levels of a different pyrethroid in the children’s urine samples was linked to a greater risk of “externalizing” behaviors, such as being defiant and aggressive.
Children with the highest levels of these insecticides were around three times as likely to display abnormal behavior, according to the study.
Pyrethroids, which work by damaging the nerves, are found in a wide variety of products including head lice treatments, scabies creams, mosquito repellents and flea control for pets.
One insecticide in the pyrethroid family known as permethrin is contained in Johnson & Johnson’s Lyclear Scabies Cream, iNova’s Pyrifoam Lice Breaker, and Orion’s Quellada Headlice Treatment.
The Centers for Disease Control and Prevention (CDC) estimates that up to 12 million head lice infestations happen each year in the U.S. among children aged 3 to 11.
Pyrethroids are among the most common treatments for lice, but other compounds can be used, such as alcohol-based treatments and an anti-parasitic drug called ivermectin.