Reports of a French study claimed last week that exposure to certain insecticides during pregnancy were linked to “abnormal behaviour” in children.

Called pyrethroids, these chemicals are present in some common treatments for dog ticks and head lice.The Daily Mail zeroed in on this fact with the headline:

“Poor behavior is linked to head lice treatments: Chemicals used to tackle the problem may effect [sic] nerve activity in the brain.”

The study’s lead author, Professor Jean-François Viel, told The Conversation he was surprised media reporting focused on head-lice treatment rather than “the overall exposure to pyrethroid insecticides we attempted to address”. You can view his full response at the end of the article.

Published in the Occupational and Environment Medicine journal, the study suggests an association between exposure to pyrethroids in pregnancy and behavioural issues in six-year-olds. But an association isn’t the same as causation – and as far as associations go, the one in this study was pretty weak.

We asked a chemistry expert to explain, and a toxicologist to review the analysis.

How was this study conducted?

The study had two aims. The first was to test the effect of children’s exposure to pyrethroids in utero; the second tested for exposure to pyrethroids during childhood.

Researchers randomly selected 571 pregnant women from a sample of 3,421 women from an agricultural region of France, who were recruited for a broader study. Of the 571 pregnant women selected, 287 agreed to a neurological (nervous system) and chemical and psychological follow-up when their children were six years old.

Mothers completed a list of 25 questions, drawn from a French version of the International Strengths and Difficulties Questionnaire, to describe their children’s behaviour over the previous six months. Children’s behaviours were also assessed by visiting psychologists.

Researchers tested mothers’ urine at weeks six to 19 of gestation, and their children’s at age 5.99 to 6.27 years. They analysed the urine samples for pyrethroid metabolites.

Pyrethroid metabolites were absent from the urine of 82 mothers and four of their children. So the study only reported results for the remaining 205 women and 283 children. Metabolite concentrations in these women and children were very low – typically in the region of sub-micrograms per litre.

One metabolite (cis-DCCA) was detected in almost all the women and children. Another (3-phenoxy benzoic acid) was detected in some samples, but its concentrations were very low. And in as many as 36% of the childhood urine samples, its levels were not detectable. 

Different metabolites arise from different pyrethroids, so the presence or absence of a metabolite is likely to be a consequence of exposure to different pyrethroids rather than different metabolism.

Psychologists assessed children for altruism (social behavior), internalizing disorders (inability to share problems and ask for help) and externalising disorders (defiant and disruptive behavior). Mothers also sent in reports on their children’s behavior.

What were the results?

Concentrations of the most commonly observed metabolite (cis-DCCA) in the urine of pregnant mothers in the first trimester was positively correlated with internalising difficulties – such as being anxious or withdrawn – of their six-year-olds.

The variant of cis-DCCA, called trans-DCCA, in the urine of the six-year-olds was associated with reduced externalising behaviors – such as being aggressive or defiant. This is a counter-intuitive finding for which the researchers had no explanation.

Authors also report childhood exposure to the metabolite 3-phenoxy benzoic acid was associated with “increased odds of behavioral disorders”. But this was the metabolite that, in 36% of the urine samples, was below the level of detection.