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Exposure to certain insecticides linked to childhood behavioural difficulties
Pyrethroids widely used in pest control for pets, people, and crops
Exposure to a particular group of chemicals widely used in pest control for people, pets, and crops, may be linked to behavioural difficulties in 6 year olds, suggests research published online in Occupational & Environmental Medicine.
Pyrethroids are synthetic chemicals which are found in a range of products, including treatments for head lice, scabies, and fleas, and some mosquito repellants.
They are a safer alternative to organophosphates. But like many classes of insecticides, they work by damaging nerves, and concerns have recently been raised about the potential impact of children’s exposure to them.
So the researchers measured levels of five pyrethroid metabolites in the urine of women between 6 and 19 weeks of pregnancy, and subsequently their 6 year olds, to see if there was any link between prenatal and childhood exposures and behaviour that might be indicative of neurodevelopmental damage.
From among 3421 pregnant women enrolled in the study between 2002 and 2006, some 571 were randomly selected to take part in the assessments of their children when they reached the age of 6: 287 of these women agreed to do so.
The mothers filled in a detailed questionnaire on socioeconomic factors, lifestyle, their child’s behaviour, and various environmental exposures.
Psychologists then visited them and their children at home to carry out behavioural assessments, and to collect dust and urine samples for analysis.
The children’s behaviour was assessed using the validated Strengths and Difficulties Questionnaire (SDQ), with a particular focus on altruism (pro-social behaviour); internalising disorders (inability to share problems and ask for help) and externalising disorders (defiant and disruptive behaviours).
Three metabolites (trans-DCCA, cis-DBCA, and cis-DCCA) showed up the most frequently in the urine samples of both the mothers (100%, 68%, and 65%, respectively) and their children (96.5%, 85%, and just under 65%, respectively).
After taking account of potentially influential factors, higher levels of cis-DCCA in the urine of the mums-to-be was associated with a heightened risk of internalising behaviours in their 6 year olds.
Levels of another metabolite (3-PBA) in the children’s urine samples were associated with a heightened risk of externalising behaviours. However, high levels of trans-DCCA were associated with a lowered risk of externalising behaviours.
But children with the highest levels of metabolites in their urine were around three times as likely to display abnormal behaviour.
By way of an explanation for these associations, the researchers suggest that pyrethroids might alter neurochemical signalling in the brain.
This is an observational study so no firm conclusions can be drawn about cause and effect, added to which accurately assessing pyrethroid exposures using urine samples is notoriously difficult because metabolites are cleared from the body in just a few days.
Nevertheless, the researchers conclude: “The current study suggests that exposure to certain pyrethroids at the low environmental doses encountered by the general public may be associated with behavioural disorders in children.”
Expert Reaction
These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.
Dr David Goddard is a senior lecturer in the Department of Epidemiology & Preventive Medicine at Monash University
The study by Viel and his team concerned pyrethroid insecticide exposure and psychological effects in 6 year olds.
I consider the positive findings to be unreliable. In other words, if another similar study were done, the findings would most likely be different.
The study by Viel and his team involved 287 mothers and their children. This sounds like a lot of people and it took much effort, yet it is a small size given the extent of analysis that the team intended.
In a standard way, the team compared several different types of exposure, with two or more different exposure levels, for mother and child one at a time with each of three different psychological effects.
More than 60 such comparisons were made.
A quirk of the statistics used means that one such comparison in twenty is likely to show a link between an exposure and a health effect simply by chance.
The authors found three links. In making 60 comparisons that’s simply par for the course – just what you’d expect by chance.
It is ethically correct for the team to publish the links that they found, but a fair interpretation of these findings is that chance is a reasonable explanation for them.
The team did not openly qualify their findings in this way. Had they done so, their article would have been less likely to have been published which would have been a pity given all the work they had done.
Instead the team used soft words in their abstract and conclusion, viz. “…suggests that exposure … may affect behaviour”.
So, is it safe to continue to use chemical head lice treatments?
What is safe? Is it safe for women to have babies? Is it safe for children to travel in motor cars? Is it safe for people to simply live with their head lice and cockroaches? In modern life, we constantly face risks and trade-offs between risks.
My opinion is the team has not made a case to affect our judgment of a risk to health from pyrethroids.
Instead, their study provides a useful reminder to us to be prudent, i.e. that our exposure to pesticides should be no more extensive or frequent than is needed to control the target pests.
Professor Ian Rae is an expert on chemicals in the environment from the School of Chemistry at the University of Melbourne. He was also an advisor to the United Nations Environment Programme on chemicals in the environment and is former President of the Royal Australian Chemical Institute
The idea that exposure of a foetus to even very low concentrations of chemicals such as pesticides can lead to developmental problems in children has been around for quite a while.
Clear demonstrations of cause-and-effect are rare.
This is especially true when the 'effect' is behavioral, as in the recently published work by French researchers.
They are operating near the limits of even modern analytical chemistry, and they report that the concentrations of the substances they are monitoring are frequently below the level of detection.
They have used the 'real' values, sometimes only 10-20 per cent of the cohort.
Understandably there is much less precision about the numbers describing behavioral traits: the reported numerical results span very wide ranges.
There is a clue to a possible reason for this in one factor that the researchers mention - that the lifetimes of pyrethroids in the body are quite variable and, in any case, quite short - and one that they don't mention - that there could be only short windows of time during gestation when the developing foetus is susceptible to 'poisoning' by a particular pesticide.
We are all exposed to them and live with traces in our tiny bodies of as many as several hundred substances produced by the chemical industry and used in domestic, industrial or agricultural settings.
So, were the pesticides to blame in this case? Who can tell? Even the researchers admit that there could be 'reverse causality' - that is, that children with behavioural problems (such as hyperactivity) might be somehow more exposed to pesticides. And, I should add, possibly more susceptible since there is considerable variation within the population.
As an example of the uncertainty thrown up by even such a carefully conducted study as this, the researchers found that the presence of one particular pesticide metabolite seems to be associated with better than normal behavioural outcomes, something they describe as 'counter intuitive' and for which they 'have no current explanation'.
Associate Professor Vincent Pettigrove is Chief Executive Officer of the Centre for Aquatic Pollution Identification and Management (CAPIM) at the University of Melbourne
These results are very concerning as these chemicals are commonly used in urban areas of Australia, especially for termite control.
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