Pesticides may increase risk of ADHD in children

A recent article published in the journal Pediatrics links exposure to certain types of organophosphate pesticides with ADHD.  This is a reasonable study and the results should be taken seriously, but as always they need to be put into context – something most media outlets are failing to do.

First – the study itself – here are the methods:

Cross-sectional data from the National Health and Nutrition Examination Survey (2000–2004) were available for 1139 children, who were representative of the general US population. A structured interview with a parent was used to ascertain ADHD diagnostic status, on the basis of slightly modified criteria from the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition.

They looked at the results of spot urine tests (a one time urine sample) for six different organophosphate metabolites, and found that two of them correlated with a statistically significant higher chance of  meeting criteria for ADHD.

One hundred nineteen children met the diagnostic criteria for ADHD. Children with higher urinary dialkyl phosphate concentrations, especially dimethyl alkylphosphate (DMAP) concentrations, were more likely to be diagnosed as having ADHD. A 10-fold increase in DMAP concentration was associated with an odds ratio of 1.55 (95% confidence interval: 1.14–2.10), with adjustment for gender, age, race/ethnicity, poverty/income ratio, fasting duration, and urinary creatinine concentration. For the most-commonly detected DMAP metabolite, dimethyl thiophosphate, children with levels higher than the median of detectable concentrations had twice the odds of ADHD (adjusted odds ratio: 1.93 [95% confidence interval: 1.23–3.02]), compared with children with undetectable levels.

There are solid results, but it should be noted that the lower end of the confidence interval (1.14 and 1.23) are only slightly greater than one, which represents only a small increase in risk. But they are statistically significant.

To put this into perspective, however, this is a correlational study only and cannot be used to conclude cause and effect. It may be possible, for example, that children with ADHD engage in behaviors that expose them to more organophosphates.

Other weaknesses acknowledged by the authors include the fact that exposure was determined by a single spot urine test – which may not reflect long term exposure. Organophosphates are generally cleared by the urine in 3-6 days.

I also noted that the prevalence of ADHD in this study, 12%, is about twice the 4-7% found by other studies. This could reflect a hidden bias in the data collection methods.

Regarding plausibility, a possible link between organophosphates and ADHD is a mixed bag. There is basic science plausibility in that organophosphates do have neurotoxicity, and specifically inhibit acetylcholine function which is involved in memory and attention.

On the other hand, toxicity is also related to dose, so the real question is are the doses detected above or below toxic levels. The EPA insists they are below safe levels, but critics contend that chronic low levels of exposure have not been adequately studied. Of course – you can always make this claim when the data does not show toxicity.

Sometimes it seems like the best we can expect from a study is that it will indicate whether or not further study might be worthwhile, and we never get to concrete results.

 A structured interview with a parent was used to ascertain ADHD diagnostic status, on the basis of slightly modified criteria from the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition.