Organophosphate and ADHD Study: More Questions than Answers

For years, scientists have looked for causes for a childhood disorder that is becoming more common among young children: attention deficit hyperactivity disorder (ADHD). Medical studies have cited several possible causes, from diet to environmental factors. A new study released in May adds organophosphate pesticide exposure to the list.

Based on diagnostic and urinalysis data collected between 2000-2004, the study concludes that organophosphate pesticides such as malathion and chlorpyrophos are associated with an increased risk of childhood ADHD. Both pesticides are used to control fruit, vegetable and human health pests and can be a tool in integrated pest management interventions.

According to the National Institute of Mental Health, a child with ADHD exhibits severe inattentiveness, hyperactivity, or a combination of both. Although all children can be inattentive and fidgety at times, children with ADHD have those symptoms at a level that interferes with daily functioning, typically for at least six months.

The study, to be published in the June 2010 issue of Pediatrics, bases ADHD diagnosis on a survey known as the Diagnostic Interview Schedule for Children, version IV (DISC-IV). The DISC-IV is a telephone interview typically conducted with parents and guardians of children with a variety of mental disorders, including ADHD. Children are diagnosed with ADHD if they meet the diagnostic criteria in two or more settings. In the 2010 organophosphate study, 148 out of an initial study sample of 1139 children met the diagnostic criteria according to the DISC-IV. Twenty-nine of the diagnosed children were taking medication for ADHD.

Dr. Maryse Brouchard of the University of Montreal led the study. She hypothesized that organophosphate exposure would be associated with increased odds of ADHD. To measure exposure, she and her team took single urinary samples (called “spot” samples) between 2000 and 2004 and tested for markers of organophosphate exposure, also known as dialkyl phosphate (DAP) metabolites.

Metabolites are degraded products of the parent pesticide. When a person eats a piece of food or breathes in air, he or she may ingest chemicals, including pesticides. Some of those chemicals include the original pesticide; others include degraded products of that pesticide. When both the parent and by-products are broken down in the body, they both become by-products that are excreted in the urine and look identical when analyzed.

Bouchard’s team tested for the by-products—or metabolites—of malathion and chlorpyrophos metabolites, known as demethyl alkylphosphates (DMAPs) and diethyl alkylphosphates (DEAPs), respectively.

Results showed that 93.8% of the original 1139 children had at least one of the six metabolites screened. Between 35.7% and 80% of the children had metabolite concentrations below the detection limit for each of the six metabolites.

According to the study, the chance of meeting the criteria for ADHD increased 10-fold with the urinary concentrations of total DAP metabolites (DMAPs and DEAPs combined). The odds increase for children taking ADHD medication. Children with creatinine-adjusted dimethyl thiophosphate (a DMAP metabolite) above the median of detectable values had twice the odds of ADHD.

The study concludes that current levels of organophosphate pesticide exposure might contribute to ADHD.

“Organophosphates disrupt the activity of acetylcholine, a neurotransmitter also implicated in ADHD,” Dr. Brouchard is quoted as saying in Medscape Medical News. “In addition, certain organophosphates affect growth factors, several neurotransmitter systems, and second messenger systems. These changes in brain activity could well result in ADHD-like symptoms.”

The Medscape Medical News story quotes another doctor, not involved in the study, as saying that the data show “that even small exposures (to organophosphate pesticides) are very harmful to kids.”

While the questions the study raises about organophosphate pesticides have value, the study itself is limited and warrants further investigation. Some concerns include:

  • Data from the urinalysis, taken from 2000-2004, was generated while EPA was reevaluating several organophosphate pesticides. The reevaluation resulted in the cancellation of several uses, including nearly all residential uses and some uses on food typically consumed by children (EPA Statement, 5/21/2010). According to a representative from EPA Region 4 in Atlanta, “in July 2005 the use of dimethoate (organophosphate pesticide) was voluntarily cancelled on seven crops. Three of the cancelled uses – apples, grapes, and spinach – were major contributors to the organophosphate cumulative dietary risk. The data analyzed (urine samples) in the referenced survey (National Health and Nutrition Examination Survey) as part of the study done by Marc Weisskopf et al. and published in “Pediatrics” were from 2000-2004. While this does not mean that organophosphate residues in food have been eliminated since then, it does indicate a significant reduction.”
  • Authors do not state whether they compared the diets of the study subjects. According to the National Institute of Mental Health, sugar and some food additives such as preservatives and coloring have been linked to ADHD. Future studies should eliminate diet as a factor.
  • Authors do not describe how they calculated the odds ratio, the key factor in determining that children with detectable amounts of metabolites were more likely to be diagnosed with ADHD. In addition, the study does not show comparisons between metabolite levels of children not diagnosed with ADHD and those diagnosed with the disorder.
  • No control group was indicated in the study. Future studies should include a control group without ADHD, and DAP levels should be compared between the control group and the ADHD-diagnosed group.
  • The authors indicate that one weakness of the study was the “spot” urine analysis. Because DAPs are eliminated from the body after three to six days, the authors hypothesized that the presence in the urine of most children indicated continuing exposure. Future studies should include multiple urine tests over time, in addition to analysis of the child’s environment, to determine if exposure is long-term or from isolated activities.
  • The diagnosis of ADHD from phone interviews with parents, without the review of medical records, needs to be further investigated (CropLife America).

The Environmental Protection Agency has stated that it is collaborating with various agencies of the National Institutes of Health and the Centers for Disease Control and Prevention to conduct a follow-up study using more recent data. The Agency recommends that consumers use preventative pest control practices such as removing sources of food, water and shelter to reduce the incidence of pest problems, and to wash and peel fruits and vegetables before consuming them.

One Response

  1. I really enjoyed reading this article. I learned so much to this stuff.
    Atlanta Pest Control

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