Exposure to organophosphate (OP) pesticides before birth can increase susceptibility to attention disorders such as attention deficit/hyperactivity disorder (ADHD), according to new research published in the journal Environmental Health Perspectives (EHP). The new study is part of a growing body of research indicating that exposure to OP pesticides can adversely affect brain development.
OP pesticides target the nervous systems of insects by affecting the activity of neurotransmitters including acetylcholine, which in humans plays a critical role in brain development and is involved in attention and short-term memory. Exposure to OP compounds may also disrupt DNA replication and the growth of nerve axons and dendrites. Infants and young children are much more vulnerable to OP exposures than adults are because their ability to produce the enzyme that detoxifies OP pesticides is still developing.
Mothers participating in the study were recruited during pregnancy by the Center for the Health Assessment of Mothers and Children of Salinas (CHAMACOS). The Mexican-American women lived in the Salinas Valley, an area of intensive agriculture where more than 235,000 kg of pesticides are applied annually. The researchers analyzed six OP metabolites in urine samples collected from the mothers during pregnancy and from their children several times after birth. The presence of these metabolites indicated exposure to OP pesticides used in the Salinas Valley, such as chlorpyrifos, diazinon, and oxydemeton-methyl.
The children's behavior was assessed at the ages of 3 and a half years (n = 331) and 5 years (n = 323) using reports from the mothers and standardized psychological tests.
The results indicated that as the concentration of OP metabolites in the urine of pregnant women increased, so did the likelihood that their children's test scores would be consistent with a clinical diagnosis of ADHD. The association was stronger at age 5 years than at 3 and a half and was more pronounced in boys than in girls. Prenatal exposures had a greater association than did exposures after birth: A tenfold increase in levels of measured pesticide metabolites in the mother's urine during pregnancy correlated to about a 500% increase in the likelihood of attention issues in their 5-year-olds, whereas a tenfold increase in levels of metabolites in the children's urine at 5 years of age corresponded to a 30% higher likelihood.
By measuring prenatal exposures and focusing on participants likely to have higher exposures to OP pesticides than the general population, this study complements research published in the June 2010 issue of the journal Pediatrics. In that study, Maryse Bouchard and colleagues measured the same six OP metabolites in 1,139 children aged 8 to 15 years selected from the general U.S. population. They found associations between OP exposure and ADHD even though those children had lower average exposures than did the children in the CHAMACOS study.
The authors of the EHP study suggest that research should continue to investigate whether genetic differences in OP metabolism affect susceptibility to developmental disorders, including ADHD. They state that "given that attention problems of children interfere with learning and social development, finding potential causes that can be remediated are of great public health importance." A companion article, also released today in EHP, explores potential genetic mechanisms behind effects associated with OP exposure.