Evidence is now mounting for the link between early exposure to pesticides and an increased risk of ADHD. A number of recent studies in the USA indicate that early exposure to organophosphates or organochlorine compounds widely used as pesticides is associated with behavioural changes and attention problems at age five, and that the effects are stronger in boys.
A recent study published in the Journal of the American Medical Association (2010, 304: 27-28) looked at 1139 children and found that those with higher levels of exposure (measured by the concentration of chemical metabolites in urine) were twice as likely to have ADHD as those with the lowest exposure levels.
Researchers also found that those children with higher than medium exposures to pesticides had about a 20% risk of being diagnosed with ADHD. According to a recent US National Health and Nutrition Examination Survey, even children with relatively lower levels of pesticide exposure had a higher risk of ADHD.
Fruits and vegetables were the most common source of pesticide exposure in children according to Professor Maryse Bouchard of Sainte-Justine University Hospital Research Centre in Montreal, Canada. Her recommendation to parents was to buy organic or locally grown produce and to carefully wash fruits and vegetables to remove water-soluble chemicals.
In another longitudinal study, a team of researchers at the University of California at Berkeley lead by Brenda Eskenazi, (professor of epidemiology and of maternal and child health) followed more than 300 children to determine the effects of prenatal pesticide exposure on a baby’s rapidly developing nervous system (Center for the Health Assessment of Mothers and Children of Salinas (CHAMACOS),.
The results suggest that “each tenfold increase in prenatal pesticide metabolites was linked to having five times the odds of scoring high on the computerized tests of attention at age 5, suggesting a greater likelihood of a child having clinical ADHD. The effect appeared to be stronger for boys than for girls”. This longitudinal study will continue to follow the children as they get older and the UC Berkeley researchers expect to present more results in the years to come.
Amy Marks, an analyst at UC Berkeley’s School of Public Health at the time of the study concluded that ” there is reason to be cautious, especially in situations where exposure may coincide with critical periods of fetal and child development.” (Aug 19, Environmental Health Perspectives–EHP ).
Genetics also increase the risk of developing ADHD in vulnerable children. In a separate study published in the same journal (EHP) researchers found that 2-year-olds with lower levels of an enzyme (paraoxonase 1 (PON1) that breaks down the toxic metabolites of organophosphate pesticides, had more neurodevelopmental delays than those with higher levels of the enzyme.
Organophosphate pesticides are designed to attack the nervous system of organisms by disrupting neurotransmitters, particularly acetylcholine, which plays an important role in sustaining attention and short-term memory. Lead and phytates commonly used in toys and plastics have also been linked to ADHD. These studies add to the mounting evidence that chemical assaults on the most vulnerable in our society at critical periods of brain development can have disastrous effects on learning and behaviour.
“High levels of the symptoms of ADHD by age 5 are a major contributor to learning and achievement problems in school, accidental injuries at home and in the neighborhood, and a host of problems in peer relationships and other essential competencies,” says Stephen Hinshaw, professor of psychology and one of the country’s leading experts on ADHD, who was not part of this study. “Finding preventable risk factors is therefore a major public health concern.”