A baby is more likely to suffer from ADHD (attention deficit/hyperactivity disorder) while it is still in the womb if the mother is exposed to organophosphate (OP) pesticides, according to a new study published in Environmental Health Perspectives. Over the last few years there has been increasing interest in determining whether OP pesticides might undermine brain development.

OP pesticides targets insects by affecting their nervous systems – they affect the activity of neurotransmitters. Neurotransmitters are chemicals that are released from nerve (brain) cells which thereby transmit an impulse from one nerve cell to another nerve cell, muscle, organ or other tissue – they are messengers of neurologic information that passes from one cell to another.

OP pesticides affect the activity of acetylcholine, a type of neurotransmitter which is vital in brain development and is involved in attention and short-term memory.

The authors explain that OP pesticide exposure may also undermine DNA replication and the growth of nerve axons and dendrites. As children have not fully developed the ability to produce an enzyme that detoxifies OP pesticides, they are much more vulnerable to the effects.

Mothers who took part in the study were recruited while they were pregnant by CHAMACOS (Center for the Health Assessment of Mothers and Children of Salinas). The women, from Salinas Valley, lived in an area of intensive farming where over 235,000 kilograms of pesticides are applied every year.

The scientists analyzed six OP metabolites in urine samples collected from the mothers during pregnancy and from their children at different intervals after birth. By detecting and measuring these metabolites, the investigators could determine levels of exposure to OP pesticides used in the Salinas Valley, such as chlorpyrifos, diazinon, and oxydemeton-methyl.

With the aid of standardized psychological tests, as well as reports from the mothers, the researchers assessed the children’s behavior when they were 3 and a half (n=331) and 5 years (n=323) old.

The scientists found a close link between the pregnant mothers’ OP metabolite urine levels and the probability of a clinical diagnosis of ADHD among the children.

The link was stronger when the children were tested at the age of 5 years than at 3-and-a-half. The authors report that ADHD symptoms were more pronounced among the boys than the girls.

They also found that exposure before birth had a bigger impact on ADHD risk than exposure 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
  • A tenfold increase in levels of metabolites in the children’s urine at 5 years of age corresponded to a 30% higher likelihood

This study complements a previous one which measured OP metabolites in the urine of 1,139 children aged 8 to 15 years, and found an association with ADHD risk. The previous study was published in the journal Pediatrics in June 2010.

The authors of the current study say that investigations should continue to determine whether genetic variations in OP metabolism affect susceptibility to developmental disorders, such as ADHD.

The researchers said:

..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.

“Organophosphate Pesticide Exposure and Attention in Young Mexican-American Children”
Amy R. Marks, Kim Harley, Asa Bradman, Katherine Kogut, Dana Boyd Barr, Caroline Johnson, Norma Calderon, and Brenda Eskenazi
Environmental Health Perspective August 2010. doi:10.1289/ehp.1002056

“PON1 and Neurodevelopment in Children from the CHAMACOS Study Exposed to Organophosphate Pesticides in Utero”
Brenda Eskenazi, Karen Huen, Amy Marks, Kim G. Harley, Asa Bradman, Dana Boyd Barr, and Nina Holland
Environmental Health Perspective August 2010. doi:10.1289/ehp.1002234

Written by Christian Nordqvist