Can Pesticide Exposure Lead to Brain Damage?

"Pesticides were designed to be neurotoxic.   Why should we be surprised if they cause neurotoxicity?"  - Bruce Lanphear, M.D, M.P.H

More than five decades since Rachel Carson alerted the world to the dangers of pesticide exposure with her classic book, Silent Spring, the longterm consequences of widespread pesticide use continues to be felt worldwide.  Despite stringent legislation designed to curb the use of pesticides, research investigating the medical consequences of exposure has raised disturbing new questions about how this exposure is affecting children.   Studies have already linked organophosphates, which are found in many pesticides and herbicides, with neurodegenerative diseases in farm workers and laboratory animals.   Linking organophosphate pesticide use to the rise in new cases of  attention-deficit hyperactivity disorder (ADHD) and learning disabilities has been more controversial, however. 

Most organophosphate pesticides work by inactivating the enzyme cholinesterase in insects.  This then leads to the breakdown of acetycholine, which acts as a neurotransmitter in many organisms (including humans).   The same neurotoxic effects which kill insects can also affect humans and animals as well.    While most commercial pesticides carry instructions regarding proper use,  the presence of pesticide residue in many foods and even local water supplies can lead to toxic effects even in minute quantities.   Symptoms of organophosphate exposure can include fatigue, dizziness, nausea, headaches, sweating, and vomiting.   More severe symptoms include abdominal cramps, diarrhea, motor impairments, and breathing problems.   Since pesticide exposure symptoms are often mistaken for other medical conditions, the true cause may go untreated until too late.   While EPA worker protection standards are in place to protect agricultural workers, they are often inadequate since they do not acknowledge the greater risk that organophosphates pose for pregnant women and their children.   At this time, only California, Ohio, Arizona and Colorado have formal testing standards in place to protect people at risk for pesticide exposure.

Among the organophosphates that have raised particular concern has been chlorpyrifos, a popular insecticide which was phased out as a home insecticide in 2001 due to neurotoxic effects.  Before being banned for household use,  researchers found detectable levels of chlorpyrifos in virtually all African-American and Dominican pregnant women living in public housing in New York City.   While home insecticides no longer contain chlorpyrifos, it continues to be widely used in agricultural settings.   As for newer insecticides for use in the home, including  pyrethroid insecticides,  evidence suggests that they are potentially neurotoxic as well 

For more than twenty years, a research program at the University of California at Berkeley  has been investigating whether  organophosphate exposure can be contributing to abnormal brain development in children living in California.   Two additonal research programs run by Mount Sinai Hospital and Columbia University have been doing similar research looking at low-income children in New York City. All three studies have found a significant link between organophosphate exposure and developmental disorders in children including lower IQs and attention deficit problems.

The studies used similar research methodology, including recruiting hundreds of pregnant women and testing them for pesticide exposure while following their children for years afterward to measure potential learning problems.    For all three studies, the children with the highest pesticide exposure showed abnormal reflexes as infants, ADHD-like behaviour by five years of age, and significantly lower IQ scores by the age of seven.    MRI research also showed significant brain anomalies in children with high prenatal pesticide exposure.   This includes significant cortical thinning in the frontal and parietal lobes which may be linked to attention problems and development delays.

 One surprising result was that even pregnant women living in urban areas showed evidence of pesticide exposure.  While largely viewed as a risk for pregnant women living in agricultural settings, 71% of the pregnant women enrolled in the study showed detectable levels of pesticide exposure.    Even in cities, organophosphate pesticides can be purchased in spray form to use on indoor pests such as roaches and ants and women living in low-income housing remain especially vulnerable due to high levels of pesticide use.

As for the long-term consequences of widespread pesticide exposure,  researchers are only beginning to discover the true scope of the medical and social costs involved.   As organophosphates  known to be neurotoxic are phased out of general use, companies are often replacing them with newer chemicals with properties that are less well-understood.   So long as pesticides continue to be used in large quantities to control pests, the potential medical risks will likely remain. 

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