Last year, there was considerable news coverage on the Environmental Protection Agency’s (EPA) decision to not ban chlorpyrifos, a pesticide which is believed to cause negative effects on brain development in children. While a lot of the media coverage focused on the nature of the decision, little was reported on the science itself. Thus, I have summarized below EPA’s assessment on the health effects of the pesticide on humans.
Chlorpyrifos is widely used to control insect pests in crop fields, golf courses, greenhouses, nurseries, etc. It can be applied aerially to control mosquitoes as well. While the residential or indoor use of chlorpyrifos was banned in 2000, it is still used in baits to control cockroaches and ants. Chlorpyrifos acts by inhibiting an enzyme which is essential for proper cell communications in our brain. This can lead to drooling, nausea, dizziness, muscle cramps, tremors, weakness and loss of coordination in humans. In severe cases, it can cause unconsciousness, convulsions, breathing difficulties and paralysis.
Whether exposure to chlorpyrifos would have a strong adverse effect on a child’s brain development, is still under discussion. Several studies have shown that children exposed to chlorpyrifos during pregnancy had delayed brain development, attention problems, autism and reduced IQ. These effects seemed to occur at concentrations which caused less than 10% inhibition of the essential enzyme. However, there have been few studies which showed no link between chlorpyrifos exposure and brain development.
In the end, EPA largely relied on one chlorpyrifos study where 265 children were exposed to chlorpyrifos prior to birth and subsequently showed a decrease in IQ at the age of 7. The children were born prior to 2000, when residential applications were common. Chlorpyrifos was detected in the 100% of the homes of pregnant mothers and in 70% of the umbilical cord blood collected from their babies.
The group which conducted the prenatal chlorpyrifos exposure study. This study was central to EPA’s assessment on chlorpyrifos risks to humans.
EPA estimated the amount of chlorpyrifos that pregnant women and children would have gotten exposed to, prior to its indoor use elimination in 2000. They used a computer model to predict this and considered all exposure routes (food, drinking water, inhalation and touch). They found that the highest chlorpyrifos concentration that pregnant women and children could get exposed to would be less than the concentration that causes inhibition of the essential enzyme. This means that chlorpyrifos causes adverse health effects even at doses that were previously thought to be safe.
EPA then assumed that a concentration which is 100 times lesser than that estimated by the model would be the maximum concentration that is safe for humans. They did this because it is unknown what concentration of chlorpyrifos is safe and because some humans could be more susceptible to effects of chlorpyrifos than others.
A Scientific Advisory Panel, which acted as an independent advisory to EPA, noted that there are several uncertainties in this assessment. EPA majorly relied on a single study, which was not replicated. The scientists in that study declined to share the original data with EPA, citing privacy concerns as human subjects were involved. Also, the children in this study were exposed to other chemicals like lead and it is unclear if chlorpyrifos alone caused the adverse effects. And the mechanism through which chlorpyrifos causes decreased IQ is unknown.
After the year 2000, possible exposure to chlorpyrifos might occur from treated food, drinking water, mosquito sprays, treated golf courses, application of cockroach baits in homes and drift of chlorpyrifos from agricultural fields. EPA calculated the combined chlorpyrifos concentration humans could get exposed to from all these routes. They found this concentration to be greater than the concentration they consider safe. Thus, in 2015, EPA revoked its previously determined safe limits for chlorpyrifos exposure. However, in 2016, EPA decided not to ban the pesticide. It will continue evaluating effects of chlorpyrifos on brain development and come up with a final assessment in 2022.