Chlorpyrifos is an organophosphate insecticide that has been used since 1965 to kill insects on farms, in homes, and in commercial buildings. It works by attacking the nervous system of insects, but that same mechanism can affect humans, which is why it has become one of the most controversial pesticides in the world. Once among the most widely applied insecticides in the United States, its use has been sharply restricted in recent years due to evidence linking it to brain development problems in children.
How Chlorpyrifos Works
Chlorpyrifos kills insects by blocking an enzyme that breaks down a chemical messenger called acetylcholine. Acetylcholine is what nerves use to signal muscles and other nerves. When the enzyme is blocked, acetylcholine builds up at nerve junctions, causing uncontrolled, continuous nerve firing. For an insect, this leads to paralysis and death. The problem is that humans and other mammals rely on the same signaling system, which means chlorpyrifos can interfere with our nervous systems too.
The pesticide doesn’t do this directly. Once inside the body, liver enzymes convert chlorpyrifos into a more potent form called its “oxon,” which is the compound that actually locks onto and permanently disables the enzyme. This conversion happens in insects and mammals alike, though the dose needed to cause serious harm is much higher in humans than in insects.
Acute Symptoms of Exposure
The symptoms of chlorpyrifos exposure reflect what happens when acetylcholine floods the nervous system. At low levels, breathing it in or ingesting it can cause headaches, blurred vision, watery eyes, a runny nose, dizziness, confusion, nausea, and diarrhea. Muscle weakness and tremors are also common. Heart rate may change suddenly.
At high levels, the effects escalate to severe sweating, loss of bowel control, intense muscle tremors, seizures, loss of consciousness, and potentially death. These severe poisoning cases are most often seen in farmworkers who handle the chemical directly or in cases of accidental ingestion. The speed and severity of symptoms depend on how much was absorbed and whether it was inhaled, swallowed, or absorbed through the skin. Workplace safety limits for airborne chlorpyrifos are set extremely low, at 0.2 milligrams per cubic meter of air, and the chemical carries a skin absorption warning, meaning it can enter the body through contact alone.
Effects on Children’s Brain Development
The most alarming research on chlorpyrifos involves its effects on developing brains. A study published in Environmental Health Perspectives tracked children from before birth to age seven and found that prenatal exposure was associated with measurable drops in IQ and working memory. For each increase in exposure equivalent to one standard deviation, full-scale IQ declined by about 1 to 1.8 points and working memory scores dropped by 1.6 to 3.7 points. Those numbers may sound small on an individual level, but across a population of exposed children, they represent a significant shift.
Earlier results from the same research group had already found that prenatal exposure was linked to mental and motor delays by age three, with exposed children scoring 3.5 to 6 points lower on developmental scales. There was also evidence of increased attention problems and behaviors associated with ADHD in toddlers whose mothers had higher exposure during pregnancy. These findings were central to the push to restrict chlorpyrifos in food production.
Where People Encounter It
Chlorpyrifos was originally used in homes, lawns, and gardens as well as on farms. Residential use in the U.S. was phased out in 2000, but agricultural use continued for years on crops like apples, citrus, corn, soybeans, and a range of vegetables. Internationally, it remains widely used. A study of produce in Bangladesh found chlorpyrifos residues in 80% of eggplant samples, 68% of cabbage samples, and 44% of cauliflower samples tested. The majority of positive samples exceeded maximum residue limits. Residues were found not just in fields but also in produce purchased at local markets.
For most people who don’t work on farms, food is the primary route of exposure. Farmworkers and their families face higher risk through direct handling, drift from sprayed fields, and residues tracked into homes on clothing and shoes.
What Happens in the Environment
Chlorpyrifos binds tightly to soil particles, which limits how far it travels in its original form. Its persistence in soil varies depending on temperature, sunlight, and microbial activity. However, when it breaks down, its primary byproduct is more water-soluble than the parent chemical and more likely to leach into groundwater. This means that even after the pesticide itself degrades, its breakdown product can continue to pose a contamination risk to drinking water sources.
Regulatory Status
The EPA revoked all food-use tolerances for chlorpyrifos in 2022, effectively banning its use on crops grown for human consumption in the United States. This came after years of legal battles and mounting scientific evidence about neurodevelopmental harm. Several states, including California, Hawaii, and New York, had already moved to ban or restrict it before the federal action. The European Union banned chlorpyrifos in 2020.
In many other countries, chlorpyrifos remains legal and widely available. This means imported produce may still carry residues, though shipments are subject to testing and tolerance limits at the border.
Reducing Your Exposure
If you’re concerned about pesticide residues on produce, a few simple steps can help. Wash all fruits and vegetables thoroughly under running water, rubbing them briskly with your hands. For firm-skinned produce like apples and cucumbers, use a vegetable brush. Peeling removes more residue than washing alone, particularly for root vegetables and waxy fruits. For leafy greens like cabbage and lettuce, separate the leaves and rinse them individually, discarding the outer leaves, which tend to carry more residue.
The FDA advises against using commercial produce washes, as their effectiveness hasn’t been standardized. A vinegar rinse (half a cup of white vinegar per cup of water, followed by a clean water rinse) has shown some ability to reduce surface contaminants, though it may affect taste. Eating a wide variety of fruits and vegetables also helps limit repeated exposure to any single pesticide. No washing method removes all residues completely, but consistent rinsing under running water makes a meaningful difference.