Are organophosphate pesticides like chlorpyrifos harmful to children's brain development?

What they are. Organophosphates block acetylcholinesterase, an enzyme that switches nerve signals off⁷. At high doses they cause acute poisoning. At lower doses over time, studies have linked prenatal exposure to cognitive and behavioral problems in children⁷. Chlorpyrifos, used since 1965, is the best known².

How children are exposed. Homes were once a major source. In 2000 manufacturers agreed to eliminate most homeowner uses of chlorpyrifos², and New York newborns born after January 2001 had much lower exposure and no link to birth size³. Today the routes are food residues, drinking water and drift near farmland^2,4.

What the cohorts found.

• New York (Columbia): children with the highest cord-blood chlorpyrifos scored 6.5 points lower on a psychomotor index and 3.3 points lower on a mental development index at age 3⁵. At age 7, each standard-deviation rise went with 1.4% lower full-scale IQ and 2.8% lower working memory⁸.
• California (farmworker families): children of mothers in the highest fifth of urinary organophosphate metabolites averaged 7.0 IQ points lower at age 7⁶.
• New York (Mount Sinai): prenatal metabolites went with poorer perceptual reasoning, especially in children whose mothers had a slow-clearing enzyme variant¹⁰.
• Cincinnati: no link to thinking skills at ages 1 to 5 in 327 mother-child pairs⁹.

Reviews disagree. A systematic review found that most prenatal studies showed lower mental development and more attention problems¹¹. A review funded by the pesticide industry's trade association said the human studies did not strongly implicate any one pesticide as a cause¹². Exposure is hard to measure: urine metabolites are not specific and can come from residues already on food⁹.

US rules. In 2017 the EPA said the science on neurodevelopment was unresolved. A court ordered action, and in 2021 the agency revoked all chlorpyrifos food tolerances. An appeals court vacated that rule in 2023 and the tolerances returned. Food uses have since been canceled except for 11 crops, and in December 2024 EPA proposed revoking all other tolerances; its page, last updated 24 August 2026, reported no final rule².

What to do. Wash produce and choose organic when you can (see pesticide residues on produce). For other farm chemicals, see glyphosate and atrazine.

New York cohort

The Columbia cohort measured chlorpyrifos in umbilical cord plasma. Among 254 children, those above 6.17 pg/g scored 6.5 points lower on the Bayley Psychomotor Development Index and 3.3 points lower on the Mental Development Index at 3 years; delayed psychomotor development was 5 times and delayed mental development 2.4 times more common, and attention, ADHD and pervasive developmental problems were more likely⁵. At 7 years in 265 children, each standard-deviation rise (4.61 pg/g) went with a 1.4% fall in full-scale IQ and a 2.8% fall in working memory, adjusted for maternal education, maternal IQ and home environment⁸.

California and Mount Sinai cohorts

In the CHAMACOS birth cohort of farmworker families, 329 children had IQ tested at 7 years. Maternal dialkylphosphate metabolites averaged over pregnancy went with poorer working memory, processing speed, verbal comprehension, perceptual reasoning and IQ; the highest quintile had a 7.0-point deficit. Children's own urinary metabolites were not consistently linked, and maternal levels were higher than typical but within the general US range⁶. In Mount Sinai's cohort, prenatal metabolites went with a decrement in mental development at 12 months among Black and Hispanic children, with enhanced effects in mothers carrying the PON1 Q192R QR/RR genotype, and with lower perceptual reasoning at ages 6 to 9 in mothers with the QQ genotype¹⁰.

The null result

The Cincinnati cohort measured six dialkylphosphates twice in pregnancy in 327 mother-infant pairs and tested children from age 1 to 5. It found no associations. Mothers with higher metabolite levels had higher socioeconomic status and ate more fresh fruit and vegetables, and the authors suggested a healthier diet may protect the fetus, or that dietary metabolites are harmless and a poor measure of exposure to the parent compound⁹.

Systematic reviews

A review of 20 studies (7 prenatal, 8 postnatal, 5 both) found that most prenatal studies observed worse mental development and more attention problems, postnatal evidence was less consistent, and designs varied so much that standardized methods are needed before firm conclusions¹¹. The industry-funded review concluded that, as a whole, the epidemiology did not strongly implicate any particular pesticide as causally related to adverse outcomes and that animal effects generally appeared at doses at or above those in regulatory studies¹². Its funder was the European Crop Protection Association.

Exposure and regulation

Among 314 mother-newborn pairs, each log-unit rise in cord chlorpyrifos went with 42.6 g lower birth weight, and the association was strong before the 2000 to 2001 residential phase-out but absent in newborns born after January 2001, when exposure was substantially lower³. In Washington State, a urinary dimethyl organophosphate metabolite (DMTP) was higher in farmworkers (71 versus 6 µg/L) and their children (17 versus 8 µg/L) than in non-farmworker households, and fell 20% for each mile from farmland⁴. The EPA's 2020 assessment judged chlorpyrifos residues in food not of concern but found drinking water added significantly to risk. Courts then reshaped the legal outcome: the Ninth Circuit forced the 2021 revocation, and the Eighth Circuit vacated it in November 2023, noting EPA should have considered modifying tolerances, with 11 crops as candidates².

Where the evidence conflicts

Most data are urinary metabolites that are not specific to one pesticide, and mothers with higher levels may differ in income and diet. The cohorts differ in design and results, one is null, and a randomized trial of this exposure would be unethical. In the New York cohort, exposure fell sharply after the residential bans, so today's exposures are probably lower than in the earliest cohorts³.

Bottom line

Confidence is 3 of 5. Several independent birth cohorts and systematic reviews point to harm from prenatal exposure at levels people actually had. The exposure measures are imperfect, and current exposures are probably lower, so sensible precautions in pregnancy are justified while the science and rules keep moving.