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Pregnancy & Organophosphate Pesticides

How Many Farmworkers Live within a Kilometer of Organophosphate Applications while Pregnant – And What Are the Health Implications?

Organophosphate pesticides (OPs) are a class of insecticides applied widely across agricultural regions in the United States. A 2025 study published in BMC Public Health found that 7.5% of pregnant Californians lived within a kilometer of fields treated with OPs during their pregnancy. In Monterey County, however, that number jumps to 50.1%.  Known as the “Salad Bowl of the World,” Monterey County produces significant amounts of the nation’s leaf lettuce (61%), celery (57%), head lettuce (56%), broccoli (48%), spinach (38%), cauliflower (30%), and strawberries (28%), among other crops.   One in five households works in agriculture. Many of these crops are labor intensive, requiring care and harvesting by hand, and consequently the region has a high number of farmworker families living near fields. 

One Kilometer Buffer Zone

Monterey County has long served as a location for research on the impact of pesticide exposure on farmworker children, including data collection for the groundbreaking Center for the Health Assessment of Mothers and Children of Salinas (CHAMACOS) studies. In 2017, CHAMACOS researchers evaluated prenatal residential proximity to five potentially neurotoxic pesticide groups -- organophosphates, carbamates, pyrethroids, neonicotinoids, and manganese fungicides -- as well as five individual organophosphates: acephate, chlorpyrifos, diazinon, malathion, and oxydemeton-methyl. Researchers then conducted a cognitive assessment when the children of those pregnancies reached seven years of age. The CHAMACOS researchers found a decrease of 2.2 points in IQ and 2.9 points in Verbal Comprehension for each standard deviation increase in toxicity-weighted use of OPs.   Numerous additional studies from CHAMACOS and others have shown associations between prenatal OP exposure and cognitive, behavioral, sensory, motor, and morphology issues in children,   including autism spectrum disorders  and attention-deficit/hyperactivity disorder.   The 2025 Monterey Co. study, “Temporal trends of agricultural organophosphate pesticide use in California and proximity to pregnant people in 2021,” chose the one-kilometer buffer zone to be consistent with the CHAMACOS study, to demonstrate the ongoing use of OPs in California even after the 2020 ban of the most used OP, chlorpyrifos.

Organophosphates in California

OPs are not limited to the crops common in Monterey Co. OPs are used across many crops, including almonds, citrus, grapes, walnuts, alfalfa, and cotton.  In 2019, the California Environmental Protection Agency banned the use of chlorpyrifos with a use-end date of December 31, 2020. Chlorpyrifos had already been banned from residential use since 2001. Both bans were a result of research indicating that chlorpyrifos is associated with brain and neurological development impairment among children at low exposure levels. 

By the time of the ban, chlorpyrifos use in California agriculture had already declined more than 50%, from two million pounds in 2005 to about 900,000 pounds in 2017,  perhaps as growers sought safer alternatives. Despite the decline, chlorpyrifos was still the most widely used OP in California before the ban.  The 2025 Monterey Co. study sought to examine the continued use and distribution of non-chlorpyrifos OPs in California, noting that:

The US Environmental Protection Agency (EPA) is reviewing the OP class of pesticides and is projected to make regulatory determinations for individual pesticides and a cumulative assessment of the risk from combined exposure to the entire class before October 2026. Additional assessments of non-chlorpyrifos OP pesticides have not been announced in California, nor have there been studies evaluating replacement pest control practices. Although multiple scientific reviews have found evidence of developmental neurotoxicity for individual OP pesticides and combined OP pesticide exposures, there remains considerable variability in the consideration of developmental neurotoxicity in regulatory assessments among countries and within the United States.

The most common OPs used within one kilometer of prenatal residences after the chlorpyrifos ban were acephate (59% of the applications within one kilometer), malathion (40%), naled (16%), dimethoate (16%), bensulide (15%), and diazinon (10%). (The total is above 100% because between one and six different OPs were used within one kilometer of a residence during pregnancy; as the authors note in the quote above, the health consequences of overlapping or ongoing OP exposures during pregnancy are unknown.)  The study found that Hispanic people had the highest number and greatest proportion of the population exposed to OPs, California-wide. While Monterey Co. had the highest percentage of prenatal residential proximity to OP use, eight other agricultural counties had higher than 20% of pregnancies with OP use within one kilometer of residence: Santa Barbara (43.8%), San Benito (35%), Santa Cruz (29%), Merced (28.6%) Sutter (24.5%), Ventura (24%), Imperial (23%), and Tulare (23%).

The study is intended to be a first step in identifying what OPs are used and where, to better assess the long-term health implications of the rapid decline of chlorpyrifos and the implemented alternatives, including other OPs, on vulnerable populations like pregnant farmworkers. “This study offers important information to consider, particularly for women of reproductive age working in and/or living near agricultural areas,” said Amy K. Liebman, MPH, Chief Program Officer of Migrant Clinicians Network, who has facilitated national pesticide safety programs and trainings for clinicians serving farmworkers for decades. “Given studies showing associations between prenatal OP exposure and cognitive, behavioral, sensory, motor, and morphology issues in children, finding safer alternatives, limiting exposure, and taking other steps to protect farmworker women is needed.” 

Resources

MCN will feature several pesticide-related webinars this spring and summer. 

Authors

Claire

Seda

Director of Communications

MCN