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Pesticides in combination increase toxicity: New study from UCLA

Pesticides in combination increase toxicity: New study from UCLA

By Amy K. Liebman, Director of Environmental and Occupational Health, Migrant Clinicians Network

Before a pesticide is approved for use, regulators test the health effects of a pesticide as a single agent. But, on fields across America, pesticides are often applied as mixtures, and research has lagged in determining the interactive effects of pesticides. A new report from the Sustainable Technology and Policy Program out of the University of California, Los Angeles determined that three pesticides, used in combination, became more toxic. The three fumigants -- chloropicrin, 1,3-dichloropropene, and metam salts -- are often used simultaneously on common crops like strawberries, tomatoes, peppers, and nuts. The report, which focuses on California agriculture, notes that there were thousands of applications of the pesticides, totalling  millions of pounds of the fumigants applied in California alone. 

The study determined that the pesticides in combination have a “reasonable likelihood” to be more toxic to humans by decreasing the body’s ability to detoxify, attacking and damaging DNA, and disabling DNA repair and expression enzymes. “Taken together, these three possible interactive effects would result in a greater likelihood of unrepaired mutations and/or uncontrolled cell growth. This would potentially increase the likelihood of cancer,” the authors note in their case study summary.

California agricultural workers and agricultural communities, the authors argue, are often exposed to the fumigants in combination. Because the tested pesticides are fumigants, they are “likely to volatilize and drift away from the application site and expose people in surrounding schools, houses, businesses, and fields,” the summary notes. As an illustration, the researchers presented data from chemical levels at Rio Mesa High School in Oxnard, CA, which is bordered by farmland. 

The article notes that regulators like California’s Department of Pesticide Regulation (DPR) do not sufficiently regulate the application of multiple pesticides to determine cumulative effects on human health, but are required to do so under California law to protect public health. The researchers recommend that regulators should: test all pesticides sold as a mixture for synergistic toxic effects before approval for use; require testing on chemicals in combination or impose “stringent regulations” on the use of chemicals in combination on the fields when there is “a scientifically reasonable hypothesis of synergistic effects”; and include synergistic effects of pesticides in establishing risk management requirements. 

Access the complete report, the summary, and an interactive pesticide map at