High exposure to agricultural pesticides as a result of living close to fields increases the risk of adverse birth outcomes, according to a study published in the journal Nature Communications. Researchers from the University of California, Santa Barbara, have gathered new evidence on the negative effects of pesticide exposure on birth outcomes, such as weight, gestation and abnormalities. For the study, they used individual birth and demographic characteristics for over 500 000 birth observations between 1997–2011 in the agriculturally dominated San Joaquin Valley of California. The San Joaquin Valley is the state’s most productive agricultural region, growing an abundance of high value, high chemical input, and labor-intensive fruit, vegetable, and nut crops. The researcher coupled the birth statistics with pesticide use data from the California Department of Pesticide Regulation which includes detailed temporal and geographical information on agricultural pesticide use, the date of pesticide application, pounds of active ingredients and the method of application. The scientists were then able to determine if residential agricultural pesticide exposure during gestation - by trimester and by toxicity - influenced birth weight, gestational length or birth abnormalities.

For the majority of births, there was no statistically identifiable impact of pesticide exposure on birth outcome. But especially for the mothers exposed to very high levels of pesticides, the researchers found negative effects for all birth outcomes - birth weight, low birth weight, gestational length, preterm birth, birth abnormalities. “Mothers exposed to extreme levels of pesticides, defined here as the top 5 percent of the pesticide exposure distribution, experienced between 5 and 9 percent increases in the probability of adverse outcomes with an approximately 13-gram decrease in birth weight,” said lead author Ashley Larsen, an assistant professor in UCSB’s Bren School of Environmental Science & Management. The top 5th percentile group was exposed to 4,200 kilograms of pesticides applied in the 1-square-mile regions encompassing their addresses during pregnancy.

The authors say that there is still a lack of research on the adverse health impacts of agricultural pesticide, which may in part be due to logistical challenges of health research. Since controlled studies are clearly unethical, much of the available evidence relating pesticides to adverse health outcomes comes from occupationally exposed groups, such as certified pesticide applicators which may not reflect exposures that are relevant for the broader agricultural community. While the authors describe their findings as “the most comprehensive to date”, they were unable to isolate the roles of individual chemicals and their mixtures in driving the negative outcomes. “We don’t have a good understanding of how different chemicals interact with each other in the environment,” Larsen said. “Additional work is needed to understand which chemicals or combinations of chemicals are most dangerous to human health.” (ab)