Photo: Soloviova Liudmyla/Shutterstock

Monterey County, in Northern California, is one of those places that appear to tell a tale of two Americas. The part that runs along the Pacific coastline, from Pebble Beach and Carmel in the north down to Big Sur, is breathtaking and breathtakingly affluent. Travel 20 miles inland, over a narrow ridge of mountains, and you end up in the Salinas Valley, informally known as the Salad Bowl of the World, which is greened by endless rows of lettuce, cauliflower, and broccoli erupting out of rich brown soil and tended by a predominantly immigrant and poor community of farmworkers. Some of those fields run practically up to the main entrance of Natividad Medical Center in the town of Salinas, where, more than two decades ago, anenvironmental epidemiologist named Brenda Eskenazi came to study the effects of pesticides on children’s brain development. If there is a comforting illusion that barriers — be they a ridge of mountains or sheer wealth or a “wall” — can somehow seal off the dangers of modern life, the data that Eskenazi and her colleagues at the University of California, Berkeley, have produced tell a different story.

Sitting in an office on the grounds of the Natividad center recently, Eskenazi, by turns caustic and conscientiously precise, jokes that she could not have picked a more inconvenient group of experimental subjects. More than half of the primarily Latina mothers she and her team began studying in 1999 lived at or below the poverty level, 85 percent of them came from Mexico