Early one winter morning, as Brian Lilla was riding his bike through Napa, California’s hills and meadows, he spotted farmworkers driving ATVs through rows of vines. They hauled huge canisters of the weedkiller Roundup. As the workers sprayed vines, a chemical smell shot through the air.

Having moved to Napa from Oakland nine years ago with his wife to start a family in the seemingly healthy environs of the country, Lilla found himself paying more attention to the use of Roundup after his daughters, now 8 and 5, were born. One day, he recalled, he saw the herbicide sprayed in a vineyard across the street from his daughters’ school.