Nearly 20 million people in China are exposed to high levels of arsenic in the water they use for drinking and cooking, a new model based on geological andhydrological data and well samples shows. The model predicts high arsenic concentrations (10 micrograms per liter or greater) across more than 580,000 square kilometers, according to Chinese and Swiss researchers, who published their findings inScience. Researchers had long known that some regions had high arsenic concentrations, but it would have taken several decades to test the millions of wells in China. The new prediction combined the most recent tests with data about the underlying geology, soil characteristics, and topography.