Arsenic-laden groundwater used for cooking and drinking could pose a risk to the health of almost 20 million people across China. This is shown by a study carried out by Eawag scientists in collaboration with Chinese colleagues and published today in Science. The estimates are based on a risk model incorporating geological and hydrological data, as well as measurements of arsenic in wells. The study is being adopted by the authorities in the national groundwater monitoring programme. Most of the risk centers in a few provinces—Xinjiang, Inner Mongolia, Henan, Shandong, and Jiangsu—but the total population exposed to arsenic levels above 10 micrograms per liter could be upwards of 19 million people.