Arsenic-contaminated groundwater may threaten the health of 50 to 60 million people in Pakistan. This is shown by a study in which data from 1,200 groundwater samples was analyzed and combined with hydrological parameters to generate a hazard map. This reveals for the first time the full extent of the risks to which the population of Pakistan is exposed. In addition, there is growing evidence that natural arsenic levels are increased by extensive irrigation.