It is called the largest mass poisoning in history. Wells drilled in the Indian subcontinent in the 1970s exposed millions to arsenic leaching into their drinking water from surrounding rocks and soil; this has caused skin lesions, cancer, cardiovascular diseases, and neurodevelopmental delays. A new study now suggests that up to sixty million Pakistanis might also be exposed to contaminated water. The World Health Organisation warned in the 1990s that up to 77 million people in Bangladesh were in danger of drinking water with unsafe levels of arsenic, and 200 million people in Nepal, Bangladesh, India and Vietnam were exposed to concentrations exceeding the recommended limit. Experts hope this new study will motivate Pakistan’s authorities to test wells in high-risk areas and to warn communities. If people are aware of the risks they could use water from deeper aquifers that are in contact with older sediments, or invest in treating groundwater to remove arsenic.