Around the world thousands of people live in rural areas and less-developed countries and they rely on wells and other potentially untreated sources of water for drinking, cooking and cleaning. However, these sources are at a high risk of contamination. Often when one hears the words water contamination it is assumed to come in the form of biological sources, such as bacteria, protozoa or parasire. Indeed, this is a major concern and has led organisations like the United Nations Children's Fund (UNICEF) and the World Health Organization (WHO) to campaign for the drilling of deep water well because they are less likely to be contaminated with waterborne diseases. However, the move to deeper wells and the growing availability of private wells in places where economic conditions improved has resulted in the uncovering of a hidden health problem - arsenic poisoning. Unlike biological waterborne diseases, such as cholera or dysentery, arsenic is a chemical contaminant of water. Arsenic is naturally found in clay sediments and can leak into well-water regardless of the depth. Consequently, widespread arsenic contamination has occurred around the world, and, in particular, in several regions in Asia. It can be difficult to know if and when a water source is contaminated with arsenic because unless the dose is high the common symptoms associated with poison or waterborne disease, such as abdominal pain, vomiting or diarrhoea, do not occur. But, even at lower doses arsenic causes serious long-term health problems, including skin lesions and cancer. Therefore, wells must be tested before they are used and this testing must continue periodically throughout the lifetime of the well. This is of course difficult in resource-poor settings and in the cases of private wells, government- or NGO-assisted screening programmes may not cover all wells. These two factors have made arsenic contamination a widespread and often unknown health issue for many. Takahiko Yoshida, professor at the Asahikawa Medical University in Hokkaido, Japan, has studied this problem for decades. Yoshida and his colleagues are focused on prevention measures, but they stress that a better understanding of the chronic and long-term health effects of arsenic poisoning is first required.