NEW DELHI: About 239 million people across 153 districts in 21 states drink water that contains unacceptably high levels of arsenic. In effect, they are being slowly poisoned. Calculations based on information provided by the water resources ministry in response to a question in the Lok Sabha reveal that 65% of Assam ’s population, or about 21 million people, is drinking arsenic-contaminated water, while it’s 60% in Bihar and 44% in West Bengal In terms of absolute numbers, however, Uttar Pradesh has the largest number of people, over 70 million, exposed to arsenic-contaminated water.How did we get to these numbers? The response to the question contained a list of the 153 districts. We then got the population figures for each district from the census.The census also gives details of the proportion of households in each district that depend on different sources of water for drinking. Hence, we took only those households that are dependent on groundwater sources like wells, tubewells and hand pumps. The population figures were then adjusted for 2016 based on the census’s own projections for states.The World Health Organisation has warned that longterm intake of such water leads to arsenic poisoning or arsenicosis, with cancer of skin, bladder, kidney or lung, or diseases of skin (colour changes, and hard patches on palms and soles), or blood vessels of legs and feet. Fresh evidence indicates a possible association between intake of contaminated water and onset of diabetes, hypertension and reproductive disorders, states a WHO document.States like Maharashtra, Kerala, Uttarakhand, Goa, Tripura and Arunachal have not reported arsenic contamination in groundwater in any district. Odisha and Rajasthan are among the least affected states, with just one district in each having high arsenic levels in groundwater.WHO’s guideline value for acceptable levels of arsenic in drinking water is 0.01 mg/litre. In these 153 districts, the arsenic level in groundwater is above this benchmark.For India, the WHO has said that, in the absence of an alternative source of water, arsenic of 0.05 mg/l could be treated as a permissible level. How many districts are above even this level? The answer given in Parliament unfortunately does not have that information. Where does this arsenic come from? The main source in drinking water is arsenic rich rocks through which the water filters.It may also occur because of mining or industrial activities. Drinking the water, crops irrigated with it, and food prepared with contaminated water are the sources of exposure. Fish, shellfish, meat, poultry, dairy products and cereals could also be dietary sources of arsenic.