Thirty million people living in rural West Bengal are at risk from arsenic poisoning as the chemical is leaching out of the bedrock into wells and bore-holes.
Since the problem was identified in the 1990s hundreds of thousands of people in the area have developed the skin lesions associated with chronic arsenic poisoning.
The long-term effects of arsenic poisoning include cancer of the skin and other organs, progressive damage to the nervous system and muscles and, eventually death.
Calcutta Rescue started providing safe drinking water in one of the affected districts, Malda, on the border with Bangladesh, in 2003.
It has now installed and maintains arsenic filters in 12 villages and in 2019 brought back into use a derelict government filter in another village. These filters provide safe drinking water to 2,000 families.
The water is analysed regularly to ensure it is below the safe level of 0.05 mg/l laid down by the World Health Authority.
All the filters have been installed with electric pumps which makes them more popular with the villagers.